Title of Invention

HETEROLOGUS EXPRESSION OF TRYPANOTHIONE REDUCTASE FROM LEISHMANIA DONOVANI IN A PROKARYOTIC SYSTEM

Abstract

The present invention relates to a process for heterologous expression and large-scale production of functionally active enzyme trypanothione reductase of Leishmania donovani in a prokaryotic system. The prime purpose of the invention is to obtain L. donovani TR in very high yield and at very low cost. This was achieved by expressing L. donovani TR in E.coli bacterial cells. The enzyme can be purified from the parasite itself but the process is very expensive, cumbersome and therefore, time consuming. Moreover, the yield was very low. The present invention provides a prokaryotic expression system for the expression of the said enzyme which is cheapest and easiest for large-scale yield.

Full Text

Field of the invention The present invention relates to a process for heterologous expression and large-scale production of functionally active enzyme trypanothione reductase of Leishmania donovani in prokaryotic system.\\\\\\\\\\\\\\\\\\\Background Information Human leishmaniasis has been rated as the second target next to malaria among the six major diseases identified by the WHO for intensive research and control efforts (1). Leishmania, a trypanosomatid parasite caused a wide spectrum of infection ranging from self-curing ulcer to often-fatal visceral diseases. Further, the disease has been recognized as an opportunistic infection in immunocompromised individuals particularly in patients infected with HIV (2). There is no vaccine in routine use and chemotherapy still relies on antimonial-based drugs, first used in early 20th century. The pentavalent antimonials (SbV) drugs Sodium stibogluconate (SAG) and N-methylglucamine antimoniate are the only anti leishmanials chemotherapeutic compounds with a certain degree of efficacy and safety. Sodium stibogluconate (Stibanate) first became available in 1945 (3). Two formulation of pentavalent antimonials are available, sodium stibogluconate solution (Pentosam, Wellcome Foundation, UK) containing 100 mg SbV /ml and meglumine asciculat solution (Glucantine, Rhone Poulence, France) containing 85 mg SbV /ml, and former is used in Indian subcontinent. Currently recommended dose of SbV is 20 mg/kg/day (MKD) for 30 days (4). The drug can be used via intramuscular or intravenous route. Response to relatively small daily dose (600 mg. Max.) for short duration (6-10 days) of SbV had been excellent until the early 1980 (5), when reports of treatment failure appeared, and modifications for SbV treatment were suggested to overcome the drug failure (6). The WHO revised its recommendations twice, resulting in an increase in the daily dose (from 10-20 mg/kg) and duration (from 6-10 days to 20-40 days) (7, 8). However, non-responsiveness to SAG is on increase especially in epidemic areas of visceral Leishmaniasis (9). Leishmania protein are generally insoluble in nature and tend to form aggregate i.e. inclusion bodies upon expression in prokaryotic hosts e.g. Adenylate kinase 2 of L. donovani (10), Methionine adenosyl transferase (MAT 2) of L. donovani (11), Cysteine protease type A & B (CPA & CPB) genes of I. infantum (12), Glucose regulating protein 94 (GRP 94) of I. infantum (13), Myristoyl- CoA N Myristoyl transferase of L major (14). To get active protein from inclusion bodies is a tedious process and requires lot of laboratory work and time. Some researchers has worked upon it and tried many conditions to get active protein in the soluble fraction but the yield is too low to work upon this e.g. L. mexicana aminotransferase was expressed to 1 mg./l bacterial culture (15), L. donovani recombinant chitinase Ld CHT1 (16), Ornithine decarboxylase (ODC) was expressed at 0.2 % of the soluble protein in E.coli. While Thymidylate synthase dihydrofolate reductase was expressed at a level of 2% of the soluble protein in E.coli. (17). The soluble functionally active enzyme must be substrate specific and should have other kinetic parameters in accordance with the natural enzyme. Therefore, a process needs to be developed for large-scale heterologous expression and purification of functionally active leishmanial proteins. Also, there is an increasing failure of insecticides to control vector. This has led to research on the basic studies to evaluate the significant differences between host and parasite, which leads to development of logical approaches towards new chemotherapeutic agents and vaccines. To validate new targets as well as new molecules active against the parasite requires lot of native target enzyme thereof and it is therefore necessary to get the target enzyme / protein in large amount by developing a heterologus expression system. As parasites, the trypanosomatids are inevitably exposed to various reactive oxygen species, such as superoxide radicals, hydrogen peroxide and myeloperoxidase products generated during the host defense reaction. However there ability to cope with such oxidative stress appears to be surprisingly weak. They lack both catalase & peroxidase and totally dependent upon unique Trypanothione reductase redox system to overcome this stress. Trypanothione Reductase (TR) is an NADPH-dependent flavoprotein oxidoreductase central to thiol metabolism in the leishmanias and trypanosomatids (18). The uniqueness of this cascade of oxidoreductase offers an opportunity to inhibit the parasite metabolic pathway without causing adverse effects in the host organism therefore make this enzyme an attractive target site for the development of the antileishmanials. The importance of trypanothione and trypanothione reductase in defending trypanosomatids against nitrosative and oxidative stress was established by disabling the function of trypanothione reductase gene by gene disruption. A double knock out of TR gene in Leishmania species could not be achieved which indicated that null mutants are not viable hence proved importance of the gene in parasite survival (Dumas, C., Quellette, M., Tovar, J., Cunningham, M. L., Fairlamb, A. H., Tamar, S., Olivier, M. and Papadopoulou,B. (1997), EMBO J., 16, 2590-2598). Mutants in which the TR activity had been reduced to half that of wild type by disrupting one member of allelic pair were not able to survive in macrophages that were capable of respiratory burst. In another approach which relies on the fact that the active enzyme is dimeric in nature an expression vector was constructed bearing inactive mutant of T.cruzi TR gene. This expression construct was heterologously expressed in L donovani. In the resultant recombinants, a large proportion of the TR consisted of inactive heterodimer with the result that TR activity was reduced to approximately 15% of normal cells (Tovar, J., Cunningham, M.L., Smith, A.C., Croft, S.L.andd Fairlamb, A.H. (1998)PNAS, 95, 5311-16). These recombinant L.donovani cells expressing very less active TR were incapable of suriving in IFN-gamma activated macrophages. This further confirms that parasite with much less TR activity than wild type were more vulnerable to oxidative and nitrosative stress". Hetrologous expression means the expression of a gene from one organism into another organism. In the paper by Tovar et al, 1998, an inactive mutant of T. cruzi TR was expressed in L.donovani cells resulting recombinant Leishmania cells expressing inactive TR. While, we have expressed the Leishmanial TR gene in E. coli cells resulting recombinant bacterial cells expressing active Leishmanial TR which was not done before. These recombinant bacterial cells were used to purify Active Leishmanial enzyme. Though in both cases the expression of TR was heterologous but in first case it was expression of inactive mutant TR of T.cruzi in L.donovani while in second it was expression of active L.donovani TR in E.coli cells. The main objective of this patent is to get L. donovani TR in very high yield and at very low cost. This was achieved by expressing L. donovani TR in E.coli bacterial cells. The enzyme can be purified from the parasite itself but the process is very expensive, cumbersome therefore time consuming. Moreover the yield was very low [Cunningham, M.L. and Fairlamb, A.H.(1995) Eur. J. Biochem., 230, 460-468]. To grow one liter of promastigotes of L. donovani in liquid 199 medium involves an expenditure of approximately $57.00 which will generate approximately 2.5g parasite while to grow 1L bacterial culture that comes only $1.69 generating 12.5 g cells. Further purification steps to process 1L parasite culture will cost $ 1011.8 resulting in total yield of only 0.47mg protein while purification from bacterial 1L culture will cost only 131$ with a total yield of approximately 16mg protein (Table). The expression vectors used in the present study have far more advantage in obtaining the desired results as revealed by the present study. In addition to pET41a we tried four more expression vectors but the yield of active protein was very-very less. Almost all of the expressed protein was inactive in the form of inclusion bodies. The best yield of the active recombinant Leishmanial TR was observed with pEt41a vector expressing in E.coli cells strain BL21. So far Trypanothione reductase has been isolated from C. asciculate itself (19) and from Trypanosoma cruzi (2.2 mg. TR / 33 g wet weight of cultured epimastigotes)(20). Briefly, 33.4g cells were suspended in 100 ml of buffer A (50mM Potassium phosphate, ImM EDTA, pH7.0 at 25°C) in presence of ImM digitonine. After 10 min. of gentle stirring the suspension was freeze thawed for 2 cycles and centrifuged for 30min at gravitational force, 4000g and temperature 4°C. The supernatant was combined with 50 ml of 2'5' ADP sepharose pre-equilibrated with buffer A . The suspension was shaken for 2 h and transferred to a chromatography column. Column was washed first with 150 ml of buffer A (at 4°C) followed by 60 ml of buffer B (RT) TR was eluted using 75 ml of 0.3mM NADPH in buffer B. Active fractions were combined to give TR in 2 electron-reduced form(EH2) which is susceptible to nonspecific auto oxidation. TR (EH2) was reduced by addition of TS2 and GSSG to a final cone. Of 1.6 µM and 1 mM respectively. Reaction was allowed to proceed for 30 min. at 25°C resulting in fraction 2. Fraction 2 was applied on to DEAE sephadex A- 50 column pre-equilibrated with buffer B. Column washed with buffer B and yellow band of TR was eluted with 0.5M KC1. Active fractions were combined. TR was precipitated with slow addition of solid ammonium sulfate to 60% saturation. After 24 h of precipitation at 4°C the sample was centrifuged at gravitational force,6000g for 10 min at 4°C temperature. The yellow precipitate was washed thrice with 45% ammonium sulfate in buffer A resulting in 95% pure trypanothione reductase. In 1989, first time, the cloned TR gene from Trypanosoma congolens had been expressed in E. coli, where the yield was only 1% of the total soluble fraction of the bacterial cells (17). The cell pellet obtained from 1L of culture of induced cells were suspended in 25 ml buffer A (20 mM phosphate buffer pH7.2, 5mM BME, ImM EDTA), sonicated six times for 1 min each and centrifuged. Nucleic acids were removed by addition of protamine sulphate to 0.4% concentration. The cloudy solution was then brought to 40% saturation with ammonium sulfate. Precipitated proteins and nucleic acid were removed by centrifugation and TR in supernatant was precipitated with 60% ammonium sulphate. The resulting pellet of TR was dissolved in buffer B (20mM Tris, 5mM BME, ImM EDTA pH7.2) and dialyzed extensively. The dialyzed solution was applied to DEAE sephacel (pre-equilibrated) column. The TR was eluted at 0.1M KC1 as yellow protein, which was dialyzed against buffer A. The yellow protein solution was applied on to 2'5' ADP sepharose pre-equilibrated column. The column was washed with 100 ml of buffer A and pure TR was eluted with 5mM NADP+ in buffer A. The enzyme was dialyzed against buffer A and stored at 4°C after concentration. Thus, the purification process was quite lengthy and involves several steps of purification. TR from Trypanosoma cruzi was cloned and expressed in E.coli (21). However, all the steps for the purification of recombinant TR were almost same as described for T. congolens (17) with a yield of approximately 6 mg/1 bacterial culture. TR from L. donovani Ethiopian strain has been cloned ,over-expressed in parasite itself (22). However, purification of the enzyme from overexpressing parasite resulted in very poor yield to work upon (23). No reports are available for heterologous expression of Leishmanial trypanothione reductase in prokaryotic system. We have presented a comparative data in table 1 which shows the present process is far more advantages than the earlier known methods. Further table 2 shows another comparison cost effectiveness of the present method than the known methods. Tablel: Comparison of the present invention with the prior art (Table Removed) Comparison of the cost in US$ required to purify Img of LDTR enzyme from I L.donovani promastigotes and recombinant E.coli cells (Table 2) Table 2 (Table Removed) Objects of the invention: The main object of the present invention provides a process for heterologus expression and large-scale production of functionally active enzyme trypanothione reductase of Leishmania donovani in prokaryotic system. Another object of the present invention provides characterization of the protein and that they can be prepared by means of and/ or isolated from a species of the family Trypanosomatidae. Still another object of the present invention provides a prokaryotic expression system for the expression of the said enzyme which is cheapest and easiest for large-scale yield. Yet another object of the present invention provides expressed recombinant enzyme as biologically active and as a soluble fraction of the total lysate. One more object of the present invention provides the enzyme in a soluble fraction which requires no further treatment for down stream applications. Still another object of the present invention provides higher total yield of the purified fraction than the earlier reported methods. Another object of the present invention provides the purified active enzyme which is stable at low temperature for long periods. One more objective of the present invention relates use use of primers having SEQ ID No.3 and SEQ ID No.4 for isolating Open Reading Frame (ORF) of trypanothione reductase from Leishmania donovani strain Dd8 having accession ATCC 50212 and its heterologus expression in a prokaryotic system. Yet another object of the present invention relates to a use of cloning and expression vectors for isolating Open Reading Frame (ORF) of trypanothione reductase from Leishmania donovani strain Dd8 having accession ATCC 50212 and its heterologus expression in a prokaryotic system. Still another object of the present invention relates to obtain L. donovani TR in very high yield and at very low cost by expressing L. donovani TR in E.coli bacterial cells Brief Description of Accompanying Figure/Drawings Fig.l. describes the PCR amplification of Leishmania donovani Trypanothione reductase ORF using L.donovani genomic DNA as a template: Lane 1- 4: Different concentration of L.donovani genomic DNA Lane 5-7: Master mix Lane 8: Molecular weight marker {100 bp ladder (GIBCO BRL)} Fig.2. describes the nucleotide sequence of cloned Leishmania donovani Trypanothione reductase of L. donovani Fig.3. describes the amino acid sequence encoded by cloned Leishmania donovani Trypanothione reductase Fig.4. describes the sequence alignment of Leishmania donovani Trypanothione reductase with other known Trypanothione Reductase sequences. Fig.5. describes the Leishmania donovani Trypanothione Reductase construct in pET 41 a expression vector. Fig.6. describes the SDS PAGE analysis of expressed Leishmania donovani Trypanothione Reductase and purified recombinant Leishmania donovani Trypanothione Reductase under reducing conditions on 10% gel. Lane 1: Cp - Crude E.coli supernatant Lane 2: F - Flow through Lane 3:W-Wash 2 Lane 4:M - BSA (marker) Lane 5-7:TR M, 54.6 Kd purified fractions Lane 8: TR and GST fractions Fig.7. describes the fold purification of Recombinant Leishmania donovani Trypanothione Reductase. Fig.8. describes the substrate specificity of recombinant purified Leishmania donovani Trypanothione Reductase. GSSG- Oxidised Glutathione, TS2- Oxidised Trypanothione Fig.9. describes the kinetics of Trypanothione Reductase with NADPH and trypanothione. Km - Michelies-Menten constant, Vmax. - Max. Velocity, TS2 - Oxidized Trypanothione, NADPH - Nicotinidine adenine dinucleotide phosphate Fig. 10. describes the effect of inhibitors on TR activity- (NX- Nifurtimox, NF- Nitrofurazone, MO-Melarsen oxide, MP- Melarsoprol, Sblll- Potassium antimony tartrate, SAG- Sodium antimony gluconate, BSO- DL- Butathionine-sulfoximine, BCNU- carmustine, CDNB- 1 chloro, 2-4dinirobenzene) Summary of invention Heterologous expression denotes expression of a gene isolated from one organism/cells into another (different) organisms/cells to get large amount of the expressed protein. Selected organism should express the protein not only in large amount but also in functional form. The three available systems used for heterologous expression are bacterial, yeast and mammalian cells. Most preferred is bacterial system. Trypanothione reductase(TR)enzyme of any Leishmania species has not been heterologously expressed in bacterial system due to various limitations described in prior art (Table 1) . For the first time expression TR enzyme of L. donovani (in functionally active form) in E. coli expression system is reported. Primers were designed to pull out TRORF from L. donovani genome using PCR, which were novel and have not used before therefore non-obvious. The expression vector used in this invention, pET 4la, has His and GST tags. This vector has not been used before for the expression of TR of even any other trypanosomatid parasite including Leishmania. Therefore selection of the vector was novel and non-obvious from the literature. Protein is expressed as fusion protein with GST, which helps in expression of protein in soluble form as well as reduces conventional several step purification processes to single step. GST could be cleaved onto column so the protein could be obtained in single band form even in first elution. Use of this vector for expression of TR has made the purification process very easy and has reduced time and chemical consuming lengthy process to single step. This is the novelty of the invention. Conditions are developed to get more than 80% expression of recombinant enzyme/protein in soluble form. Temperature of the culture, concentration of IPTG (isopropyl-D-thiogalactopyranoside) and time of induction play most important role in solublization of enzymes/protein in prokaryotic expression. So far studies reported maximum 10-20% of the total protein expressing in soluble form. This invention has developed the conditions, which resulted in more than 80% expression of TR enzyme in soluble form which are again novel and non-obvious. Simple steps for the maximum binding and elution of recombinant enzyme/protein from the column are defined that are not obvious from the prior art and are therefore novel. Detailed description of the invention The present invention provides a process for heterologus expression and large scale production of functionally active enzyme trypanothione reductase of Leishmania donovani in prokaryotic system, which comprises; (a) amplification of open reading frame of trypanothione reductase of strain Dd8(WHO ref no MHOM/IN/80/Dd8) ATCC No 50212, causative agent of Indian Kala azar, using polymerase chain reaction;(b) cloning of the amplified product (Ld TR ORF) in TA cloning vector, sequence analysis of cloned TR;(c) subcloning of LdTRORF in prokaryotic expression vectored) development of experimental conditions for maximum expression of cloned LdTRORF in soluble native form; (e)purification of the enzyme to homogeneity using affinity chromatography;(f) analysis of fold purification and specific activity;(g) biochemical characterization to determine Vmax and Km and (h)effect of known TR inhibitors, thiol depletors and antileishmanial drug(s) on recombinant purified Ld TR. Accordingly, the main embodiment of the present invention relates to a process for heterologus expression of functionally active enzyme trypanothione reductase of Leishmania donovani strain Dd8 having accession ATCC 50212 in a prokaryotic system, said process comprising the steps of: (a) amplifying, identifying and isolating the Open Reading Frame (ORF) of trypanothione reductase having DNA SEQ ID No. 1 and corresponding protein SEQ ID No. 2 using forward primer having SEQ ID No. 3 and reverse primer having SEQ ID No. 4, (b) cloning of the amplified product of step (a) in cloning and expression vector, (c) subcloning of the amplified product of step (b) in cloning and expression vector, (d) transforming the cloned ORF of steps (b) & (c) in a prokaryotic heterologus cloning and expression system, (e) studying the expression of the ORF transformed in step (d) under different in vitro conditions, and (f) purifying the enzyme from step (d) to homogeneity, and (a) analyzing physical and biochemical properties of the enzyme to establish the heterologus expression of soluble native form of ORF Leishmania donovani strain Dd8 having accession ATCC 50212 in a prokaryotic system. Another embodiment of the present invention relates to oligonucelotide primers having SEQ ID No. 1 and SEQ ID No.2 for isolating Open Reading Frame (ORF) of trypanothione reductase from Leishmania donovani strain Dd8 having accession ATCC 50212 and its heterologus expression in a prokaryotic system. Yet another embodiment of the present invention relates to cloning and expression vectors for isolating Open Reading Frame (ORF) of trypanothione reductase from Leishmania donovani strain Dd8 having accession ATCC 50212 and its heterologus expression in a prokaryotic system. Still another embodiment of the present invention relates to enzyme trypanothione reductase, wherein the enzyme trypanothione reductase, is central to thiol metabolism of the parasite and essential for the survival of the parasite in the macrophage system. Still another embodiment of the present invention relates to enzyme trypanothione reductase, wherein the enzyme trypanothione reductase is a drug target for prevention and treatment of Leishmania donovani. Still another embodiment of the present invention relates the amplification conditions in step (a) are single cycle at 95°C for 9 min followed by 25 cycles of 95°C for 1-2 min., 45°C -47°C for 1 min. and 72°C for 2-3 min and final single cycle of 72°C for 4 min. Still another embodiment of the present invention relates the complete open reading Frame (ORF) having nucleotide SEQ ID NO.l and corresponding protein SEQ ID NO.2 isolated in step (a) was amplified using forward oligonucelotide primer having SEQ ID No.3 and reverse oligonucelotide primer having SEQ ID No. 4. Still another embodiment of the present invention relates the cloning vectors, wherein cloning vectors in step (b) is selected from group comprising of TOPOII, TA and pGEM-T. Still another embodiment of the present invention relates to the cloning vector wherein, wherein cloning vector selected is pGEM-T. Still another embodiment of the present invention relates to a process as claimed in step (a), wherein cloning conditions in step (b) are: Vector and insert ratio 1:3 (1.5 (il: 4.5 \i\; 120ng of insert DNA), T4DNA ligase lµl(10U/nl), 10X phosphate buffer, Ligation conditions 14°C for 18 hrs. Still another embodiment of the present invention relates to a process in step (c), wherein expression vectors in step (c) is selected from group comprising of pQE 30, pET 2Id, pET 28b, pET 41 a and pGEX4T vectors. Still another embodiment of the present invention relates to the expression vector, wherein expression vector selected is pET41a Still another embodiment of the present invention relates to the prokaryotic system for cloning in step (d) are is from group comprising of JM 109 E.coli cells. Still another embodiment of the present invention relates to prokaryotic expressions in step (d) are selected from group comprising of BL21 DE3 or M-15 E.coli cells. Still another embodiment of the present invention relates to prokaryotic expression system is selected is BL21 DE3 E.coli cells. Still another embodiment of the present invention relates to, wherein the ORF sequence (designated as LDTR or LddTRpET41aDE3) of trypanothione reductase isolated from Leishmania donovani strain Dd8 is different from other known such sequences (Fig.4). Still another embodiment of the present invention relates to the purification steps in step (f) said purification are as following: (a) growing the E.coli cells containing LdTRORF overnight, (b) harvesting cells of step (a) and suspending them in lysis buffer, (c) sonicating the harvested cells and removing the debris from the harvested cells of step (b), (d) resuspending the cells again in phosphate buffer, (e) loading the suspended cells of step (d) in pre-equilibrated column of glutathione sepharose 4B column and incubating for about 5 hrs. at 22-20 min, (f) washing the column during incubation in step (e) twice with chilled PBS buffer, and adding thrombin at concentration of 1U/100 µg of loaded protein, and. (g) eluting the recombinant enzyme from step (f) by adding 20-25 ml of elution buffer. Still another embodiment of the present invention relates to growth temperature of E.coli cells, wherein the E.coli cells are grown at temperature of about 22-27°C. Still another embodiment of the present invention relates to composition of lysis buffer, wherein the lysis buffer comprises of potassium phosphate buffer (10 mM) pH 7.2, 10 mM EDTA, 0.01% triton X 100, 0.1 mM PMSF Still another embodiment of the present invention relates to the sonication, wherein sonication is done 3-5 times for 30 sec to 1 min pulse with 1-2 min. cooling interval. Still another embodiment of the present invention relates to the removal debris by centrifugation, wherein the debris is removed by centrifugation at 9000-120000 g fro 15-20 min. Still another embodiment of the present invention relates to the elution buffer, wherein the elution buffer is 50 mM Tris-HCl, pH 8.0. Still another embodiment of the present invention relates to the recombinant protein (LdTR), wherein the recombinant protein is isolated is having molecular mass of 54.6 kd. Still another embodiment of the present invention relates to the recombinant protein, wherein recombinant protein is having specific activity of 12.5 U/mg. Still another embodiment of the present invention relates to the total yield of recombinant protein, wherein the total yield of the recombinant protein is 16 mg/litre. Still another embodiment of the present invention relates to the Vmax of recombinant protein, wherein Vmax of recombinant protein with TS2 is 200 µM/ml/min and with NADPH2isl25^M/ml/min. Still another embodiment of the present invention relates to the Km pf recombinant protein , wherein Km of recombinant protein with TS2 is 50 µM and with NADPH2 is 20µM. The following examples are given by the way of illustrations and therefore should not be construed to limit the scope of the present invention. EXAMPLES Examplel Cloning and sequencing of TR of Leishmania donovani Dd8 strain Cloning and Sequencing: Trypanothione reductase (TR) gene is a single copy gene in Leishmanial genome and is located on a 1.1 MB chromosome (23). Complete Open reading frame of the said protein was amplified using genomic DNA (100-200ng/reaction) as a template and primers designed from known sequences flanking the TR ORF. The template DNA was initially denatured at 94°C for 6-9 min. then allow to amplify using 25-30 cycles at 95°C/l-2 min., 45-47°C/l min. and 72°C/2-3 min. The purified PCR product was cloned in any TA cloning vector like TOPOII vector (Invitrogen), pGEM-T (Promega) and TA cloning vector of Stratagene. The plasmid was then transformed and propagated in suitable E.coli competent cells. In total 5-7 clones were sequenced using dideoxy method (Sanger et al) in both direction to confirm the sequence of the amplicon and a 1508 nucleotide long ORF was submitted in EMBL genesequence (AJ415162). Two different restriction sites were incorporated in forward (TR-ATG) and reverse (TR-TAA) primers for directional subcloning in E.Coli expression vector. LdTRORF was amplified using these primers. The purified PCR product was digested with restriction enzymes to provide cohesive ends and ligated to suitable expression vector which can be pQE 30, pET 2Id, pET 28b, pET41a and in pGEX4T expression vectors. The ligated mixture was first transformed into the suitable (compatible to vector) E. coli competent cells and sequenced the insert. After confirmation of the sequence, the construct was further transformed again into the suitable host expression cells for heterologus expression of the protein. Host cells could be BL21DE3, M-15 or BL21DE3 (p Lys) depending upon the expression vector. Cell culture and DNA extraction: L.donovani promastigotes (WHO ref no MHOM/IN/80/Dd8) ATCC No 50212, regularly maintained at Central Drug Research Institute in Golden hamsters and grown in vitro in NNN medium (26) is used for extraction of genomic DNA. The cells were harvested by centrifugation for 15 min. at 5OOOrpm, washed twice with saline(0.9%NaCl) and suspended in 5ml buffer(0.2M TrisHCl pH 8.0, 0.2M EDTA, 0.5%SDS, 1mg /ml proteinase K). Cell lysate was incubated for 5 hours at 50°C and then extracted once with equal volume of phenol : chloroform : isoamylaalcohol (25:24:1) by mixing very slowly followed by centrifugation (10,000 rpm X 10 min. at 4°C). Aqueous layer was further extracted once with chloroform : isoamylalcohol (24:l).Genomic DNA was precipitated with sodium acetate (3M, pH 5.2 (1710th vol. of the aqueous phase) and 2.5 Vol. of absolute ethanol). Primers, PCR amplification, cloning and sequence analysis: Primers were designed from L.donovani, (Ethopian strain) sequences flanking the TR ORF. Forward primer: 5'CTCGCGAAAATTCTTCG3', Reverse primer: 5'GAGATGAAGAAGAAGGGCCTAA3'. Polymerase chain reaction was performed using 200ng of L.donovani genomic DNA as template, 45 µ,l of PCR mix.(Gibco BRL) and 60ng of each primer. The PCR conditions were 95°C/9min. (initial), 95°C/1 min., 47°C/1 min. and 72°C/2 min. for 25 cycles. The amplified product was analyzed by gel electrophoresis. Single band of 1.5kb was obtained (figl describes the PCR amplification of Leishmania donovani Trypanothione reductase ORF using L.donovani genomic DNA as a template. Lane 1- 4 depicts amplification of 1.5kb LDTR band from different concentration of L. donovani genomic DNA). The band was purified from gel using (QIAGEN Gel extraction Kit) and was ligated in pGEMT cloning vector (Promega). Ligation mixture consists of vector and insert in 1:3 ratio, 1.5ul and 4.5ul respectively(120ng DNA), T 4 DNA ligase, 1 µl (10U/ uL, GIBCO BRL), 10 X buffer, 1 ul and TDW,2ul. The mixture was incubated at 14°C for 18hr. 4ul (around 50 ngDNA) of ligated mixture was transformed and propagated in E.coli JM 109 competent cells. Transformed cells were selected by Ampicilline (lOOug/ml) resistance. Plasmids were purified using QIA prep spin plasmid kit (Qiagen Inc.). About 20ug plasmid DNA was purified from 5ml of culture. 2ug (4ul) of DNA was digested with Not l(lul) in presence of 1 OXbuffer(2ul) in a total reaction volume of 20ul and analyzed on 1% agarose gel for the presence of 1.5kb insert. In total 5 clones were sequenced using dideoxy method (Sanger et al) in both directions to confirm the sequence of amplicon and a 1508 nucleotide long ORF was submitted in EMBL genesequence (AJ415162). The full length encoding DNA and deduced amino acid sequence is shown in fig.2 (the nucleotide sequence of cloned trypanothione reductase of L. donovani) and Fig.3 (the amino acid sequence encoded by cloned Leishmania donovani trypanothione reductase). Sequence alignment analysis was performed using Meg-align software of DNA star. Fig.4 showed Clustalw linear alignment of TR ORF amino acid sequence of L. donovani (Dd8) with other TR sequences of different origin (LV9-L. donovani LV9 strain; LMA- L. major, DD8- L. donovani Dd8; CFA- C. fasciculata; TBU- T. brucei; TCO- T. congolense; TCR- T. cruzi). The L.donovani nucleotide sequence is most similar to Ethopian strain with 96% homology, while L. major TR is 92%, C. fasciculata 81%, T.congolens 82% and T. brucei is 84% identical. The identity at amino acid level is little lesser and it is 84% with Ethiopian strain, 81% with L. major, 68% with C. fasciculata, 59 % with T. congolens and 60% with T. cruzi. Example 2 Heterologous expression of LDdTR in E.coli: Expression and purification: A single colony of E.coli containing LdTRORF was inoculated in to 5-10 ml LB medium with ampicilline (75-100 µg/ml) and kanamycin (50-100 µg/ml) and allowed to grow at 22-3 7°C overnight. The overnight grown culture was then transferred to 100ml LB in 1:50 orl: 100 ratios containing ampicilline (50-100µg/ml) and kanamycin (50-100 µg/ml) and incubated further at 20°C-35°C. The culture was grown to the culture density of (O.D.)600 = 0.5-0.8 and induced with 0.1-2mM IPTG and then incubated at 20-28°C for 6-18 hrs. The cells were harvested by centrifugation at 3000-5000rpm for 20 min at 4° C. The wet cell pellet was suspended into 20ml of lysis buffer (l0mM K+Po4~ buffer pH 7.2, l0mM EDTA, 0.01% tritonXlOO, O.lmM PMSF) and sonicated with a power at 50W(3-5 times 30sec to 1min. pulse with 1-2 minute cooling interval). The cell homogenate was centrifuged at gravitational force 9000- 12000g for 15- 20 min. After removing debris clear supernatant was loaded to a pre-equilibrated column of Nickel agarose or Glutathione sepharose 4B. The column was then washed twice with l0x bed volume of chilled PBS. Then thrombin at the concentration of approx 1U/100 µg of loaded protein (in 50mM Tris-HCl, pH8.O) was applied on the column (if used glutathione-sepharose affinity chromatography) and incubated for 5 hours at 22-30°C.The recombinant enzyme was eluted with elution buffer (50mM Tris-Cl, pH8.O) in 20-25 ml. The fractions with higher activity were pooled together. Purity of the eluted protein was checked on SDS PAGE. Concentration of the protein was determined by Bradford method using BSA as standard (24) Subcloning in Expression vector: The forward (TR-ATG) and reverse (TR-TAA) primers were designed from LDdTR sequence that contain Ncol and BamHl restriction sites (bold and underlined) respectively to facilitate cloning directionally into pET-41a expression vector (Novagen). The primer sequences were, TR-ATG 5'GCATATCCATGGCCCGCGCGTACGACCTCGTG (forward); TR-TAA: 5' CGCGCGGATCCTCAGAGGTTGCTGCTGAGCTT (Reverse). Amplification was performed as described above using cloned LDdTR as template. The annealing was kept at 50°C with final extension at 72°C for 4min. The purified coding region of Ldonovani TR was digested with Ncol and BamHl to provide cohesive ends and ligated to pET41a (Novagen) treated with same enzymes and dephosphorylated. The resulting construct was first transformed into the DH5a competent cells and selected by kanamycin (50(µ.g/ml) resistance (fig5 describes the Leishmania donovani Trypanothione Reductase construct in pET 41 a expression vector). After confirmation of the sequence of LddTRpET41a, the construct was transformed again into the BL21DE3 expression cells for heterologous expression of the protein. Clones were selected by kanamycin (50µ.g/ml) resistance. Glycerol stock of clone LddTRpET41aDE3 was prepared in 15% glycerol in LB and stored at -80°C. Expression: Single clone of E.coli cells containing plasmid with the LdTR gene (LdTRpET41aDE3) was inoculated in 10 ml LB medium with kanamycin (50µ.g/ml) at 25°C overnight. The overnight grown culture was then transferred to 100ml LB in 1:100 ratio containing 50µig/ml kanamycin and incubated further at 22°C and 200rpm. The culture was grown upto the (O.D.) 600 = 0.4-0.5 and induced with ImM IPTG (isopropyl-D-thiogalactopyranoside) and further incubated at 22°C overnight at 200rpm. E.coli BL21 (DE3) containing pET41a plasmid alone was grown in the same way. The cells were harvested by centrifugation at 6000rpm for 20 min. The wet cell pellet was suspended into 20ml of lysis buffer (l0mM K+Po4" buffer pH 7.2, l0mM EDTA, 0.01% tritonXl00, 0.lmM PMSF) and sonicated with a power at 50W(six times thirty sec. pulse with one minute interval). The cell homogenate was centrifuged at 12000g for 20 min. After removing debris clear supernatant was tested for TR activity. Marked activity of TR was present in lysate of experimental cells while no activity was present in control cells. Glutathione reductase activity was almost same in control and experiment E. coli lysate. Protein concentration was determined by Bradford method using BSA as standard. Example 3 Purification Single clone LddTRpET41aDE3 was inoculated in 10 ml LB medium with kanamycin (50µ.g/ml) at 25°C overnight. The overnight grown culture was then transferred to 100ml LB in 1:100 ratio containing 50µg/ml kanamycin and incubated further at 22°Cand 200rpm. The culture was grown upto the (O.D.) 6oo = 0.4-0.5 and induced with ImM IPTG (isopropyl-D-thiogalactopyranoside) and further incubated at 22°C overnight at 200rpm. The cells were harvested by centrifugation at 6000rpm for 20 min. The wet cell pellet was either stored at -20°C or suspended into 20ml of lysis buffer (lOmM K+Po4" buffer pH 7.2, lOmM EDTA, 0.01% tritonXlOO, O.lmM PMSF) and sonicated with a power at 50W(six times thirty sec. pulse with one minute interval). The cell homogenate was centriftiged at 12000g for 20 min. After removing debris clear supernatant (20 ml) was loaded to a pre-equilibrated resin (SXwashed with 10ml and re-suspended in 8ml of 10 mM Potassium phosphate buffer pH 7.2), glutathione sepharose 4B column. The column was then washed twice with lOx bed volume (25 ml each time) of chilled PBS at flow rate Iml/min. Then thrombin (15 IU/ 5 ml of 50mM Tris-Cl, pH 8.0) at the concentration of lU/100u.g protein of E.coli lysate was applied on the column and incubated for 5 hours at 22°C. The recombinant enzyme was eluted with five bed volumes of elution buffer (5 X 5ml of 50mM Tris-Cl, pHS.O). Final elution was performed with 5ml of lOmM glutathione. The fractions with higher activity (Fractions 1 & 2) were pooled together and purity was checked on SDS PAGE. The expressed LdTR showed a molecular mass of about 54.6 Kd in SDS-PAGE (fig6 describes the SDS PAGE analysis of expressed and purified recombinant Leishmania donovani trypanothione reductase under reducing conditions on 10% gel. Lane lis Crude E. coli supernatant,Lane 2, Flow through; Lane 3,Washing of the column; Lane 4, BSA (marker); Lane 5-7 are 54.6 Kd purified LdTR fractions; Lane 8, LdTR and GST fractions). The molecular mass determined by MALDIJTOFF was 54.6 Kd and was in full agreement with the amino acids composition. Example 4 Enzyme assay, kinetics and inhibition studies Trypanothione reductase activity was routinely assayed spectrophotometrically at 340 nm, as previously described (25). The reaction mixture contained 20-100mM HEPES pH 7.2-7.8, 20µM-lmM EDTA, 80-200µM NADPH and 15-50µM TS2. One unit of enzyme activity is defined as that amount of enzyme required to convert 1 µmol of NADPH to NADP per minute at 25°C. Protein was assayed according to the method of Bradford (1976) as supplied by Bangalore Genei with Bovine serum albumin as a standard. For comparative kinetic studies, assay was performed at 50µM NADPH and at 5- lOOµM concentrations of TS2 and at 50µM TS2 and at 10- 200µM concentrations ofNADPH. Effect of various leishmanicidal (Sodium stibogluconate (SAG), sodium antimony (SbIII), trypanosomaticidal (Nifurtimox (NX), Nitrofurazone (NF), melarson oxide (MO), melarsoprole (MP), known inhibitors of glutathione reductase (carmustine BCNU), 1 chloro, 2-4dinirobenzene (CDNB) and antileishmanial moiety of antimony (Sblll) was studied on recombinant protein at 10-100µM concentrations. Trypanothione reductase activity was routinely assayed spectrophotometrically at 340 nm, as previously described (25). The reaction mixture contained lOOmM HEPES pH 7.8, 20µM EDTA, 80µM NADPH and 50µM TS2. The reaction was allowed to proceed for 5 min. and change in O.D. was monitored every after 30 sec.One unit of enzyme activity is defined, as that amount of enzyme required to convert 1 µmol of NADPH to NADP per minute at 25°C. The purified recombinant enzyme has a specific activity of 12.5 U/mg.protein. Purification procedure leads to 20fold purification from the soluble content of E.coli lysate. The total yield was 16mg/lit (fig7 describes the fold purification of Recombinant Leishmania donovani Trypanothione Reductase). The purified protein specifically catalyzed reduction of oxidized trypanothione reductase and did not catalyze reduction of oxidized glutathione (fig 8 describes the substrate specificity of recombinant purified Leishmania donovani Trypanothione Red). For comparative kinetic studies, assay was performed at 50µM NADPH and varying concentration of TS2 (20,40,60,80 &100) and at 50µM TS2 and varying concentration of NADPH (20,40,60,80,100 &150). The Vmax was found to be 200 µM/ml/min, 125 µM/ml/min and the Km value were 50 µM &20 µM respectively and in agreement to purified enzyme from the parasite (19,27) (fig 9 which describes the kinetics of Trypanothione Reductase with NADPH and trypanothione). Effect of Leishmanicidal compounds (Sodium stibogluconate, Antimony potassium tartrate), trypanosomicidal compounds (Melarson oxide, Melarsoprol, Nifurtimox, Nitrofuran), glutathione metabolism inhibitors (BSO, BCNU and CDNB) was studied on the activity of recombinant LDdTR at 50 and lOOµM concentration. Sblll the trivalent antimonial, active form of the antimony is having profound effect on TR activity in comparison to SAG, the pentavalent form of the antimony. Among trypanosomaticidal compounds Melarsen Oxide inhibited TR more than 95% at 50 uM concentration.This compound forms an adduct with trypanothione thus inhibits the enzyme maximally. The other compound namely Nifurtimox, Nitrofurazone and Melarsoprol , the known subversive substrate for TR also inhibited the Leishmanial TR to 20-60%, which is in accordance with the previous reports (26), The Glutathione metabolism inhibitors did not show much effect on TR activity(fig.lO describes the effect of inhibitors on Leishmanial TR activity). Thus we can infer that the described invention provide a simple and most efficient method for the heterologous expression of Leishmanial proteins in prokaryotic system Advantages The main advantages of present invention are: The present invention is aimed for the large-scale production of leishmanial protein trypanothione reductase in its native form in E.coli, which is otherwise present in very small quantity in Leishmania parasite. The lengthy and difficult procedure for purification of the enzyme from Leishmania parasite is replaced with very simple and single step purification from bacterial cells. The expressed recombinant enzyme is biologically active and in soluble fraction of the total lysate. The enzyme is in soluble fraction and requires no further treatment for down stream applications. Total yield of the purified fraction is much higher than the reported methods. The purified active enzyme is stable at 4°C for more than three months. The protein / enzyme can be utilized for large scale screening of antileishmanials compounds in target based high throughput screening in drug development program. Provided below is the sequence listing information SEO ID Nos. 1.2.3.4 SEQUENCE LISTING GENERAL INFORMATION APPLICANT: CSIR TITLE OF THE INVENTION: Heterologus expression of Trypanothione Reductase from Leishmania donovani in a prokaryotic system NUMBER OF SEQUENCES : 04 CORRESPONDENCE ADDRESS : Central Drug Research Institute, Chattar Manzil Palace, Lucknow-226001 (U.P.), India INFORMATION FOR SEQUENCE ID No:l 1. SEQUENCE CHARACTERISTICS : 1. LENGTH: 1507bp. 2. TYPE : DNA 5'atgtcccgcgcgtacgacctcgtggtgcttggcgccggatctggaggtctggaggcgggatggaacccgg ccgtcacgcacaaaaagaaggtcgggccgtcgtcgatgtgcaggcgacgcacggtccgccgctcttcgctcg gcggcacgtgcgtgaacgtcggctgcgtgccaaagaaactcatggtgacaggtgcccagtacatggacctgat ccgtgagtctggcggcttcgatgggagatggaccgcgaatcgctctgcccccactggaagacgctcatcgccg cgaagaacaaggtggtgaacagcatctacgagagctacaagagcatgttcgctgatacggagggcctcagctt tcacatgggcttcggtgccatcaatacgctcacccggtggtggtgcgcaagtcggaagacccacacagcgac gtgctgggaccctcgacacggattacatcctcattgccaccggctcttggccgacgcgcctcggagtccccgg cgacgagttctgcatcacgagcaacgaggcttctacctcgaggatgcccccaagcggatgctgtgcgtcggcg gctgctacatcgccgttgagtttgccggcatcttcaacggctacaagccccagggtggctatgtcgacctgtgct accgcggcgatcttattttgcgcggcttcgatacagaggtgcgcaagagcctgacgaagcagctgggggcga acggaataagagtgcgtacaaacttgaacccgacgaagatcacgaagaatgaggacggctcgaatcacgttc acttcaacgatggcacggaggaggactacgatcaggtcatgctcgcgatcggtcgcgtgccgcgctcgcagg cactacagctcgccaaggccggcgtccgaacaggaaagaacggtgccgtgcaggtcgacgcgtattcgaag acatcggtggacaacatctacgccatcgccatcggcgacgtgacgaaccgcgtgatgttgacgccggtggcc atcaacgaaggcgccgccttcgttgaaaccgtcttcggtggcaagccccgcgccaccgaccacaggaaggtc gcgtgccgcgtgttctccataccgccgatcggcacgtgcggcatgacggaggaggaggcggcgaagaacta cgaaaccgtcgccgtgtacgcgagctctttcacgccccttatgcacaacatcagcggcagcaagcacaaggaa ttcacgatccgcatcatcacgaacgaatccaacggcgaggttctgggtgttcacatgctcggcgacagtgcgcc tgagatcatccagagcgtcggcatttgcatgcagatgggcgccaagatcagcggcttccacagcaccatcgga gtccacccgacgagcgccgaggagctctgctccatgcgcactccagcgtacttctacgagagtggcaagcgc g tcgaaaagct cagcagcaac ctctgaagag ggaggagagatgaagaagaacgcgtcaaS' 3. ORGANISM: Leishmania donovani 4. IMMEDIATE: Natural sequence 5. NAME / KEY: DNA sequence 6. SEQUENCE ID # 1 INFORMATION FOR SEQUENCE ID No: 2 1. SEQUENCE CHARACTERISTICS: 1. LENGTH: 491 bp. 2. TYPE : Protein MSRAYDLVVLGAGSGGLEAGWNPAVTHKKKVGPSSMCRRRTVRRSS LGGTCVNVGCVPKKLMVTGAQYMDLIRESGGFGWEMDRESLCPHW KTLIAAKNKVVNSIYESYKSMFADTEGLSFHMGFGAINTLTRWWCAS RKTHTATCWDPRHGLHPHCHRLLADAPRSPRRRVLHHEQRGFYLEDA PKRMLCVGGCYIAVEFAGIFNGYKPQGGYVDLCYRGDLILRGFDTEV RKSLTKQLGANGIRVRTNLNPTKITKNEDGSNHVHFNDGTEEDYDQV MLAIGRVPRSQALQLAKAGVRTGKNGAVQVDAYSKTSVDNIYAIAIG DVTNRVMLTPVAINEGAAFVETVFGGKPRATDHRKVACRVFSIPPIGT CGMTEEEAAKNYETVAVYASSFTPLMHNISGSKHKEFTIRIITNESNGE VLGVHMLGDSAPEIIQSVGICMQMGAKISGFHSTIGVHPTSAEELCSMR TPAYFYESGKRVEKLSSNL 3. ORGANISM: Leishmania donovani 4. IMMEDIATE: Natural sequence 5. NAME / KEY: Amino acid sequence 6. SEQUENCE ID # 2 INFORMATION FOR SEQUENCE ID No: 3 1. SEQUENCE CHARACTERISTICS: 1. LENGTH :32bp. 2. TYPE : DNA 5 'GCATATCCATGGCCCGCGCGTACGACCTCGTG3' 3. ORGANISM: Leishmania donovani 4. IMMEDIATE: Artificial sequence 5. NAME / KEY: Oligonucelotide primer (forward) 6. SEQUENCE ID # 3 INFORMATION FOR SEQUENCE ID No: 4 1. SEQUENCE CHARACTERISTICS: 1. LENGTH: 32 bp. 2. TYPE : DNA 5'CGCGCGGATCCTCAGAGGTTGCTGCTGAGCTT3' 3. ORGANISM: Leishmania donovani 4. IMMEDIATE: Atificial sequence. 5. NAME / KEY: Oliognucelotide primer (Reverse Primer) 6. SEQUENCE ID # 4 References 1. WHO report on Leishmaniasis, 1993 * 2. WHO report on Leishmaniasis, 1994 3. Berman, J.D. (1988) Chemotherapy for Leishmaniasis: Biochemical mechanisms clinical efficacy and future strategies; Rev. Inf. Dis.; 10; 560- 586 4. Herwaldt, B.L. and Berman, J.D. (1992). Recommendations for treating leishmaniasis with sodium stibogluconate (pentostam) and review of pertinent clinical studies; Am. J. Trop. Med. Hyg; 46; 296-306 5. Peter, W.(1981), The treatment of kala-azar; New approach to an old problem, Indian journal of medical research, 73 (suppl.), 1-18 6. Thakur, C.P., Kumar, M., Singh, S.K. et al (1984), Comparison of regimes of treatment with sodium stibogluconate in India: a randomized study. British medical journal; 26, 21-25 7. WHO (1984) The Leishmaniasis: report of a WHO expert committee, WHO technical report series; 701, 99-108, 8. WHO (1990) The control of Leishmaniasis report of an expert committee WHO technical report series; 793,50-55 9. Shyam Sundar (2001) Drug resistance in Indian visceral leishmaniasis, Tropical medicine and international health; 6 (11), 849-854 10. Villa H., Perez-Peatejoy, Garcia- Estradac, Reguera, R.M., Reguena, J.M., Tekwani, B.L., Balana-Fouce, R., Ordonez, D. (2003), Molecular and functional characterization of Adenylate kinase2 gene from L.donovani, European J. Biochem., 270 (21) 4339-47 11. Perez-Pertejoy, Reguera, R.M., Villa, H., Garcia-Estrada, C., Balana- Fouce, R. Pajares, M. A. and Brdonez, D. (2003), Leishmania donovani Methionine adenosyltransferase - Role of cysteine residue in the recombinant enzyme, European J. Biochem., 270 (1), 28-35 12. Rafati, S., Nakhaee, A., Taheri, T., Ghashghaii, A., Salmanian, A. H., Jimnez, M., Mohebali, M., Masina, S. and Fasel, N. (2003) Expression of Cysteine Proteinase type land II of Leishmania infantum and their recognition by sera during canine and visceral leishmaniasis, Exp. Parasitol; 103(3-4), 143-151 13.Larreta, R., Soto, M., Alones, C. and Requena, R. M. (2000), Leishmania infantum: Gene cloning of the GRP 94 homologue, its expression as recombinant protein and analysis of antigenicity, Exp. Parasitol., 96(2), 108-115 14. Helen P Price, Malini R. Mehon, Chrysoula Panethymitaki, David Goulding, Paul G. Mekean and Deberoh F. Smith (2003), J.Bio.Chem., 278(9), 7200- 7214 15. Javier Vernal, Juan Jose Cazzulo and Cristina Nowicki (2003), Cloning and heterologus expression of broad specificity aminotransferase of Leishmania promastigotes, FEMS microbiology Letters 229, 217-222 16. Abdel Razek-Desouky, Charles A. Specht, Lynn Soong and Joseph M. Vinetz (2001), Leishmania donovani expression and characterization of E.coli. expressed recombinant Chitinase Ld CHTl,Exp.Parasitol. 99(3-4), 220-225 17. Francis X. Sullivan, Spencer L. Shames and Christopher T. Walsh (1989), Expression of Trypanosoma congolens Trypanothione reductase in E.coli. Over production, purification and characterization, Biochemistry, 28, 4986- 4992 18. Shames, S.L., Fairlamb, A.H., Cerami, A. and Walsh, C.T. (1986). Purification and characterization of trypanothione reductase from Crithidia fasciculata; a newly discovered member of family of disulphide containing flavoprotein reductases; Biochemistry; 25; 3519-3526 19. Shames, S.L., Fairlamb, A.H., Cerami, A. and Walsh, C.T. (1986). Purification and characterization of trypanothione reductase from Crithidia fasciculata; a newly discovered member of family of disulphide containing flavoprotein reductases; Biochemistry; 25; 3519-3526 20. R.Luise KRAUTH-Siegel, Burkhard Enders, Graemeb HENDERSON, A.H.FAIRLAMB and R. Heiner SCHIRNER (1987), Trypanothione reductase from Trypanosoma Cruzi, European JBiochem. 164, 123-128 21. Francis X. Sullivan and Christopher T. Walsh (1991)Cloning, sequencing, overproduction and purification of trypanothione reductase from T. cruzi, Mol.Biochem.Parasitol.,44, 1991, 145-148 22. Taylor, M.C., Kelly, J.M., Chapman, C.J., Fairlamb, A.H. and Miles, M.A. (1994). The structure, organization, and expression of the Leishmania donovani gene encoding trypanothione reductase. Mol. Biochem. Parasitol.; 64;293-301 23. Cunnigham, M. L. and Fairlamb, A. H> (1995) Trypanothione redutase from L. donovani: purification , characterization and inhibition by trivalent antimonials, Eur. J. Biochem, 230,; 462-468 24. M.M.Bradford (1989), A rapid and sensitive method for the quantitation of microgram quantities of proteins utilizing the principle of protein dye binding, Anal.Biochem. 177, 248-254 25. Cunningham,M.L., Zvelebil,M.J.J.M. & Fairlamb,A.H. (1994)Mechanism of inhibition of trypanothione reductase and glutathione reductase by trivalent organic arsenials, Eur. J. Biochem. 221, 285-295 26. Lemma, A. and Schilter, E.L. (1964). Extracellular cultivation of the Leishmanial bodies of species belonging to the protozoan genus Leishmania; Exp. Parasitol.; 15; 503-513 27. Borges, A.,. Cunnigham, M. L., Tovar, J. and Fairlamb, A.H. (1995), Site directed mutagenesis of the redox active cysteines of Trypanosoma cruzi trypanothione reductase, European J. Biochem. 228, 745-752 We Claim: A. process for heterologus expression of functionally active enzyme trypanothione reductase of Leishmania donovani strain Dd8 having accession ATCC 50212 in a prokaryotic system, said process comprising the steps of: a) amplifying, identifying and isolating the Open Reading Frame (ORF) of trypanothione reductase having DNA sequence represented by SEQ ID No. 1 and corresponding protein sequence represented by SEQ ID No. 2 using the forward primer of SEQ ID No. 3 and reverse primer of SEQ ID No. 4; b) cloning the amplified product of step (a) in cloning and expression vector selected from group comprising of TOPOII, TA and pGEM-T; c) subcloning of the product of step (b) in cloning and expression vector selected from group comprising of pQE 30, pET 21d, pET 28b, pET 41a and pGEX4T vectors; d) transforming the cloned ORF of steps (b) & (c) in a prokaryotic heterologus cloning and expression system, e) studying the expression of the ORF transformed in step (d) under different in vitro conditions, f) purifying the enzyme obtained from step (d) to homogeneity by the processes of the kind such as herein described, and g) analyzing physical and biochemical properties of the enzyme to establish the heterologus expression of soluble native form of ORF Leishmania donovani strain Dd8 having accession ATCC 50212 in a prokaryotic system. 2. A process as claimed in claim 1, wherein the amplification conditions in step (a) are single cycle at 95°C for 9 min followed by 25 cycles of 95°C for 1-2 min, 45°C -47°C for 1 min and 72°C for 2-3 min and final single cycle of 72°C for 4 min. 3. A process as claimed in claim 1, wherein the cloning conditions in step (b) are: (a) vector and insert ratio 1:3 (1.5 µ1: 4.5 µl; 120ng of insert DNA), (b) T4 DNA ligase 1µ (10 U/µl), (c) 10X phosphate buffer, (d) ligation conditions 14°C for 18 hrs. 4. A process as claimed in claim 1, wherein the prokaryotic cloning system in step (d) is preferably JM 109 E.coli cells. 5. A process as claimed in claim 1, wherein the prokaryotic expression system in step (d) is preferably BL21 DE3 or M-15 E.coli cells. 6. A process as claimed in claim 1, wherein the prokaryotic expression system is preferably BL21DE3 E.coli cells. 7. A process as claimed in claim I, wherein the purification steps in step (f) comprise: (e) growing the E.coli cells containing LdTRORF overnight, (f) harvesting cells of step (a) and suspending them in lysis buffer, (g) sonicating the harvested cells and removing the debris therefrom, (h) resuspending the cells as obtained in step (c) in phosphate buffer, (i) loading the suspended cells of step (d) in pre-equilibrated column of glutathione sepharose 4B column and incubating for 5 hrs at 22-20 degree C, (j) washing the column during incubation in step (e) twice with chilled PBS buffer, and adding thrombin at concentration of 1U/100 µg of loaded protein, and (k) eluting the recombinant enzyme from step (f) by adding 20-25 ml of elution buffer. 8. A process as claimed in claim 7, wherein the E.coli cells in step (a) are grown at temperature of about 22-27°C. 9. A process as claimed in claim 7, wherein the lysis buffer in step (b) comprises of potassium phosphate buffer (10 mM) pH 7.2,10 mM EDTA, 0.01% triton X 100,0.1 mM PMSF. 10. A process as claimed in claim 7, wherein sonication in step (c) is done 3-5 times for 30 sec to 1 min pulse with 1-2 min cooling interval. 11. A process as claimed in claim 7, wherein the debris in step (c) are removed by centrifugation at 9000-120000 g for 15-20 min. 12. A process as claimed in claim 7, wherein the elution buffer in step (g) is 50 mM Tris-HCl, pH 8.0. 13. A process for heterologus expression of functionally active enzyme trypanothione reductase of Leishmania donovani strain Dd8 having accession ATCC 50212 in a prokaryotic system substantially as herein described with reference to examples and drawings accompanying the specification.

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