ROLE OF GLUTATHIONE IN DRUG METABOLISM BY SCHISTOSOMES

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Schistosomiasis is estimated to afflict well over 200 million people worldwide. Until a practical vaccine is available, chemotherapy will continue to be important both for treating infected individuals and in programs for controlling transmission within populations. Significant advances toward the rational development of new, more effective antischistosomal drugs will be facilitated by a detailed understanding of the biochemical mechanisms underlying drug-parasite interactions. Such interactions include not only the effects of a drug on the parasite, but also effects of the parasite on the drug. Thus, differences in the capacity of schistosomes to enzymatically detoxicate xenobiotic compounds could explain some observed strain or species differences in drug susceptibility. This proposal is designed to test the hypothesis that conjugation with reduced glutathione (GSH) is a pathway of xenobiotic metabolism in schistosmes and may be involved in the detoxication of some drugs by these parasites. Studies will focus on schistosomal glutathione S-transferases (GST) and their role in catalyzing such conjugation reactions. The subsequent fate of putative GSH conjugates will also be considered. Specific experimental objectives are as follows: 1) Using existing assays for GST, survey the types of GSH-dependent reactions catalyzed by cell-free Schistosoma mansoni preparations; 2) Purify GST from adult S. mansoni by affinity chromatography and high pressure liquid chromatography; characterize the purified enzyme/isoenzymes with respect to physicochemical properties and catalytic and ligand binding functions using standard biochemical techniques; 3) With model substrates and specific antisera, document changes in the expression of GST in S. mansoni during development from cercaria to adult; 4) Similarly, explore whether schistosomal GST is inducible by treatment with certain xenobiotics; 5) compare GST in S. mansoni and S. hematobium to determine whether species differences in the capacity to metabolize dichlorvos, the active form of metrifonate, can account for the differential species susceptibility to that drug; and 6) With a radiolabeled model drug and high pressure liquid chromatograhic analysis, determine whether adult S. mansoni excrete GSH thioether conjugates directly or whether these are further metabolized to other drug products such as mercapturic acids.