Leishmania donovani survives and proliferates in hydrolytic environments throughout its digenetic life cycle. How the parasite avoids destruction is unknown. Cell surface macromolecules, such as glycoconjugates, most likely play a key role in the survival of the parasite in hostile environments and in confrontation with host-immune responses. We have found that L. donovani expresses a major cell surface glycoconjugate called lipophosphoglycan (LPG). On the basis of our results, the structure of this unique macromolecule is proposed. In the ensuring years of this grant period, we will focus on three aspects concerning LPG: (1) The entire structure of the glycoconjugate will be completed. The structure of the phosphoheptasaccharide core and the location of the terminal galactose residue remain to be elucidate. Sufficient quantities of LPG fragments can be obtained by both chemical and enzymatic digestions and these fragments will be examined by 1H-NMR and GC- MS to complete the overall structure. Also, the hydrophilic form of LPG that is released into the culture medium as well as the hydrolytic enzyme responsible for its release will be characterize. (2) The function of LPG will be examined. Experiments are designed to pursue evidence that implicates LPG as in inhibitor of protein kinase C, the enzyme that is believed to be responsible for induction of the oxidative burst in phagocytic cells. In addition, mutant parasites with altered surface LPG will be analyzed to gain insights regarding its function. Failure of the mutant parasites to thrive in expected environments would be a major step in determining the function of LPG. (3) The pathway of biosynthesis of the glycoconjugate will be investigated. Emphasis will be focused on two fundamental aspects of LPG biosynthesis: addition of an unactelyated glucosamine residue to the lyso-alkyl-PI lipid and polymerization of the repeating disaccharide units. From these studies, we hope to contribute to the understanding of the role LPG plays in the pathogenesis of leishmaniasis and to provide a biochemical rationale for the design of chemotherapeutic regimens that exploit the unique structure of LPG.
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