Altered lipid composition and enzyme activities of plasma membranes from Trypanosoma (Schizotrypanum) Cruzi epimastigotes grown in the presence of sterol biosynthesis inhibitors

The accepted mechanism for the antiproliferative effects of sterol biosynthesis inhibitors (SBI) against the protozoan parasite Trypanosoma (Schizotrypanum) cruzi, the causative agent of Chagas' disease, is the depletion of specific parasite sterols that are essential growth factors and cannot be replaced by cholesterol, the main sterol present in the vertebrate host. However, the precise metabolic roles of these specific parasite steroles are unknown. We approached this problem by subjecting T. cruzi epimastigotes to two types of SBI, inhibitors of sterol C-14 demethylase and Δ 24(25) methyl transferase, and investigating the modification of lipid composition and enzyme activities in the plasma membranes of the parasite. We found in purified plasma membrane from SBI-treated cells that, together with the expected changes in the sterol composition, there was also an inversion of the phosphatidylcholine (PC) to phosphatidylethanolamine (PE) ratio and a large increase in the content of saturated fatty acids esterified to phospholipids. The modification of the phospholipid headgroup composition correlated with a 70% reduction in the specific activity of the membrane-bound PC-PE- N-methyl transferase SBI-treated cells; it was shown that this inhibition was not due to a direct effect of the drug on the enzyme. Finally, the specific activity of the Mg 2+-dependent, vanadate-sensitive ATPase present in the membranes was also inhibited by ca. 50% in SBI-treated cells. The results suggest that one of the primary effects of the depletion of endogenous sterols induced by SBI in T. cruzi is a modification of the cellular phospholipid composition as a consequence of a reduced activity of PE-PC- N-methyl transferase and probably of the acyl δ 9 and Δ 9 desaturases; this, in turn, could affect the activity of other enzymatic and transport proteins.