EFFECTS OF AN AZASTEROL INHIBITOR OF STEROL 24-TRANSMETHYLATION ON STEROL BIOSYNTHESIS AND GROWTH OF LEISHMANIA-DONOVANI PROMASTIGOTES

Leishmania donovani promastigotes were cultured in the presence of an azasterol (20-piperidin-2-yl-5 alpha-pregnane-3 beta,20-diol) to deter mine the effects on sterol biosynthesis and cell proliferation. Inhibi tion of growth increased gradually with azasterol concentrations up to 5 mu g/ml; concentrations of azasterol exceeding 5 mu g/ml were letha l, Sterol biosynthesis was affected by the azasterol when administered at concentrations as low as 100 pg/ml. The primary site of action was the alkylation at C-24 of a Delta(24)-sterol precursor. The 24-alkyla ted sterols [ergosta-5,7,24(24(1))-trien-3 beta-ol and ergosta-5,7,22- trien-3 beta-ol] of the protozoan were replaced by Delta(24)-cholesta- type sterols which then accumulated in the cells, Administration of th e azasterol together with a bis-triazole inhibitor of the 14 alpha-met hylsterol 14-demethylase reaction, which operates in sterol biosynthes is, resulted in depletion of 24-alkylsterols and their replacement wit h predominantly 14 alpha-methylsterols lacking a 24-alkyl group. Conti nuous subculture of promastigotes in the presence of the azasterol res ulted in gradual depletion of 24-alkylsterols and their complete repla cement by Delta(24)-cholesta-type sterols. Transfer of the azasterol-t reated cells to medium lacking azasterol resulted in a gradual restora tion, after several subcultures, of the normal 24-alkylsterol pattern. The results indicate that, although 24-alkylsterols are normally prod uced by the protozoan, it can nevertheless survive with sterols posses sing only the cholestane skeleton. Thus there is no absolute requireme nt for 24-alkylsterols to fulfil some essential 'sparking' role associ ated with cell growth in promastigotes.