Inhibition of sterol transmethylation by S-adenosylhomocysteine analogs

Structural analogs of S-adenosylhomocysteine were tested in vitro for inhibition of the yeast S-adenosylmethionine:delta 24-sterol-C-methyltransferase enzyme. A wide inhibitory range by these compounds was observed, suggesting which structural features of the parent compound are important for binding to the enzyme. No analog tested had inhibitory activity specific only for this enzyme. The most active compound was sinefungin, a metabolite of Streptomyces griseolus, which was also able to inhibit growth of yeast cultures. Sterol extracts of cells grown in the presence of sinefungin revealed a dramatic increase in the levels of zymosterol, the sterol substrate in the transmethylation under study, and a concomitant decrease in the levels of ergosterol. Evidence is presented that sinefungin is transported inside the cell by the same permease as S-adenosylmethionine. We conclude that sinefungin is blocking the in vivo methylation of sterols in yeast. The implications of this finding are discussed.