Azasterol inhibitors in yeast. Inhibition of the Δ24-sterol methyltransferase and the 24-methylene sterol Δ24(28)-reductase in sterol mutants of Saccharomyces cerevisiae

Abstract

The effects of several monoazasterols on sterol biosynthesis were examined in the ergosterol deficient mutants erg 2, erg 3, and erg 5 of Saccharomyces cerevisiae. When the mutants were aerobically cultured in the presence of 1 μM 23-azacholesterol, the 24-methylene sterol Δ24(28)-reductase was essentially blocked and the immediate Δ24(28)-unsaturated precursor of the final sterol metabolite in each respective erg strain was found to accumulate. Total sterol production was enhanced in the cultures grown in the presence of 1 μM 23-azacholesterol. In cultures which were grown in the presence of 1 μM 25-azacholesterol which effectively blocked the Δ24-sterol methyltransferase, all three erg strains accumulated zymosterol as the major sterol component with lesser quantities of predicted terminal sterols. Mutant erg 2 (block at Δ8 → Δ7 isomerase) grew poorly in the presence of 1 μM 25-azacholesterol and produced low levels of cholesta-5,8,24-trienol and cholesta-5,8,22,24-tetraenol, which were isolated and characterized for the first time. Compared with controls, erg 2 treated with 1 μM 23-azacholesterol produced increased amounts of ergosta-5,8,22,24(28)-tetraenol, which was hitherto unidentified as a yeast sterol. In erg 5 (block at Δ22-dehydrogenase) treatment with 1 μM 25-azacholestanol effectively blocked the Δ24-sterol methyltransferase and resulted in increased total sterol production. Cholesta-5,7,24-trienol accounted for 27–29% of the sterol pool in 25-azasterol inhibited erg 5 cultures. The 25-azasteroi inhibited erg 5 mutant thus provides a source of cholesta-5,7,24-trienol, a potential provitamin D3 substitute.