Azasterol inhibition of .DELTA.24-sterol methyltransferase in Saccharomyces cerevisiae

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Abstract
The inhibition of the .DELTA.24-sterol methyltransferase (24-SMT) of S. cerevisiae by side-chain azasterols is related to their nuclear skeleton and side chain N position. Inhibitory power [I50 (.mu.M)] was in the order of 25-azacholesterol hydrochloride salt (0.05) > 25-aza-24,25-dihydrozymosterol (0.08) > 25-azacholesterol .apprxeq. 25-azacholestanol (0.14) > (20R)- and (20S)-22,25-diazacholesterol (0.18) > 24-azacholesterol (0.22) > 25-aza-24,25-dihydrolanosterol (1.14) > 23-azacholesterol (4.8). In the presence of azasterols, S. cerevisiae produces increased amounts of zymosterol, decreased amounts of ergosterol and ergostatetraenol, and the new metabolites cholesta-7,24-dienol, cholesta-5,7,24-trienol and cholesta-5,7,22,24-tetraenol. Kinetic inhibition studies with partially purified 24-SMT and several azasterols suggest the azasterols act uncompetitively with respect to zymosterol and are competitive inhibitors with respect to S-adenosyl-L-methionine (SAM). These results are consistent with at least 2 kinetic mechanisms. One excludes competition of azasterol and zymosterol for the same site, whereas a second could involve a ping-pong mechanism in which 24-SMT is methylated by SAM and the methylated enzyme reacts with sterol substrate.