STEREOCHEMISTRY OF YEAST DELTA(24)-STEROL METHYL TRANSFERASE

S-Adenosyl-1-methionine:Delta(24)-sterol methyl transferase (24-SMT) m ediates introduction of the C-28 carbon of yeast sterols. It has been shown that sulfonium analogues of the presumptive cationic intermediat es of the methylenation reaction are potent in vivo and in vitro inhib itors of this process. In the presence of these inhibitors, cultures o f yeast produced increased proportions of zymosterol, the natural subs trate of the enzyme, while proportions of ergosterol and ergostatetrae nol were decreased. New C27-sterol metabolites were also found. The in vivo inhibitory power of the analogues [I-50 (mu M)] was determined f rom the proportion of C-24 methylated sterols to C-24 nonmethylated st erols in treated cultures to be in the following order: 25-thiacholest erol iodide (0.07) > 24(S)-methyl-25-thiacholesteryl iodide (0.14) > 2 4(R)-methyl-25-thiacholesteryl iodide (0.25). Kinetic inhibition as re vealed by radiolabeled S-adenosyl-1-methionine (SAM), crude enzyme and 25-thiacholesteryl iodide revealed this inhibitor to be uncompetitive with respect to zymosterol and competitive with respect to SAM. The g reater inhibitory power of 24(S)-methyl-25-thiacholesteryl iodide comp ared to 24(R)-methyl-25-thiacholesteryl iodide suggests that methyl do nation to Delta(24) occurs from the si face. When considered in conjun ction with Arigoni's previous work, the present results infer the meth ylenation mediated by yeast 24-SMT proceeds by alkylation from the si face of Delta(24) followed by migration of a hydrogen from C-24 to C-2 5 across the re face and final loss of a hydrogen from C-28 on the re face. (C) 1997 Elsevier Science Ltd.