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.