Structure-activity study of new inhibitors of human betaine-homocysteine S-methyltransferase

Betaine-homocysteine S-methyltransferase (BHMT) catalyzes the transfer of a methyl group from betaine to L-homocysteine, yielding dimethylglycine and L-methionine. In this study, we prepared a new series of BHMT inhibitors. The inhibitors were designed to mimic the hypothetical transition state of BHMT substrates and consisted of analogues with NH, N(CH₃), or N(CH ₃)₂ groups separated from the homocysteine sulfur atom by a methylene, ethylene, or a propylene spacer. Only the inhibitor with the N(CH₃) moiety and ethylene spacer gave moderate inhibition. This result led us to prepare two inhibitors lacking a nitrogen atom in the S-linked alkyl chain: (RS,RS)-5-(3-amino-3-carboxypropylthio)-3-methylpentanoic acid and (RS)-5-(3-amino-3-carboxypropylthio)-3,3-dimethylpentanoic acid. Both of these compounds were highly potent inhibitors of BHMT. The finding that BHMT does not tolerate a true betaine mimic within these inhibitors, especially the nitrogen atom, is surprising and evokes questions about putative conformational changes of BHMT upon the binding of the substrates/products and inhibitors.