We described recently the purification and preliminary characterization of human hepatic betaine: homocysteins S-methyltransferase (Skiba, W. E., Taylor, M. P., Wells, M. S., Mangum, J. H., and Awad, W. M., Jr. (1982) J. Biol. Chem. 258, 14944-14948) where it was shown that isovalerate and 3,3-dimethylbutyrate, analogs of dimethylglycine and betaine, respectively, were good inhibitors. The present study demonstrates that butyrate is a modest competitive inhibitor, binding at the betaine site. This led to the consideration and synthesis of a putative dual-substrate analog, S(delta-carboxybutyl)-DL-homocysteine, which bound with high affinity to the active site of the methyltransferase; presumably this effect is due to the L-isomer only. Homologs, S(gamma-carboxypropyl)-DL-homocysteine and S(beta-carboxyethyl)-DL-homocysteine, do not inhibit at concentrations 100-fold higher than where inhibition is noted with the dual-substrate analog, indicating the latter's specificity. These findings support the hypothesis that methyl transfer in this enzyme occurs directly from one substrate to the other.