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The configuration at the asymmetric sulfonium pole of S-adenosyl-L-methionine (SAM) necessary for optimal enzymatic binding and methyl donation has been elucidated in this study. For the transmethylations catalyzed by catechol O-methyltransferase, phenylethanolamine N-methyltransferase, histamine N-methyltransferase, and hydroxyindole O-methyltransferase, it was demonstrated that only the natural (-) enantiomer of SAM was active as a methyl donor. The corresponding (+)-SAM, which was prepared by enzymatic resolution of synthetic (+/-)-SAM, was shown to be inactive as a methyl donor in these enzymatic reactions. The (+)-SAM was found, however, to be a potent inhibitor of each of these enzyme-catalyzed transmethylations. These results suggest that the (+) enantiomer offers a nonproductive configuration for the methyl-transfer reaction itself; however, this configuration fails to hamper enzymatic binding. These results are discussed relative to the geometric requirements necessary for the methyl-transfer reaction and the requirements for enzymatic binding.

Structural analogues of S-adenosyl-L-homocysteine (L-SAH), with modifications in the amino acid or base portions of the molecule, have been synthesized and evaluated for their abilities to inhibit the transmethylations catalyzed by catechol O-methyltransferase (COMT), phenylethanolamine N-methyltransferase (PNMT), histamine N-methyltransferase (HMT), hydroxyindole O-methyltransferase (HIOMT), and indoleethylamine N-methyltransferase (INMT). From these studies some interesting and potentially useful differences in the structural features of L-SAH needed to produce maximal binding to these methyltransferases were detected. This paper provides evidence that 8-azaadenosyl-L-homocysteine is a potent and selective inhibitor of HIOMT, whereas N6-methyladenosyl-L-homocysteine and N6-methyl-3-deazaadenosyl-L-homocysteine are selective inhibitors in INMT. In contrast, it was found that S-tubercidinyl-L-homocysteine was a fairly potent, but nonselective inhibitor of all of the methyltransferases studied. The differences and the similarities in the requirements for the binding of SAH to methyltransferases which were detected in this study and earlier studies from our laboratory, are described. The possibilites of utilizing differences in binding requirements for the design of SAH analogues as specific inhibitors of methyltransferases are discussed.

Structural analogs of S-ADENOSYL-L-HONOCYSTEINE (L-SAG), WITH MODIFICATION IN THE RIBCOSE PORTION OF THE MOLECULE, HAVE BEEN SYNTHESIZED AND THEIR ABILITIES TO INHIBIT CATECHOL O-METHYLTRANSFERECE(COMT), phenylethanolamine N-methltransferase (PNMT) histamine N-methyltransferase (HMT),and hydroxyindole o-methytransferase (HIOMT) have been investigated. From these studies it was concluded that, in general, the 2'-hydroxyl and 3'-hydroxyl groups of the ribcose moiety of SAH play crucial roles in the binding of this molecule to most methyltransferases. However several interesting exceptions to this strict structural specificity have been observed. While S-3'-DEOXY-ADENOSYL-L-HOMOCYSTEINE PRODUCED NO INHIBITION OF HMT and HIOMT, it produced strong inhibition of the transmethylation catalyzed by PNMT and COMT. Likewise, S-2'-DEOXYADENOSYL-L-HOMOCYSTEINE AND S-5'-(9-(arabinofuranosyl)adenyl)-l-homocysteine had little or no effect of COMT, HMT, and HIOMT but were potent inhibtors of PNMT. The significance of these data relative to the nature of the SAH binding sites and the potential inhibitors of PNMT. The significance of these data relative to the nature of the SAH binding sites and the potential for in vivo differential inhibition of methyltransferases will be discussed.