Speculations about the binding sites of S-adenosyl-l-homocysteine and some of its synthetic analogues to histamine methyltransferase

Conclusions

None of the synthetic analogues ofS-adenosyl-l-homocysteine was a more potent inhibitor of the enzyme thanS-adenosyl-l-homocysteine itself. Several requirements very probably had to be fulfilled for inhibition of gastric histamine methyltransferase by compounds similar in structure to S-adenosyl-l-homocysteine: (1) The side chain linked to the 5′-position of ribose must bear an amino acid residue; (2) the chain-length of this residue must be the same as that of homocysteine; (3) the heterocyclic base has to be a purine base with a nucleophilic center at position 6; (4) this nucleophilic center must not be sterically hindered by substitutes; (5) the purine base must have a nitrogen atom in position 3. These requirements indicate, that the binding sites, proposed byZappia et al. [2] for various methyltransferases were identical with those found for the fixation ofS-adenosyl-l-homocysteine or its analogues to histamine methyltransferase.