title: The kinetic properties and reaction mechanism of histamine methyltransferase from human skin.
abstract: The substrate kinetic properties of histamine methyltransferase from human skin were studied at limiting concentrations of both histamine and S-adenosylmethionine. Substrate inhibition by histamine was observed at concentrations above 10 microM. Primary plots showed evidence of a sequential reaction mechanism. The Michaelis constants were derived from secondary plots of slopes from the primary plots ([S]/v versus [S]) versus reciprocal of the second substrate concentration. The mean Km values for histamine and S-adenosylmethionine were 4.2 and 1.8 microM respectively. Histamine in concentrations of 25-100 microM inhibited enzyme activity uncompetitively with respect to S-adenosylmethionine. No substrate inhibition was observed with S-adenosylmethionine. To elucidate the reaction mechanism further, inhibition by the two products, S-adenosylhomocysteine and 1-methylhistamine, was studied. S-Adenosylhomocysteine inhibited non-competitively with respect to histamine and competitively with respect to S-adenosylmethionine. 1-Methylhistamine inhibited non-competitively with respect to histamine and to S-adenosylmethionine. These results are interpreted as providing evidence for an ordered sequential Bi Bi reaction mechanism, with the methyl-group donor S-adenosylmethionine as the first substrate that adds to the enzyme and histamine as the second substrate. 1-Methylhistamine is the first product to leave the enzyme and S-adenosylhomocysteine is the second. The results are discussed in terms of the possible role that this enzyme could play in the modulation of histamine-mediated reactions in skin.
mesh_terms: D006632:Histamine / Q000378:metabolism; D006637:Histamine N-Methyltransferase / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008780:Methyltransferases / Q000378:metabolism*; D011485:Protein Binding; D012436:S-Adenosylmethionine / Q000378:metabolism; D012867:Skin / Q000201:enzymology*
chemical_list: D012436:S-Adenosylmethionine; D006632:Histamine; D008780:Methyltransferases; D006637:Histamine N-Methyltransferase