Francis D M, Thompson M F, Greaves M W
Biochem J. 1980 Jun 1;187(3):819-28. doi: 10.1042/bj1870819.
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.
在组胺和S-腺苷甲硫氨酸的极限浓度下，研究了人皮肤中组胺甲基转移酶的底物动力学特性。在浓度高于10 microM时，观察到组胺对底物的抑制作用。初始图显示了有序反应机制的证据。米氏常数是从初始图的斜率的二级图（[S]/v对 [S]）与第二种底物浓度的倒数得出的。组胺和S-腺苷甲硫氨酸的平均Km值分别为4.2 microM和1.8 microM。浓度为25-100 microM的组胺对S-腺苷甲硫氨酸的酶活性具有非竞争性抑制作用。未观察到S-腺苷甲硫氨酸对底物的抑制作用。为了进一步阐明反应机制，研究了两种产物S-腺苷同型半胱氨酸和1-甲基组胺的抑制作用。S-腺苷同型半胱氨酸对组胺具有非竞争性抑制作用，对S-腺苷甲硫氨酸具有竞争性抑制作用。1-甲基组胺对组胺和S-腺苷甲硫氨酸均具有非竞争性抑制作用。这些结果被解释为为有序的顺序Bi Bi反应机制提供了证据，甲基供体S-腺苷甲硫氨酸作为第一个添加到酶上的底物，组胺作为第二个底物。1-甲基组胺是第一个离开酶的产物，S-腺苷同型半胱氨酸是第二个。根据该酶在调节皮肤中组胺介导的反应中可能发挥的作用对结果进行了讨论。
