Substrate specificity and heterogeneity of N-methyltransferases

Abstract

Histamine N-methyltransferase ( EC. 2.1.1.8 ) from cat intestine and guinea pig brain was compared with indoleamine N-methyltransferase from rabbit lung and chick brain. Histamine N-methyltransferase, regardless of its source, methylated specifically histamine and not other imidazoles or aromatic amines. In contrast, indoleamine N-methyltransferase from rabbit lung had different substrate specificity from that of chick brain enzyme. The best substrate for lung indoleamine N-methyltransferase was N-methyltryptamine whereas serotonin was the best substrate for the chick brain enzyme. Both histamine N-methyltransferase and indoleamine N-methyltransferase appeared to occur in these species in multiple forms. They had different kinetic parameters, different pH optimum values, displayed different response towards inhibitors, and had different heat stability. © 1972.