Gastric histamine methyltransferase: Different methylation rates for enantiomers of side-chain methylated histamine analogues using a highly purified enzyme preparation

Abstract

The inhibitor/activator and substrate properties of enantiomers of two methylated histamines (MH) were investigated using a histamine methyltransferase preparation which was purified 1207-fold from pig fundic mucosa by ultracentrifugation, ion-exchange chromatography on DEAE-cellulose and preparative electrofocusing. In 1-100 microM concentrations, S-alpha-MH and R-alpha-MH were acceptor substrates as good as histamine itself. When substrate concentrations were increased to 1 mM these substances were methylated to an even greater extent than histamine, since they did not exert substrate inhibition on HMT. Introduction of a further methyl-group into the N alpha-position reduced acceptor substrate properties drastically. A difference in methylation was then seen since R-alpha,N alpha-DMH was a better substrate than S-alpha,N alpha-DMH. Whereas alpha-MH's could not activate HMT the alpha,N alpha-DMH's did. The poorer the substrate affinity of the investigated substances was, the better they were able to activate HMT.

Inhibition of Rat Kidney Hisstamine-N-Methyltransferase by Biogenic Amines

“…HMT preparations from pig antrum mucosa, pig liver, guinea-pig brain and rat kidney were shown to methylate not only his tamine but also various synthetic histamine analogues [12,18,19,[21][22][23][24][25][26][27], Furthermore, rat kidneys were shown to contain a catechol-O-methyltransferase [28,29], The HMT used in the present study did not catalyze a methyl group transfer from SAM to any of the other biogenic amines, including the catechols adrenaline and arterenol. Although only chloroform-extractable methylation products would have been detected, methylated amines have been shown to be extracted into organic solvents [30], Thus, although the en zyme was only partially purified by a single DEAE ion exchange chromatography step (additional purification caused considerable loss of activity, as has been found by others [22]) it appeared to be specific for the meth ylation of histamine.…”