Rat and mouse brain histamine N-methyltransferase: modulation by methylated indoleamines.

A purification procedure for rat and mouse brain histamine N-methyltransferase (HMT. EC 2.1.1.8) is described which achieves the preparation of 87-fold purified rat brain and 166-fold purified mouse brain enzyme. The purified HMT (MW 29.000) is inhibited by a number of physiologically and pharmacologically active amines. among them several methylated indoleamines. at concentrations above 5 x M. At concentrations below 1 x lo-’ M. most of the methylated indoleamines stimulate HMT, provided histamine is maintained at, or close to, its optimal concentration as an HMT substrate, namely 1 x to -$ M. A study of the nature of the inhibitory process revealed a non-competitive inhibition of HMT by dopamine as against a competitive inhibition of the enzyme by most methylated indoleamines. Increasing the concentration of histamine beyond the optimal value, i.e. to inhibitory levels, resulted in less stimulation. The findings support the notion that MSO elicits the formation in selected brain cells of supranormal amounts of several methylated indoleamines which are able to stimulate HMT (and possibly other methyltransferases, see SALAS et al., 1977), thereby causing the depletion of the cerebral levels of S-adenosyl-L-methionine, reported previously (SCHATZ & SELLINGER, 1975b). THE PRINCIPAL catabolic pathway for histamine in rodent brain is by methylation to 1-methyl histamine2 and by further oxidation of the latter t o l-methylimidazoleacetic acid. As we have recently shown (SCHATZ & SELLINGER, 1975a), the activity catalyzing the methylation of histamine, histamine N-methyltransferase (HMT, EC 2.1.1.8) becomes elevated in both mouse and rat brain following the administration of the convulsant agent L-methionine-dl-sulfoximine (MSO). Since this treatment also leads to a marked depletion of cerebral S-adenosyl-L-methionine (SCHATZ & SELLINGER, 1975b), we proposed (SCHATZ & SELLINGER, 19750) that MSO elicits the accumulation within selected brain cells of methylated HMT-stimulating substances. To begin testing this notion, we decided to examine first the effectiveness of several naturally occurring and biologically potent ( ROSENGARTEN & FRIEDHOFF, 1976; GILLIN et al., 19760) methylated indoleamines as HMT modulators in uitro. To ensure a high specificity of interaction between the methylated indoleamines and the HMT protein, we purified HMT from mouse and rat brain and we also tested the effect of several of the amines found active with ’ Supported by a United States Public Health Service grant. NINCDS 06294. Abbreuiations used: HMT. histamine N-methyltransferase; MSO. L-methionine-dl-sulfoximine; PNMT. phenylethanolamine N-methyltransferase; DTT. dithiothreitol; COMT. catechol 0-methyltransferase. By the ‘histamine nomenclature‘ (see BARTH e f al.. 1973). In the ‘histidine nomenclature’ (see SCHAYER & REILLY. 1975) this is 3-methylhistamine. H M T on another methyltransferase, phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28). In addition, we purified HMT from brains of MSOtreated animals and compared its interaction with N,N-dimethyltryptamine to that of the control enzyme. A preliminary account of some of the findings has appeared (SELLINGER el al.. 1976). EXPERIMENTAL PROCEDURES Animals. Adult, male Sprague-Dawley rats (200-250 g) and male Swiss-Webster mice (20-25 g) (Spartan Research Animals, Haslett, MI) were killed by decapitation and the brains (minus the cerebellum) rapidly excised and weighed. Usually 1S21 g of rat and 9-14g of mouse brain were worked up. Chemicals. The following substances were purchased from Sigma Co. (St. Louis. MO): dithiothreitol (DTT), ammonium sulfate (enzyme grade). sucrose (enzyme grade), DL-norepinephrine HCI. L-epinephrine, tryptamine HCI. imidazole, 3-hydroxytyramine HCI. 2-methylimidazole. N,N-dimethyltryptamine, 5-hydroxytryptamine. 5-hydroxyDL-tryptophan, 5-hydroxyindoleacetic acid, 3-methoxytyramine, ~-3-methoxytyrosine, L-I-methylhistidine. N,Ndimethyl-5-methoxytryptamine HCI. 5methoxytryptamine HCI, L-tryptophan, a-methyl-DL-histidinediHCI, 5-methoxy-DL-tryptophan and L-3-methylhistidine. Regis Chemical Co., Morton Grove, IL supplied N-methyl-3-hydroxytyramine HCI, N-methyl-5-hydroxytryptamine hydrogen oxalate, m-octopamine HCI. 4-methoxy-3-hydroxyphenethylamine HCI. 4-methoxyphenethylamine HCI, 3,4dimethoxyphenethylamine HCI. synephrine. melatonin. ~~-3-m-dimethyl-tyrosine methyl ester HCI. N-methyltryptamine and N-acetyl dopamine. Tyramine HCI. Ldihydroxyphenylalanine and phenylethanolamine were from