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

Tissue distribution of histamine N-methyltransferase-like immunoreactivity in rodents

The rat kidney histamine N-methyltransferase was purified to homogeneity from Escherichia coli transfected with its recombinant cDNA. An antiserum to the enzyme was raised in rabbit by immunization with the purified protein. Western blot analysis of rat tissues with the antiserum revealed a band with identical mobility to that of purified enzyme in the extracts of kidney, jejunum, and brain, where the enzyme activity was detected. The antiserum cross-reacted with a 32K protein in mouse liver, brain, stomach, kidney and lung, and a 33K protein in guinea pig brain, stomach jejunum, spleen, lung, and kidney. The intensity of the staining in western blotting correlated well with the enzyme activity in all the tissues in these three species, suggesting that our antiserum is useful for quantifying histamine N-methyltransferase protein in rodent tissues.

Inhibition of histamine-N-methyltransferase (HNMT) by fragments of 9-amino-1,2,3,4-tetrahydroacridine (tacrine) and by beta-carbolines

Histamine-N-methyltransferase (HNMT), the major enzyme for the metabolism of histamine in rat brain, is potently inhibited by 9-amino-1,2,3,4-tetrahydroacridine (tacrine). Structural fragments of tacrine were less potent inhibitors of rat brain HNMT than was tacrine itself. Harmaline and a number of other beta-carbolines inhibited HNMT with IC50 values in the range of 1-10 microM. HNMT inhibition by harmaline was competitive with respect to both substrates, S-adenosylmethionine and histamine (Ki = 1.4 microM). These findings are discussed in the context of mechanisms for HNMT inhibition.

Regulation of the activity of histamine-N-methyltransferase from guinea pig skin by biogenic amines

Histamine-N-methyltransferase, a major histamine-degrading enzyme in the skin, was purified from guinea pig skin about 150-fold. The enzymological characteristics including pH optimum, Km values for substrates, and molecular weight were almost consistent with those reported in the brain. Regulatory mechanism of the enzyme activity by biogenic amines was investigated using the purified specimen. Serotonin, tryptamine, and 5-methoxytryptamine intensely inhibited the activity while tryptophan, melatonin, N-acetylserotonin, tryptophol, and 5-hydroxyindole acetic acid had no significant effects. Dopamine, tyramine, 3-methyltyramine, and phenylethylamine also inhibited the activity while no particular effects were obtained by adrenaline, noradrenaline, tyrosine, and DOPA. Spermidine and cadaverine caused significant but weaker inhibition. These amines acted competitively with respect to histamine, although varying manners were observed with respect to S-adenosyl-L-methionine. From these results, it was concluded that the enzyme activity was inhibited by such compounds in which a certain chemical structure, CH2-CH2-NH2 group neighboring the hydrophobic group, was contained. A possible mechanism of inhibition by the amines is postulated, and possible roles of such compounds in the inflammation by impairing the histamine metabolism is discussed.

Purification and kinetic properties of ox brain histamine N-methyltransferase

Histamine N-methyltransferase (EC 2.1.1.8) was purified 1100-fold from ox brain. The native enzyme has an Mr of 34800 +/- 2400 as measured by gel filtration on Sephadex G-100. The enzyme is highly specific for histamine. It does not methylate noradrenaline, adrenaline, DL-3,4-dihydroxymandelic acid, 3,4-dihydroxyphenylacetic acid, 3-hydroxytyramine or imidazole-4-acetic acid. Unlike the enzyme from rat and mouse brain, ox brain histamine N-methyltransferase did not exhibit substrate inhibition by histamine. Initial rate and product inhibition studies were consistent with an ordered steady-state mechanism with S-adenosylmethionine being the first substrate to bind to the enzyme and N-methylhistamine being the first product to dissociate.

Guinea pig brain histamine N-methyltransferase: purification and partial characterization

Histamine N-methyltransferase (EC 2.1.1.8) was purified 4400-fold in 12% yield from guinea pig brain. The basic steps in the purification included differential centrifugation, calcium phosphate adsorption, DEAE-cellulose chromatography, and affinity chromatography on an S-adenosylhomocysteine-agarose matrix. The resulting protein was homogeneous by gel electrophoresis and was stable for at least 3 months at -80 degrees C. It had an apparent molecular weight of 29,000 +/- 1000 as determined by both gel filtration through Sephadex G-100 and by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. The isoelectric point of the protein was found to be 5.3. The pH optima for methylation of histamine were determined to be 7.5 and 9.0; the KmS for histamine and S-adenosyl-L-methionine were 13.57 +/- 0.74 microM and 6.1 +/- 0.12 microM, respectively; the Ki for S-adenosyl-L-homocysteine was 24.5 +/- 1.45 microM.

Differences in activity in cerebral methyltransferases and monoamine oxidases between audiogenic seizure susceptible and resistant mice and deermice

The specific acitivity of cerebral histamine N-methyltransferase (HMT) was significantly lower in the audiogenic seizure-susceptible (SS) 21-day old DBA/2J mouse when compared to the non-susceptible 70-day old DBA/2J mouse but not when compared to the seizure resistant (SR) C57B1/6J mouse at 21 days of age. There was no significant difference between the two strains at 70 days of age. The lower HMT specific activity was also observed in a SS mutant of the deermouse Peromyuscus maniculatus, relative to the SR, wild-type animal. The activity of cerebral catechol-O-methyltransferase (COMT) was significantly lower in the DBA/2J mice relative to the C57B1/6J at 21 and 70 days while, in Peromyscus, it was higher in the SS mutant than in the SR animal. The activity of MAO, B was lower in the 21-day old, relative to the 70-day old DBA/2J and the 21-day old C57B1/6J mice. There were no differences in MAO A or B between SS and SR Peromyscus. The findings raise the possibility that cerebral methylation may operate at characteristic rates in SS animals.