Mouse kidney histamine N-methyltransferase: assay conditions, biochemical properties and strain variation

Characterization of a unique catechol-O-methyltransferase as a molecular drug target in parasitic filarial nematodes

BACKGROUND

Filarial nematodes cause severe illnesses in humans and canines including limb deformities and disfigurement, heart failure, blindness, and death, among others. There are no vaccines, and current drugs against filarial nematodes infections have only modest effects and are prone to complications.

METHODOLOGY/PRINCIPAL FINDINGS

We identified a gene (herein called DiMT) encoding an S-adenosyl-L-methionine (SAM)-dependent methyltransferase with orthologs in parasite filarial worms but not in mammals. By in silico analysis, DiMT possesses catalytic sites for binding SAM and catecholamines with high affinity. We expressed and purified recombinant DiMT protein and used it as an enzyme in a series of SAM-dependent methylation assays. DiMT acted specifically as a catechol-O-methyltransferase (COMT), catalyzing catabolic methylation of dopamine, and depicted Michaelis Menten kinetics on substrate and co-substrate. Among a set of SAM-dependent methyltransferase inhibitors, we identified compounds that bound with high affinity to DiMT's catalytic sites and inhibited its enzymatic activity. By testing the efficacy of DiMT inhibitors against microfilariae of Dirofilaria immitis in culture, we identified three inhibitors with concentration- and time-dependent effect of killing D. immitis microfilariae. Importantly, RNAi silencing of a DiMT ortholog in Caenorhabditis elegans has been shown to be lethal, likely as a result of excessive accumulation of active catecholamines that inhibit worm locomotion, pharyngeal pumping and fecundity.

CONCLUSIONS/SIGNIFICANCE

Together, we have unveiled DiMT as an essential COMT that is conserved in parasitic filarial nematodes, but is significantly different from mammalian COMTs and, therefore, is a viable target for development of novel drugs against filarial nematode infections.

Chronic Intake of Japanese Sake Mediates Radiation-Induced Metabolic Alterations in Mouse Liver

Sake is a traditional Japanese alcoholic beverage that is gaining popularity worldwide. Although sake is reported to have beneficial health effects, it is not known whether chronic sake consumption modulates health risks due to radiation exposure or other factors. Here, the effects of chronic administration of sake on radiation-induced metabolic alterations in the livers of mice were evaluated. Sake (junmai-shu) was administered daily to female mice (C3H/He) for one month, and the mice were exposed to fractionated doses of X-rays (0.75 Gy/day) for the last four days of the sake administration period. For comparative analysis, a group of mice were administered 15% (v/v) ethanol in water instead of sake. Metabolites in the liver were analyzed by capillary electrophoresis-time-of-flight mass spectrometry one day following the last exposure to radiation. The metabolite profiles of mice chronically administered sake in combination with radiation showed marked changes in purine, pyrimidine, and glutathione (GSH) metabolism, which were only partially altered by radiation or sake administration alone. Notably, the changes in GSH metabolism were not observed in mice treated with radiation following chronic administration of 15% ethanol in water. Changes in several metabolites, including methionine and valine, were induced by radiation alone, but were not detected in the livers of mice who received chronic administration of sake. In addition, the chronic administration of sake increased the level of serum triglycerides, although radiation exposure suppressed this increase. Taken together, the present findings suggest that chronic sake consumption promotes GSH metabolism and anti-oxidative activities in the liver, and thereby may contribute to minimizing the adverse effects associated with radiation.

Periodic properties of the histaminergic system of the mouse brain

Brain histamine is involved in the regulation of the sleep-wake cycle and alertness. Despite the widespread use of the mouse as an experimental model, the periodic properties of major markers of the mouse histaminergic system have not been comprehensively characterized. We analysed the daily levels of histamine and its first metabolite, 1-methylhistamine, in different brain structures of C57BL/6J and CBA/J mouse strains, and the mRNA level and activity of histidine decarboxylase and histamine-N-methyltransferase in C57BL/6J mice. In the C57BL/6J strain, histamine release, assessed by in vivo microdialysis, underwent prominent periodic changes. The main period was 24 h peaking during the activity period. Additional 8 h periods were also observed. The release was highly positively correlated with active wakefulness, as shown by electroencephalography. In both mouse strains, tissue histamine levels remained steady for 24 h in all structures except for the hypothalamus of CBA/J mice, where 24-h periodicity was observed. Brain tissue 1-methylhistamine levels in both strains reached their maxima in the periods of activity. The mRNA level of histidine decarboxylase in the tuberomamillary nucleus and the activities of histidine decarboxylase and histamine-N-methyltransferase in the striatum and cortex did not show a 24-h rhythm, whereas in the hypothalamus the activities of both enzymes had a 12-h periodicity. These results show that the activities of histamine-metabolizing enzymes are not under simple direct circadian regulation. The complex and non-uniform temporal patterns of the histaminergic system of the mouse brain suggest that histamine is strongly involved in the maintenance of active wakefulness.

Autonomous histamine metabolism in human melanoma cells

Melanoma cells constitutively produce various cytokines as well as growth factors and express their corresponding receptors. Exogenous histamine is known to be a growth factor for some tumours while in other cases histamine inhibits tumour growth, and acts on G protein-coupled H1 and H2 histamine receptors. In previous studies we have detected the expression of the l-histidine decarboxylase (HDC) gene and the presence of HDC protein in human melanoma cell lines. In the present study, the activities of the histamine-forming enzyme HDC and of the degrading enzymes diamine oxidase (DAO) and histamine N-methyltransferase (HNMT) were measured in primary (WM35 and WM983) and metastatic (M1 and HT168) human melanoma cell lines. HDC activity was found in WM35 and WM983 cell lines, while detectable HNMT activity was measured in WM983, M1 and HT168 lines. In contrast, DAO showed very low activity in melanoma cell lines. Melanoma cells release a detectable amount of histamine into the medium without external stimuli. These findings support the possibility of autonomous histamine metabolism in melanoma cells. Our results suggest that not only exogenous histamine but also histamine produced and released by the melanoma cells and acting as an autocrine and paracrine factor may influence cell proliferation and modulate the in situ immune response of the host.

Histamine N-methyltransferase: inhibition by monoamine oxidase inhibitors

Histamine N-methyltransferase (HNMT) catalyzes the N tau-methylation of histamine. N tau-Methylhistamine can then undergo oxidation catalyzed by the mitochondrial enzyme monoamine oxidase (MAO). Addition of an MAO inhibitor such as pargyline to tissue preparations can increase the HNMT activity assayed --presumably as a result of inhibition of N tau-methylhistamine metabolism by MAO. However, pargyline-dependent "activation" of HNMT may also occur in tissue preparations that lack mitochondria. Our experiments were performed to determine whether MAO inhibitors, like many other amine compounds, could directly increase the activity of partially purified HNMT, and, if so, to study the mechanism of activation. Human kidney HNMT was partially purified by sequential ion exchange and gel filtration chromatography. The activity of the purified HNMT was increased approximately 50% in the presence of pargyline. However, enzyme kinetic experiments showed that pargyline, like many other amines, was a competitive inhibitor of HNMT. Apparent activation of the enzyme resulted from sequential shifts of histamine substrate curves to higher Vmax values in the presence of increasing concentrations of pargyline. Other acetylenic MAO inhibitors, clorgyline and the two stereoisomers of deprenyl, were also competitive inhibitors of purified human kidney HNMT. Inhibition kinetic experiments performed in the presence of varying concentrations of histamine demonstrated that Kis values for pargyline, clorgyline, (R)-deprenyl and (S)-deprenyl were 0.126, 0.144, 0.217, and 0.627 mM, respectively. When the concentration of the cosubstrate for the reaction, S-adenosyl-L-methionine, was varied in the presence of variable concentrations of pargyline, inhibition of HNMT by pargyline was noncompetitive with regard to the methyl donor, with Kii and Kis values of 1.23 and 0.95 mM, respectively. Finally, several amine compounds related structurally to pargyline were also found to be inhibitors of HNMT.