The histamine methylating enzyme system in liver

Mutations in the histamine N-methyltransferase gene, HNMT, are associated with nonsyndromic autosomal recessive intellectual disability

Histamine (HA) acts as a neurotransmitter in the brain, which participates in the regulation of many biological processes including inflammation, gastric acid secretion and neuromodulation. The enzyme histamine N-methyltransferase (HNMT) inactivates HA by transferring a methyl group from S-adenosyl-l-methionine to HA, and is the only well-known pathway for termination of neurotransmission actions of HA in mammalian central nervous system. We performed autozygosity mapping followed by targeted exome sequencing and identified two homozygous HNMT alterations, p.Gly60Asp and p.Leu208Pro, in patients affected with nonsyndromic autosomal recessive intellectual disability from two unrelated consanguineous families of Turkish and Kurdish ancestry, respectively. We verified the complete absence of a functional HNMT in patients using in vitro toxicology assay. Using mutant and wild-type DNA constructs as well as in silico protein modeling, we confirmed that p.Gly60Asp disrupts the enzymatic activity of the protein, and that p.Leu208Pro results in reduced protein stability, resulting in decreased HA inactivation. Our results highlight the importance of inclusion of HNMT for genetic testing of individuals presenting with intellectual disability.

Metabolism of [c]-histamine in heart-lung-liver preparations of cats

The metabolism of injected [(14)C]-histamine was studied in heart-lung-liver preparations and heart-lung preparations from cats. When the liver was included in the circulation the injected histamine was rapidly eliminated from the blood, some of its metabolites appearing in the blood within 2 min after the injection. More than 70% of the injected histamine was metabolized to 1-methylimidazol-4-ylacetic acid. In the heart-lung preparations [(14)C]-histamine disappeared much more slowly from the blood and the major metabolite was 4-(2-aminoethyl)-1-methylimidazole (methylhistamine). Less than 4% of the injected (14)C was recovered in the form of imidazol-4(or 5)-ylacetic acid. In none of the experiments was there any measurable formation of acetylhistamine.

EFFECTS OF CHLORPROMAZINE AND BROMOLYSERGIC ACID DIETHYLAMIDE ON GASTRIC SECRETION OF ACID INDUCED BY HISTAMINE IN RATS

In anaesthetized rats in which the lumen of the stomach was perfused with 0.001 to 0.00025 N-sodium hydroxide solution and the pH of effluent fluid was recorded continuously, intravenous administration of chlorpromazine caused transient inhibition of acid secretion. After acid secretion had returned to the control level the responses to histamine were greater than those before chlorpromazine was given. Aminoguanidine, iproniazid and bromolysergic acid diethylamide also potentiated the effect of histamine on acid secretion but the initial inhibition was absent. Indirect evidence from experiments in which mixtures of aminoguanidine with chlorpromazine or bromolysergic acid diethylamide and of iproniazid with chlorpromazine or bromolysergic acid diethylamide were given, suggests that chlorpromazine and bromolysergic acid diethylamide enhance responses to histamine by inhibition of imidazole-N-methyl transferase.

Histamine inhibition of mixed function oxidase activity in rat and human liver microsomes and in the isolated perfused rat liver

The imidazole ring is a common structural feature of some xenobiotics that inhibit cytochrome P-450-catalysed reactions. Histamine is a 4-substituted imidazole and a preliminary study has shown it to be an inhibitor of rat liver microsomal drug oxidation. This work has now been extended. Histamine appears to be a competitive inhibitor of the alpha-hydroxylation (HM) (Ki = 164 microM; IC50 at 20 microM = 308 microM) and O-demethylation (ODM) (Ki = 243 microns; IC50 at 20 microM = 400 microM) of metoprolol in rat liver microsomes. Of the metabolites of histamine only N-acetylhistamine showed comparable inhibitory potency to that of the parent compound. Histamine impaired the disappearance of lignocaine when incubated with rat liver microsomes. This was accompanied by a corresponding inhibition of 3-hydroxy-lignocaine appearance. Histamine produced a type II spectral interaction with rat liver microsomes (lambda max = 432 nm, lambda min = 408 nm; Ks = 0.11 mM). When histamine was incubated alone with rat liver microsomes no loss of substrate was observed. The oxidation of metoprolol by human liver microsomes was impaired by histamine (IC50 values for ODM appearance at 25 microM: liver HL1 greater than 10, HL3 = 3.8 and HL4 = 3.7 mM). In comparison, cimetidine had an IC50 value of 1.5 mM using microsomes from liver HL3. Addition of histamine impaired the elimination of metoprolol by the isolated perfused rat liver in a dose-dependent manner (P less than 0.001, one-way analysis of variance). These data demonstrate that histamine can enter hepatocytes, interact with cytochrome P-450 and inhibit some drug oxidation reactions. The physiological relevance of inhibition of drug metabolism by histamine remains to be determined.

Reliability and practicability of the fluorometric-fluoroenzymatic histamine determination in pathogenetic studies on peptic ulcer: detection limits and problems with specificity

Histamine, among various "biologic-physiologic" abnormalities, is considered as a pathogenetic factor in chronic duodenal ulcer disease. The 10-30 per cent difference between its concentration in gastric and duodenal mucosa of patients compared to healthy controls, however, has to be demonstrated to be specific for the disease. It has to be shown to be neither a methodological artefact nor a common effect, concomitant factor or consequence. This study, after a series of pathogenetic trials examines systematic errors (biases) in the fluorometric-fluoroenzymatic histamine assay under the conditions of field studies including tests on specificity over a time period of 10 years. It concentrates on sensitivity (detection limits) and specificity of a standard technique described herein. A modified Shore procedure for large scale assays in human biopsies was developed including reference luminescence values for all reagents, cleaning material and glassware, reduction of OPD concentration to 0.05%, purification of n-heptan, omission of centrifugation steps in the extraction procedure and use of 2 ml 1 M HClO4 in the homogenization step to prevent losses of histamine due to adherence to the mechanical homogenizer. This assay was sensitive enough to measure histamine without difficulty in any biopsy taken. The detection limit was 3 ng/biopsy, but the smallest quantities of the amine ever obtained were 10.6 and 18.3 ng/biopsy (depending on both histamine content and biopsy weight). A series of problems had to be solved both in achieving and demonstrating specificity. It had to be defined not only for the assay in general, but also for assessing the difference in histamine content between ulcer patients and healthy controls. Exogenous more than endogenous fluorescing material interfering with the determination had to be excluded. A series of pitfalls were detected which had to be overcome in demonstrating the specificity of the assay by physicochemical and enzymatic tests. The specificity of the identification tests was more often impaired than the histamine assay itself. Fluorescing material interfering with the assay occurred in the homogenization, extraction and condensation steps, was found in water, OPD, the organic solvents, the cleaning material and in all kinds of plastic vessels. Plasticizers were shown by physicochemical characteristics including fluorescence spectra to be most likely responsible for this interfering material. Rules were developed to exclude such hazards in specificity in longterm pathobiochemical studies. Enzymatic identification test were applied to exclude endogenous fluorecing substances interfering with the standard technique. Simil

Alpha-methylhistamine methylation by histamine methyltransferase

Enzymatic methylation of histamine and analogues by histamine methyltransferase (HMT) was studied. Incubation of histamine or alpha-methylhistamine with HMT and S-adenosylmethionine was shown by GC-MS to yield tele-methyl and alpha-methyl-tele-methyl-histamine, respectively. Kinetic analysis of the reactions indicated that both substrates exhibited equivalent Vmax values, but that histamine's Km was ten-fold lower than that of alpha-methylhistamine. These results indicate that alpha-methylhistamine can be methylated as effectively as histamine, and may be a useful tool in further studies of HMT.

Histamine uptake and metabolism in intact isolated parietal cells

3H-Histamine binding, uptake and metabolism were investigated in intact isolated and enriched parietal cells from the dog and guinea pig. Histamine uptake was sodium dependent and followed by intracellular metabolism. The only metabolite that was detected and extracted from cytosol has been identified by TLC to N tau-methylhistamine. The histamine N-methyltransferase activity appeared to be sodium dependent and was inhibited by mepyramine and chlorpromazine, and also by higher concentrations (10(-4)--10(-3) mol/l) of cimetidine. Two blockers of the sodium channel, amiloride an aminoguanidine, also reduced the enzyme activity by an as yet unknown mechanism.

The intestinal phase hormone

The existence of a stimulatory intestinal phase of gastric acid secretion has been suspected, but largely ignored, for many years. Recently, however, it has become clear that the intestinal phase plays an important role in acid production during digestion. The intestinal phase is of additional interest in relation to the profound gastric acid hypersecretion associated with portacaval shunt (PCS). Substantial evidence indicates that PCS-related gastric hypersecretion is due to unmasking of the intestinal phase by hepatic bypass of a humoral stimulant in portal blood that is normally degraded to a considerable extent by the liver. Studies in our laboratory during the past 12 years have provided strong physiologic evidence for humoral mediation of both the intestinal phase of gastric secretion and of PCS-related hypersecretion by a hormone that arises in the small intestine, particularly in the jejunum. Furthermore, our studies have demonstrated that this intestinal phase hormone (IPH) exists in humans as well as in dogs, rats, and pigs. Additionally, recent work by a number of investigators, as well as by our group, has provided convincing evidence that IPH is different from any of the known gastric stimulatory hormones. With these physiologic observations as a background, we have used a classical method for extracting acidic peptides to prepare a hog intestinal mucosa extract (HIME) that has all of the known physiologic properties of an IPH. Specifically, HIME contains a potent stimulant of gastric acid secretion that acts according to a linear dose-response relationship; that is not gastrin in any of its immunoassayable forms; that significantly augments the maximal acid secretory responses to pentagastrin, gastrin, CCK, and histamine; and that is substantially degraded by the liver, in contrast to gastrin and CCK. Efforts at isolating the gastric stimulatory substance in HIME suggest that it is a peptide of low molecular weight. Work directed at isolating IPH in pure form and identifying it is in progress.

An improved radioenzymatic assay for histamine in human plasma, whole blood, urine, and gastric juice

A radioenzymatic method suitable for the assay of histamine in human blood, urine, plasma, and gastric juice is described. It differs from earlier methods by use of a histamine methyltransferase preparation from pig brain, of high activity tritiated S-adenosylmethionine, and of a heat precipitation step to reduce the previously noted interference from plasma constituents. The method is simpler than those requiring solvent extraction and concentration of histamine, gives recoveries in the range 80-120%, and so sliminates the need for internal standardisation. The method is sensitive and precise with coefficients of variation for blood, urine, and plasma of 5%, 6%, and 13% respectively. The mean +/- standard deviation for normal human plasma histamine is 5 +/- 4 nmol/l, for whole blood 559 +/- 193 nmol/l, and for urine 229 +/- 128 nmol/24h.

[Studies on the pre- and posthepatic plasma histamine concentrations and their possible pathophysiological effects in liver cirrhosis (author's transl)]

Fluorometric estimations of plasma histamine in the peripheral venous blood were performed in a control group of 16 subjects with normal liver values and normal liver biopsy specimen. Two further groups with liver changes were studied: Ten patients with fatty liver (stage I-II) and 22 cases suffering from liver cirrhosis, including 7 patients with portocaval shunt. Additionally, plasma histamine concentrations were determined in the blood of the portal vein, hepatic vein, cubital vein and in the femoral artery of another 11 normal subjects and 8 cirrhotic patients, some of them with portocaval shunt. The elimination of histamine by the liver was calculated. In healthy humans about two thirds (67.8 plus or minus 11.4 per cent; n=11) of the histamine content in the portal vein is eliminated by liver passage. This is due mainly to liver uptake and catabolism of histamine. It could be shown, that the elimination rate (41.0 plus or minus 15.1 per cent, n=8) is diminished in cirrhotic livers. Therefore, the plasma histamine content measured in the peripheral venous blood is significantly higher (p less than 0.001) in cirrhotic patients (1.2 +/- 0.3 ng/ml; n=22) than in healthy subjects (0.7 +/0 0.2 ng/ml; n=16). The expected pathophysiological effects of the elevated plasma histamine levels in liver cirrhosis are discussed with respect to circulatory changes ("hyperdynamic circulation") and their possible role in the development of "hepatogenic" ulcers of the stomach.

Effect of amodiaquine on gastric histamine methyltransferase and on histamine-stimulated gastric secretion

1 Amodiaquine was found to be a potent inhibitor in vitro of gastric histamine methyltransferase from human and canine corpus and from pig antrum. The ID50 for the enzyme, purified from pig antrum mucosa by ultracentrifugation and chromatography on DEAE-cellulose, was 2.5 muM. 2 In six dogs with Heidenhanin pouches the maximum secretory response to histamine (40 mug/kg i.m.) was augmented by i.m. injection of amodiaquine. The augmentation depended on the dose of amodiaquine, the optimum effect (40% increase in volume of gastric juice, 80% in acid output) being achieved with 2 mg/kg. The maximum secretory response to betazole was also enhanced by amodiaquine. 3 It was suggested that amodiaquine may enhance the histamine and betazole stimulated gastric secretion by an inhibition of gastric histamine methyltransferase in vivo.