Product inhibition of rat brain histamine-N-methyltransferase

Inhibition and Regulation of the Ergothioneine Biosynthetic Methyltransferase EgtD

Ergothioneine is an emerging factor in cellular redox homeostasis in bacteria, fungi, plants, and animals. Reports that ergothioneine biosynthesis may be important for the pathogenicity of bacteria and fungi raise the question as to how this pathway is regulated and whether the corresponding enzymes may be therapeutic targets. The first step in ergothioneine biosynthesis is catalyzed by the methyltransferase EgtD that converts histidine into N-α-trimethylhistidine. This report examines the kinetic, thermodynamic and structural basis for substrate, product, and inhibitor binding by EgtD from Mycobacterium smegmatis. This study reveals an unprecedented substrate binding mechanism and a fine-tuned affinity landscape as determinants for product specificity and product inhibition. Both properties are evolved features that optimize the function of EgtD in the context of cellular ergothioneine production. On the basis of these findings, we developed a series of simple histidine derivatives that inhibit methyltransferase activity at low micromolar concentrations. Crystal structures of inhibited complexes validate this structure- and mechanism-based design strategy.

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.

Histamine inactivation in the brain: aspects of N-methylation

This report deals with molecular and anatomical site of histamine N-methylation assumed to be the exclusive route of HA inactivation. The methyl transfer from the -S-CH3 of S-adenosyl-L-methionine to the ring (tele)-nitrogen of histamine, appears as much more complex than a one-step transformation. It seems that -S-CH3 is transformed before being transferred to the nitrogen of the acceptor probably via methanol (formaldehyde) formation. For localizations of transmethylation of neuronal histamine we assume at least a two-compartment model in which glia participate to a significant extent. The uptake of neuronal HA into glial cells might be the first step of histamine inactivation.

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.

Single-step purification of "kallikrein-resistant" kininogen from rat plasma using monoclonal-antibody immunoaffinity chromatography

Monoclonal antibody to rat plasma kininogen, obtained after immunization of mice with the kininogen prepared by conventional methods, was purified from ascites fluid and coupled to CNBr-activated Sepharose-4B. Monoclonal-antibody affinity adsorbant thus prepared provided a rapid single-step method of purifying to homogeneity plasma kininogen. Purified rat plasma kininogen showed identical molecular weight and immunological cross-reactivity to rat plasma low molecular weight (LMW) kininogen purified by conventional procedures. Rat plasma kininogen differed from LMW kininogen from other species by virtue of its resistance to cleavage by either plasma or glandular kallikreins.

Human erythrocyte histamine N-methyltransferase: radiochemical microassay and biochemical properties

A radiochemical assay for the measurement of histamine N-methyltransferase (HNMT) activity in human erythrocytes (RBCs) has been developed. This assay was developed as a first step toward testing the hypothesis that the biochemical properties and regulation of HNMT in an easily obtainable human cell, the RBC, might reflect those of the enzyme in less accessible cells and tissues. The Michaelis (Km) constant in the RBC for histamine, the methyl acceptor substrate for the reaction, was 5.0 X 10(-5) mol/l. The Km constant for S-adenosyl-L-methionine, the methyl donor, was 2.8 X 10(-6) mol/l. The assay was performed at a reaction pH of 7.4 with a potassium phosphate buffer. The product of the reaction was identified as N tau-methylhistamine by high performance liquid chromatography. The Kii for inhibition of the RBC enzyme by amodiaquine, an HNMT inhibitor, was 1.0 X 10(-7) mol/l, while the Kis value was 0.48 X 10(-7) mol/l. Blood samples obtained from 39 randomly selected adult white subjects had a mean activity of 130 +/- 30 U/ml of packed RBCs (mean +/- SD). Enzyme activities varied over a range from 74-213 U. There were no differences between men and women in mean activities, nor was there a significant correlation between RBC HNMT activity and age. The results of experiments in which lysates with 'low' and 'high' activities were mixed gave no indication that individual variations in RBC HNMT activities were due to the effects of endogenous enzyme inhibitors or activators. RBC HNMT activities measured in blood samples from 17 individual subjects four times over 6 wk were quite constant in each subject, with an average coefficient of variation of 6.2%. The availability of this assay will make it possible to test the hypothesis that individual variations in RBC HNMT activity might be used to predict individual differences in HNMT activity in other human cells and tissues.

Dural mast cells: source of contaminating histamine in analyses of mouse brain histamine levels

Histamine levels were determined in mouse brains from WBB6F1-+/+ (mast cell normal) and WBB6F1-W/Wv (mast cell-deficient) mice whose brains were dissected immediately after decapitation or after freezing the severed heads in liquid nitrogen for 10 s. In WBB6F1-+/+ mice, brains obtained from frozen heads contained significantly higher levels of histamine than those obtained from unfrozen heads. The converse was found in brains obtained from the WBB6F1-W/Wv mice. When CF-1 mice (which also contain brain-associated mast cells) were treated as described above, results very similar to those found with the WBB6F1-+/+ mice were obtained. Further, the high levels of histamine found in CF-1 mice whose brains had been frozen in situ were accompanied by an extensive degranulation of mast cells in the dura mater of these mice. Because of this degranulation of mast cells, and the fact that increased levels of brain histamine were not found in mast cell-deficient mice, it is concluded that dural mast cells are the likely source of the artifactually higher levels of histamine seen in brains frozen in situ.

The differential reaction of histamine and N tau-methylhistamine with Pauly's diazo reagent: application to assay of histamine N-methyltransferase activity

A simple nonradioisotopic fluorescent method for assay of histamine N-methyltransferase (HMT) activity was developed. After termination of the HMT reaction, the remaining excess substrate, histamine, was degraded by Pauly 's diazo reagent, whereas the product, N tau-methylhistamine (N- MeHA ), was not degraded by the reagent. Then the mixture was applied to high-performance liquid chromatography under conditions in which N- MeHA was not separated from histamine, and N- MeHA was measured fluorometrically by condensation with o-phthalaldehyde. The method would be convenient for measurement of HMT activity during enzyme purification.

A new radioenzymatic assay for histamine using purified histamine N-methyltransferase

Radioenzymatic assays for histamine (Hm) have found wide application. However, these procedures may lack the sensitivity necessary to quantify Hm in certain biological samples, such as human plasma. Purification of histamine N-methyltransferase (HNMT) has permitted the development of a new and highly sensitive radioenzymatic assay for Hm. HNMT was purified by sequential ion exchange, hydrophobic and molecular exclusion chromatography. The use of purified HNMT in the Hm assay has allowed the inclusion of high specific activity tritiated S-adenosyl-L-methionine ([3H]SAME) and the development of a simplified solvent extraction product isolation procedure. This assay has a sensitivity of approximately 2 picograms and is specific for Hm. Hm was easily quantified in human plasma and was found to be 303 +/- 81 pg/ml (mean +/- SD) in 8 male subjects. Substantial blank reduction and increased product conversion occur when purified HNMT is utilized in the Hm radioenzymatic assay, thus, increasing the sensitivity and possibly improving the specificity of this procedure.