Studies on the inhibition of histidine decarboxylase, aromatic-L-amino acid decarboxylase and acid secretion by brocresine and its metabolites

Histamine Signaling Is Essential for Tissue Macrophage Differentiation and Suppression of Bacterial Overgrowth in the Stomach

BACKGROUND & AIMS

Histamine in the stomach traditionally is considered to regulate acid secretion but also has been reported to participate in macrophage differentiation, which plays an important role in tissue homeostasis. Therefore, this study aimed to uncover the precise role of histamine in mediating macrophage differentiation and in maintaining stomach homeostasis.

METHODS

Here, we expand on this role using histidine decarboxylase knockout (Hdc^-/-) mice with hypertrophic gastropathy. In-depth in vivo studies were performed in Hdc^-/- mice, germ-free Hdc^-/- mice, and bone-marrow-transplanted Hdc^-/- mice. The stomach macrophage populations and function were characterized by flow cytometry. To identify stomach macrophages and find the new macrophage population, we performed single-cell RNA sequencing analysis on Hdc^+/+ and Hdc^-/- stomach tissues.

RESULTS

Single-cell RNA sequencing and flow cytometry of the stomach cells of Hdc^-/- mice showed alterations in the ratios of 3 distinct tissue macrophage populations (F4/80⁺Il1b^high, F4/80⁺CD93⁺, and F4/80^-MHC class II^highCD74^high). Tissue macrophages of the stomachs of Hdc^-/- mice showed impaired phagocytic activity, increasing the bacterial burden of the stomach and attenuating hypertrophic gastropathy in germ-free Hdc^-/- mice. The transplantation of bone marrow cells of Hdc^+/+ mice to Hdc^-/- mice recovered the normal differentiation of stomach macrophages and relieved the hypertrophic gastropathy of Hdc^-/- mice.

CONCLUSIONS

This study showed the importance of histamine signaling in tissue macrophage differentiation and maintenance of gastric homeostasis through the suppression of bacterial overgrowth in the stomach.

Histamine in brain--its role in regulation of seizure susceptibility

The influence of drugs affecting the turnover and levels of histamine in brain and histamine antagonists on pentetrazole (PTZ)-induced and electroconvulsive seizure threshold in mice was studied. A 1.5-2-fold rise in histamine brain concentration (induced by treatment with histidine or metoprine), led to a concomitant increase of PTZ-induced seizure threshold. A histidine decarboxylase inhibitor (brocresine) induced a depletion of brain histamine by about 75% for at least 8 h, the seizure threshold was, however, only reduced at 6 and 8 h after the injection. At shorter intervals, the seizure threshold was substantially increased. Treatment with centrally acting H1 antagonists (dimethindene and promethazine) in non-sedative dosage diminished the PTZ seizure threshold significantly; no changes were seen after treatment with H2 and H3 antagonists (oxmetidine, ranitidine, zolantidine or thioperamide) and a H3 agonist (R-alpha-methylhistamine). The electroconvulsive threshold was hardly influenced. It is concluded that histamine has a certain anticonvulsant effect which is mediated through H1 receptors.

Inhibition of Dopa decarboxylase by caffeic acid and related compounds: structure-activity relationships

The inhibition of purified Dopa decarboxylase, has been performed with dihydroxyphenyl derivatives devoided of the NH2 group and unable to form the Schiff base with pyridoxal-5' phosphate. The more efficient inhibition is displayed by caffeic acid and related compounds that present an insaturation in the chain. The relative contribution to the inhibition by the chain and ring structures has been evaluated.

Mechanism of inactivation of mammalian L-histidine decarboxylase by (S)-alpha-fluoromethylhistidine

The mechanism of inactivation by (S)-alpha-fluoromethylhistidine (FMH) of L-histidine decarboxylase (HDC, L-histidine carboxy-lyase, EC 4.1.1.22) purified from whole bodies of fetal rats was studied. FMH inhibited the activities of HDC purified from fetal HDC as well as HDCs from the brain and stomach of adult rats. The activity was not restored by extensive dialysis, indicating that the inhibition was irreversible. The inactivation of HDC was time and concentration dependent and followed pseudo first-order kinetics. L-Histidine, a substrate, protected HDC against inactivation, but D-histidine did not. Apo-HDC was not inactivated by FMH. On labeling of HDC with [3H]FMH, a correlation was found between the extent of incorporation of radioactivity into the enzyme and the degree of inactivation. Two moles of the inhibitor were incorporated into one mole of HDC (108,000 daltons). Experiments with [carboxyl-14C]FMH and [ring 2-14C]FMH showed that decarboxylation was necessary for the inactivation and that one molecule of FMH moiety was incorporated into an HDC monomer during every three decarboxylations of FMH.

Effect of alpha-monofluoromethyl histidine, an irreversible inhibitor of histidine decarboxylase, on gestation in mice

Chronic administration of alpha-monofluoromethyl histidine, a specific inhibitor of histidine decarboxylase, to pregnant mice blocked histidine decarboxylase activity and markedly depleted histamine levels in 18-day-old foetuses and in newborn mice. Such treatment had no effect on implantation or embryonic development or parturition suggesting that de novo synthesis of histamine is not a significant factor in these processes in the mouse.

Inhibitors binding to L-aromatic amino acid decarboxylase

The effect of a number of inhibitors of L-aromatic amino acid decarboxylase activity on the absorption spectrum of the enzyme-bound coenzyme has been studied. It has been observed that the compounds tested, even if devoid of the amino function and therefore unable to form the Schiff base with the coenzyme, modify significantly the enzyme spectrum, indicating their binding to the coenzyme active site. Spectral modifications suggest that at least two kinds of binding of inhibitors to L-aromatic amino acid decarboxylase may occur, depending on their structural features. Moreover, from the spectra obtained at different concentrations of the inhibitors their affinity constants have been determined: data indicate that the cathecol ring gives the largest contribution to the binding, while the presence of the carboxyl group, the aminic group and the aliphatic chain are responsible for a decrease in the binding, which could be relevant for the efficiency of the catalysis.

Peptide inhibition of mammalian histidine decarboxylase

The hypothesis that N-terminal histidine peptides might act as inhibitors to histidine decarboxylase was investigated. A murine mastocytoma was utilized as enzyme source. The crude extract of this tissue exhibits high rates of decarboxylation of both histidine and DOPA and was used to establish the specificity in the effect of the compounds tested. For kinetic analyses a highly purified histidine decarboxylase fraction was used. The effect of some representative peptides on both enzyme activities were recorded. Histidine decarboxylase exclusively was inhibited by N-terminal histidine peptides. None of the other peptides investigated interfered negatively with this enzyme. This inhibition was consistent in the purified preparation and appeared to be more pronounced with increasing hydrophobicity in the second amino acid. Histidyl-phenylalanine was found to be about 100-fold as potent as the commonly used specific histidine decarboxylase inhibitor alpha-methyl histidine. It is concluded that small peptides with histidine as the N-terminal amino acid might act as specific inhibitors for mammalian histidine decarboxylase. An analog effect of small tyrosyl or phenylalanyl peptides was not seen for the DOPA decarboxylase.