Decarboxylation Of L-histidine Research Articles

The Catalytic Mechanism of the Pyridoxal-5'-phosphate-Dependent Enzyme, Histidine Decarboxylase: A Computational Study.

The catalytic mechanism of histidine decarboxylase (HDC), a pyridoxal-5'-phosphate (PLP)-dependent enzyme, was studied by using a computational QM/MM approach following the scheme M06-2X/6-311++G(3df,2pd):Amber. The reaction involves two sequential steps: the decarboxylation of l-histidine and the protonation of the generated intermediate from which results histamine. The rate-limiting step is the first one (ΔG≠ =17.6 kcal mol-1 ; ΔGr =13.7 kcal mol-1 ) and agrees closely with the available experimental kcat (1.73 s-1 ), which corresponds to an activation barrier of 17.9 kcal mol-1 . In contrast, the second step is very fast (ΔG≠ =1.9 kcal mol-1 ) and exergonic (ΔGr =-33.2 kcal mol-1 ). Our results agree with the available experimental data and allow us to explain the role played by several active site residues that are considered relevant according to site-directed mutagenesis studies, namely Tyr334B, Asp273A, Lys305A, and Ser354B. These results can provide insights regarding the catalytic mechanism of other enzymes belonging to family II of PLP-dependent decarboxylases.

Aminooxy analog of histamine is an efficient inhibitor of mammalian l-histidine decarboxylase: combined in silico and experimental evidence

Histamine plays highlighted roles in the development of many common, emergent and rare diseases. In mammals, histamine is formed by decarboxylation of L-histidine, which is catalyzed by pyridoxal-5'-phosphate (PLP) dependent histidine decarboxylase (HDC, EC 4.1.1.22). The limited availability and stability of the protein have delayed the characterization of its structure-function relationships. Our previous knowledge on mammalian HDC, derived from both in silico and experimental approaches, indicates that an effective competitive inhibitor should be capable to form an "external aldimine-like structure" and have an imidazole group, or its proper mimetic, which provides additional affinity of PLP-inhibitor adduct to the HDC active center. This is confirmed using HEK-293 cells transfected to express human HDC and the aminooxy analog of histidine, 4(5)-aminooxymethylimidazole (O-IMHA, IC₅₀ ≈ 2 × 10(-7) M) capable to form a PLP-inhibitor complex (oxime) in the enzyme active center. Taking advantage of the availability of the human HDC X-ray structure, we have also determined the potential interactions that could stabilize this oxime in the active site of mammalian HDC.

Autoregulation of histamine synthesis through H3 receptors in isolated fundic mucosal cells.

Histamine plays an important role in the control of gastric acid secretion by activating H2 receptors located on parietal cells. In gastric mucosa, histamine is stored both in mast cells and in enterochromaffin-like cells, especially in rodents. It has been proposed that histamine may regulate its own synthesis and/or release through receptors pharmacologically distinct from H1- and H2-receptor subtypes. In this article, we studied the regulation by histamine of histidine decarboxylase (HDC) activity (enzyme responsible for the formation of histamine by decarboxylation of L-histidine) in a fraction of isolated rabbit gastric mucosal cells enriched in mucous and endocrine cells. Histamine and (R)-alpha-methylhistamine (H3 receptor agonist) dose dependently inhibited HDC activity with the same potency (mean effective concn: 32.2 +/- 0.7 and 50.5 +/- 3.1 pM, respectively) and efficacy (35 and 36% inhibition, respectively). In contrast, the H2 agonist dimaprit was devoid of effect. The H3 antagonist thioperamide was found to decrease the histamine- or (R)-alpha-methylhistamine-induced inhibition of HDC activity (mean ineffective concn = 28.3 +/- 1.8 and 9.87 +/- 0.8 nM, respectively), whereas H1 (promethazine) and H2 (ranitidine) antagonists were unable to affect HDC activity. Moreover, high concentrations of thioperamide (1-10 microns) increased histamine release from these cells. All these results allowed us to conclude that, in gastric mucosa, histamine downregulates its own synthesis (and perhaps release) through the stimulation of autoreceptors with pharmacological characteristics of H3 receptors. However, the relationship between histamine synthesis and release remains unclear and needs further investigation.

Histamine levels and clonic convulsions of electrically-induced seizure in mice: the effects of alpha-fluoromethylhistidine and metoprine.

The purpose of this study was to investigate the possible role of the central histaminergic neuron system in electrically-induced seizure in mice. For this purpose, we examined the effects of intraperitoneal (i.p.) injections of histaminergic agents, such as L-histidine, metoprine, and alpha-fluoromethylhistidine (FMH), on electrically-induced seizure. L-Histidine decreased the duration of clonic convulsion in electrically-induced seizure, but not affected that of tonic convulsion. This effect of L-histidine was antagonized by pretreatment with FMH, indicating that it was due to histamine formed by decarboxylation of L-histidine in the central nervous system. The anticonvulsive effect of L-histidine was also reduced by the H1-antagonist pyrilamine, but not by the H2-antagonist zolantidine, indicating that the effect on electrically-induced seizure is mediated through central H1-receptors. Metoprine, which increased the histamine levels in the cerebral cortex, diencephalon and midbrain of mice, decreased the duration of clonic convulsions dose-dependently. Conversely, FMH, which decreased the brain histamine levels, increased the duration of clonic convulsions. Good inverse correlations were found between the duration of clonic convulsions and brain histamine levels, especially in the diencephalon: the histamine levels were inversely proportional to the duration of clonic convulsions. No correlation was found between the duration of tonic convulsions and brain histamine levels. These results suggest that the histaminergic neuron system is important in inhibition of the duration of clonic convulsion on electrically induced seizure in mice.

The selective eosinophil chemotactic activity of histamine.

Histamine diphosphate was shown to selectively attract human eosinophils from mixed granulocyte populations when over 20% eosinophils were used in a modified Boyden chamber chemotactic assay system. This effect of histamine is abolished by incubation with diamine oxidase (histaminase) and was generated by decarboxylation of L-histidine. A linear dose dependent increase in eosinophil migration was observed between 3 X 10(-7) M and 1.25 X 10(-6) M, while higher concentrations of histamine inhibited the migration of eosinophils. The attractant activity of histamine was not inhibited by H-1 or H-2 receptor antagonists, however, the inhibition of migration observed at higher histamine concentrations was reversed by metiamine, an H-2 receptor antagonist. The effects of histamine upon eosinophil migration were demonstrable using three different assays: (a) counting cells that had traversed 5-mum pore, 12-mum thick polycarbonate filters, (b) counting cells that had migrated various distances into a 3-mum pore, 145-mum cellulose nitrate filters, or (c) measuring the number of cells that had traversed an upper polycarbonate filter and migrated into a lower cellulose nitrate filter using 15Cr-labeled cells. The ability of histamine to enhance eosinophil migration was shown to be dependent upon the presence of a concentration gradient; histamine did not cause a dose-dependent increase in random motility. Furthermore, preincubation of the eosinophils with histamine deactivate the cells to further stimulation by histamine or by C5a. It is concluded that in low doses histamine is a chemoattractant for human eosinophils, while in higher doses histamine inhibits eosinophil migration. These observations may relate to the influx and localization of eosinophils in immediate hypersensitivity reactions.

Decarboxylation of L-histidine by Clostridia

SUMMARY A porcelain plate microtest by which the decarboxylation of L-histidine by Clostridia could be detected within 6 hours is described. It was more sensitive than chromatographic procedures for detecting histamine production, using ninhydrin or Pauly's reagent as a developing agent, which were used for confirmation. All of 26 type A strains of Clostridium perfringens and the one type F strain tested decarboxylated histidine. Strongly positive tests were obtained except for some of the Hobbs type strains that gave much weaker reactions. Two type C strains gave negative tests. Fifteen of 19 strains of Clostridium bifermentans also produced decarboxylation, but the reactions were weak and inconsistent. No strains of 18 other species of Clostridia tested produced decarboxylation.