Conditions for the formation of complexes between pyridoxal phosphate and some imidazole derivatives

Properties of histidine decarboxylase from rat gastric mucosa

The properties of specific histidine decarboxylase from highly purified rat gastric mucosa preparations were studied. The kinetic parameters were pH dependent: the apparent Km value varied inversely with pH; the maximum reaction velocity was reached at pH 6.6; the optimum pH was related to substrate concentration. The enzyme was unstable below pH 5.5. The effect of temperature was investigated and the enzyme activity was optimum near 56 degrees C. The thermal inactivation of the enzyme showed the presence of several active forms displaying distinct thermostabilities. The effect of coenzyme and substrate on heat stability was established. A small amount of pyridoxal phosphate was required for maximum enzyme activity, and the Km was low. The cofactor appeared to be tightly bound to the apoenzyme; nevertheless there was a fraction more easily resolved by dialysis. With high pyridoxal phosphate concentrations non-competitive inhibition occurred. Histamine inhibited the enzyme at high concentrations, the inhibition being competitive with respect to the substrate. No metal ion was required for enzyme activity; the enzyme was inhibited by sulfhydryl reagents and heavy metal ions, and also by high concentrations of reducing agents. The tryptophan residue of the holoenzyme seemed to be essential for the catalytic process.

Phosphopyridoxal cyclic compounds with histamine and histidine. 6: The formation of phosphopyridoxal cyclic compounds with histamine and histidine in the presence of biological material

We have studied the dynamics of cyclic compound formation between histamine or histidine and pyridoxal 5'-phosphate (Hi-PLP or His-PLP) in incubates of rat gastric mucosa histidine decarboxylase (HD), rat intestinal diamine oxidase (DAO) or homogenates of either rat liver, intestine or gastric mucosa. For gastric mucosa HD, liver and gastric mucosa homogenates, the rate of cyclization was slightly decreased; however, the rate was significantly inhibited with intestinal DAO or intestinal homogenate. Binding of PLP by tissue components was measured; free PLP was bound abundantly by rat intestinal DAO and by rat intestinal homogenate. A possible mechanism by which intestinal tissues inhibit cyclic compound formation is discussed.

[Purification and detection of multiple forms of histidine decarboxylase in rat gastric mucosa (author's transl)]

A specific histidine decarboxylase from rat gastric mucosa has been obtained at high purity and good yield (purification about 600-fold). The purification procedure included double (NH4)2SO4 fractionation, ion-exchange chromatography, preparative isoelectric focusing in a granulated gel and gel filtration. Only the specific histidine enzyme was obtained by that procedure; DOPA decarboxylase, a non-specific enzyme, was absent in our final preparation. Each step of the purification was visualized by polyacrylamide gel electrophoresis and analytical isoelectric focusing. The purified enzyme was apparently homogenous by criteria of electrophoresis and gel filtration and has a molecular weight of 94 000. Several protein bands appeared after isoelectric focusing and the enzyme activity was localized in 3 distinct peaks. The gastric enzyme consists of 3 active forms which could be distinguished by their isoelectric points: 5.4, 5.75 and 6. Moleculare weights estimated by SDS polyacrylamide gel electrophoresis were 97 000, 93 000 and 90 000, and no subunits were observed. Pyridoxal phosphate was required as a coenzyme and resolution of the holoenzyme agreed with a portion of the coenzyme tightly bound to the apoenzyme. The purified enzyme was stable at low ionic strength, near neutral pH; concentrated reducing agents inhibit the enzyme.

The metabolism of [14C]histamine during pentagastrin-stimulated acid secretion in the cat

1. The metabolism of exogenous [14C]histamine has been examined in cats during gastric secretion. The appearance of 14C-labelled metabolites has been measured chromatographically in urine and gastric juice under non-secreting and pentagastrin-stimulated conditions, and in gastric mucosa after pentagastrin stimulation. 2. The main metabolites detected were (i) an acidic product, presumed to be methylimidazoleacetic acid, (ii) Ntau-methylhistamine, and (iii) an unidentified metabolite, moving rapidly in the chromatographic systems used, and designated 'preacetylhistamine'. No evidence was found for the presence of side chain (i.e. Nalpha)-methylated histamines, either in the non-stimulated or pentagastric-stimulated state. 3. During a 3 hr non-stimulated period, followed by 3 hr of pentagastrin administration, the relative proportions of 14C-labelled metabolites excreted in urine remained unchanged. During acid secretion there was an increase in the proportion of 'pre-acetylhistamine' in gastric juice, which occurred at the expense of the acidic metabolites. It is suggested however that this effect is more likely to be the result of a slow adaptation to the administration of exogenous [14C]histamine than the result of acid secretion. 4. The results of the present work do not appear to support the hypothesis of Code (1970), that a local control mechanism for acid secretion might involve the formation Nalpha-methylated histamine metabolites in gastric mucosa.

Phosphopyridoxal complexes with histamine and histidine. (5) The kinetics of cyclic compound formation between histamine and pyridoxal-5'-phosphate in the presence of pig kidney diamine oxidase and rat intestinal histaminase

Pig kidney diamine oxidase (DAO) and rat intestinal histaminase (Hi-ase) activities are inhibited in vitro by high concentrations of both a substrate (histamine) and a coenzyme (pyridoxal-5'-phosphate). This inhibition may be at least partially associated with the formation of a cyclic compound between histamine (Hi) and pyridoxal-5'-phosphate (PLP). The dynamics of this cyclic compound formation in the presence of both enzymes has been examined. In an incubation mixture containing partially purified pig kidney DAO, the rate of cyclization decreased slightly as compared with a buffer. On the contrary, in the presence of crude rat intestinal histaminase, the rate of cyclization was inhibited significantly; this inhibition was proportional to the amount of enzyme preparation present in the incubation mixture. The possible mechanism of the influence of enzyme protein on the rate of cyclic compound formation, and its possible biological significance, are discussed.

Phosphopyridoxal complexes with histamine and histidine. (4) The kinetics of complex formation between histidine and pyridoxal 5' -phosphate in the presence of bacterial histidine decarboxylase

The dynamics of the complex formation between pyridoxal 5'-phosphate (PLP) and histidine in the presence of bacterial histidine decarboxylase was examined. Since PLP is able to form a cyclic product with histidine and histamine, the possibility of complex formation between PLP and histamine formed during a decarboxylation reaction was examined too. It was found that the cyclization reaction between PLP and histidine is equimolecular and the rate of cyclic product formation is not significantly influenced by the presence of enzyme. In the presence of bacterial histidine decarboxylase both the cyclization reaction and cyclic product formation were observed. Predominance of histamine or cyclic product formation was dependent on pH and substrate concentration. In the presence of histidine and enzymatic protein, histamine formed during the decarboxylation reaction was unable to form a cyclic product with PLP.

Phosphopyridoxal complexes with histamine and histidine. (2) The influence of presumed complex on histidine decarboxylase activity in rat gastric mucosa

It was found that in vitro, histidine and histamine form a complex compound with pyridoxal-5'-phosphate (PLP). The purpose of the present experiments was to find whether formation of this complex can influence histidine decarboxylase activity. It was found that excess PLP inhibits rat's gastric mucosa histidine decarboxylase activity in vitro. The inhibitory action of PLP depends on the histidine concentration and is independent on the amount of crude enzyme preparation. The histidine-PLP complex did not influence enzyme activity. The possible mechanisms of the inhibitory action of PLP on histidine decarboxylase activity are discussed.

Phosphopyridoxal complexes with histamine and histidine. (1) The kinetics of complex formation between pyridoxal 5'-phosphate and histamine

Amines and amino acids are able to form cyclic compounds with pyridoxal 5'-phosphate (PLP). The quantitative relationship between histamine and PLP in the cyclization reaction were examined. It was found that one mole of amine reacts with one mole of coenzyme. The velocity of complex formation increased with an excess of either histamine or PLP, without any change in molar ratios. The formation of cyclic compound was confirmed by the use of isotopic method. The separation of cyclic compound from histamine, but not from PLP, was achieved by paper chromatography.

Phosphopyridoxal complexes with histamine and histidine. (3) The influence of presumed complex on activity of rat intestinal histaminase

Histamine in high concentration inhibits the rat intestinal histaminase (diamine oxidase E.C. 1.4.3.6.). The apparent Km is approximately 4.2 X 10(-5) M. This inhibition can be reversed by an addition of PLP. It was also found that excess of PLP inhibits enzyme activity. It is competitive inhibition. Histamine and other amines which were associated with enzyme inhibition form spectrophotometricaly demonstrable complex with PLP. The possible mechanism of the inhibitory action of PLP and complex with histamine and other amines on rat intestinal histaminase activity are discussed.