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

In vivo formation of histamine phosphopyridoxal cyclic compounds

A possibility of in vivo formation of cyclic compounds between histamine (Hi) given i.p. and endogenous pyridoxal (PL) or pyridoxal 5'-phosphate (PLP) has been studied. Cyclic compounds of Hi with PL or PLP were found in all tissues examined. Although an increase in Hi levels in tissues enhances the formation of cyclic compounds, no simple relationship between the rate of formation and Hi concentration has been observed. The reaction seems to be limited by endogenous PLP. The cyclic products Hi-PL and Hi-PLP were discovered in urine. It is suggested that the process of cyclic compound formation may reduce PLP resources, resulting in a modification of PLP-enzyme activities.

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.

Conditions for the formation of cyclic compounds between pyridoxal phosphate and 5-hydroxytryptamine or 5-hydroxytryptophan

5-Hydroxytryptamine (5HT) and 5-hydroxytryptophan (5HTP) form cyclic compounds (probably of the tetrahydro-beta-carboline type) with pyridoxal phosphate (PLP). In the first step of reaction a Schiff's base is formed; during incubation it is transformed into a cyclic compound with a maximum absorption spectrum at 330 nm. The degree of cyclization depends on pH and substrate concentrations. One mole of 5HT or 5HTP reacts with one mole of coenzyme. The velocity of cyclization increased with an excess of either 5HT (5HTP) or PLP, without any change in the mole to mole ratio. The formation of cyclic compounds was confirmed by the use of isotopes, separation from substrates being achieved by high-voltage electrophoresis.

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.