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.