PYRIDOXAL KINASE OF HUMAN BRAIN AND ITS INHIBITION BY HYDRAZINE DERIVATIVES

Abstract

Brain tissue was an unusually rich source of pyridoxal phosphokinase. A procedure for purification of this enzyme approximately 200-fold from human brain tissue is described. At its optimum pH of 6.5, the purified kinase was preferentially activated by Zn++(Km, 10-6 M) and less effectively by Mg++ (Km, 10-5 [image].) Values of Km for pyridoxal and adenosine triphosphate were 10-5 [image] and 5 x 10-5 [image], respectively. Bacause of its indirect role in formation of gamma-aminobutyrate, the effects of various tranquilizers and psychic energizers on activity of the kinase were determined. Chlorpromazine and l-(l-phenyl-cyclohexyl)piperidine showed a slight stimulatory action of uncertain significance; iproniazid inhibited moderately. [alpha]-Methylphenethylhydrazine and isoniazid were extremely effective inhibitors of the kinase; the actual inhibitors were the corresponding hydrazones formed between these agents and pyridoxal. The hydrazone, oxime, and semicarbazone of pyridoxal are all similarly potent inhibitors, with affinities for the enzyme well over one hundred times that of the substrate, pyridoxal. Because of the low concentrations at which they exert this effect, it is considered that the physiological effects of these carbonyl reagents in bacteria and mammals result in part from their capacity to lower or eliminate production of pyridoxal phosphate by a given tissue.