Brain pyridoxine-5-phosphate oxidase. Modulation of its catalytic activity by reaction with pyridoxal 5-phosphate and analogs.

Pyridoxine-5-P oxidase, the flavoprotein involved in the oxidation of pyridoxamine-5-P and pyridoxine-5-P to pyridoxal-5-P, has been isolated and purified to homogeneity using sheep brain tissues. Inactivation of the oxidase by bis-pyridoxal-5-P results in binding of the inhibitor to specific lysyl residues. After NaBH4 reduction of the inactivated enzyme, it was found that 1 P-pyridoxyl-pyridoxine-P residue was incorporated per enzyme dimer. After trypsin digestion of the bis-PLP modified enzyme, only one peptide absorbing at 320 nm, was separated by reverse-phase high performance liquid chromatography. The amino acid sequence of the labeled peptide was determined by automated Edman degradation. The observations reported in this paper are relevant to the mechanisms underlying the regulation of the catalytic function of pyridoxines-5-P oxidase by the product pyridoxal-5-P. It is postulated that the catalytic function of the oxidase is modulated by binding of pyridoxal-5-P to a specific lysyl residue of the dimeric structure of the protein.