The Bacterial Oxidation of Vitamin B6: V. The Enzymatic Formation of Pyridoxal and Isopyridoxal from Pyridoxine

Abstract

Abstract Two soil organisms that degrade pyridoxine by related but independent pathways were examined for enzymes involved in the initial oxidative attack on pyridoxine. Pyridoxine 4-dehydrogenase, an FAD-dependent enzyme isolated from Pseudomonas MA-1, dehydrogenates pyridoxine to pyridoxal, and utilizes either oxygen or 2,6-dichloroindophenol, an as the hydrogen acceptor. Pyridoxine 5-dehydrogenase, an FAD-dependent enzyme isolated from Pseudomonas IA, converts pyridoxine to isopyridoxal and utilizes 2,6-dichloroindophenol, but not oxygen, as hydrogen acceptor. FMN does not replace FAD for either dehydrogenase. The two dehydrogenases differ substantially in their pH optima, in their affinities for pyridoxine and FAD, and in their behavior toward substrate analogues and inhibitors. Both are quite specific for pyridoxine as substrate, and both contain essential —SH groups. Pyridoxal and isopyridoxal, the initial products formed by these enzymes, are also the initial products formed during degradation of pyridoxine by growing cultures of the bacterial strains from which these dehydrogenases were obtained.