Enzymes of Vitamin B6 Degradation

Two NAD+-dependent, highly specific pyridine5-aldehyde dehydrogenases, 5-formyl-3-hydroxy-2methylpyridine-4-carboxylic-acid (Compound 1) dehydrogenase and isopyridoxal dehydrogenase, were purified to homogeneity from Pseudomonas MA- 1 and Arthrobacter Cr-7, respectively. Both enzymes are induced in response to growth of the organisms on pyridoxine and catalyze steps in the degradation of this compound by these organisms. Compound 1 dehydrogenase (M, = 65,000) contains two subunits of equal size with methionine as the NHz-terminal amino acid and acts optimally at pH 7.8-8.5. It catalyzes with equal facility (turnover number = 400-670 s” molecule”) both the oxidation of Compound 1 (K, = 65 p~) by NAD+ (K, = 25 p~) to 3-hydroxy-2-methylpyridine-4,5-dicarboxylic acid and the reduction of Compound 1 by NADH (K, = 20 p~) to 4-pyridoxic acid and appears to act as a true dismutase. The possible advantage to the organism of its ability to act as a dismutase is discussed briefly. No oxidation of 4-pyridoxic acid by this enzyme was observed. Isopyridoxal dehydrogenase (Mr = 242,000) contains four subunits of equal size, again with methionine at the NHz terminus. At its optimal pH of 8.0-8.6, it catalyzes the oxidation of isopyridoxal (K, = 40 p~, turnover number = 10 s-l molecule”) by NAD+ (K, = 40 PM) to a mixture of 5-pyridoxic acid and 5-pyridoxolactone, which are produced in constant ratio throughout the course of the reaction. Formation of the two products, although unusual, is readily understandable in terms of (a) the structure of isopyridoxal in solution or (b) the structure of a possible acyl-enzyme intermediate in the oxidative reaction.