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The Bacterial Oxidation of Vitamin B₆

VI. PYRIDOXAL DEHYDROGENASE AND 4-PYRIDOXOLACTONASE

From the ¹ From the Department of Biochemistry, University of California, Berkeley, California 94720

Pyridoxal dehydrogenase was partially purified from extracts of Pseudomonas sp. MA grown on pyridoxamine as the sole carbon and nitrogen source, and shown to catalyze Reaction i.

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Pyridoxal dehydrogenase is specific for DPN⁺ and does not oxidize benzaldehyde, acetaldehyde, salicylaldehyde, or 5-deoxypyridoxal. In the presence of the latter compound, oxidation of pyridoxal is stimulated, apparently because of the occurrence of the dismutation reaction (ii). 2-Demethyl-2-ethylpyridoxal is dehydrogenated at the same rate as pyridoxal. At high substrate concentrations the pH

[see PDF for equation]

optimum of pyridoxal dehydrogenase is above 9.3; at pH 9.3 the K_m values for pyridoxal and DPN⁺ are 0.076 mm and 0.29 mm, respectively. Reaction i appears to be irreversible. o-Phenanthroline and some other chelating agents inhibit the enzyme, but m- and p-phenanthroline (which do not act as chelating agents) are much more potent inhibitors with K_i values of about 1.0 µm.

4-Pyridoxolactonase, which catalyzes Reaction iii, also occurs in this same organism. It acts optimally at pH 7.7,

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has a K_m value of 3.1 µm, and, at the stage of purity achieved, requires no added cofactors. m- and p-Phenanthroline also inhibit this enzyme, but only at concentrations much higher than those that inhibit pyridoxal dehydrogenase.

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				N. Yokochi, Y. Yoshikane, Y. Trongpanich, K. Ohnishi, and T. Yagi Molecular Cloning, Expression, and Properties of an Unusual Aldo-Keto Reductase Family Enzyme, Pyridoxal 4-Dehydrogenase, That Catalyzes Irreversible Oxidation of Pyridoxal J. Biol. Chem., September 3, 2004; 279(36): 37377 - 37384. [Abstract] [Full Text] [PDF]