New pathway of conversion of pyridoxal to 4 pyridoxic acid

Abstract

The only enzyme so far shown to catalyze the formation of 4 pyridoxic acid, the final excretory product of vitamin B6, has been aldehyde oxidase. In this paper evidence is presented that another enzyme, NAD+dependent aldehyde dehydrogenase, is also capable of catalyzing this reaction. Rat mutants with high, low and no aldehyde oxidase activity excreted the same amount of isotope after a single injection of3H pyridoxol in a 12 day experiment; 4 pyridoxic acid was also present in the urine of animals without aldehyde oxidase activity. No correlation was found between the proportion of excreted acid and the amount of pyridoxal excreted after the injection of a single dose of the aldehyde form. The K(m) values for pyridoxal and NAD+, at pH 9.6, were found to be 75 and 260 μmol/l respectively. NAD+dependent pyridoxal dehydrogenase was partially purified from the rat tissues, and the following specific enzyme activities (nmol x min-1x mg-1protein) were found: red blood cell 71.5 ± 3.0, intestine 64.9 ± 9.0, muscle 61.4 ± 1.6, brain 39.5 ± 6.0, liver 34.4 ± 3.3, kidney 21.1 ± 5.6, heart l8.8 ± 0.9, and lung 6.5 ± 2.0. This is believed to be the first report demonstrating pyridoxal oxidation catalyzed by NAD+dependent aldehyde dehydrogenase.