Involved in the degradation of pyridoxine (vitamin B(6)). Catalyzes the oxidation of 5-formyl-3-hydroxy-2-methylpyridine-4-carboxylate (FHMPC) by NAD(+) to 5-hydroxy-6-methylpyridine-3,4-dicarboxylate (HMPDC) (PubMed:19218190). Can also catalyze the reduction of FHMPC by NADH to 4-pyridoxic acid (PubMed:19218190). 5-formyl-3-hydroxy-2-methylpyridine-4-carboxylate + H2O + NAD(+) = 5-hydroxy-6-methylpyridine-3,4-dicarboxylate + 2 H(+) + NADH 5-formyl-3-hydroxy-2-methylpyridine-4-carboxylate + H(+) + NADH = 4-pyridoxate + NAD(+) kcat is 204 sec(-1) for the oxidation of FHMPC. kcat is 217 sec(-1) for the reduction of FHMPC. Optimum pH is 8.5. Optimum temperature is 55 degrees Celsius. Cofactor degradation; B6 vitamer degradation. Homodimer. Disruption mutant cannot grow on a synthetic medium containing pyridoxine, 4-pyridoxic acid or FHMPC as a sole carbon and nitrogen source. Belongs to the 3-hydroxyacyl-CoA dehydrogenase family.