Pathway A has 8 steps for degrading pyridoxine

Two pathways, A and B, are involved³. Pathway A has 8 steps for degrading pyridoxine. First, pyridoxine is oxidized to pyridoxal by enzyme pyridoxine 4’-oxidase². NAD-dependent pyridoxal dehydrogenase enzyme oxidizes pyridoxal to produce 4-pyridoxolactone, which also hydrolyzed². This product undergoes oxidation to form 2-methyl-3-hydroxypyridine-4-carboxylic acid in two steps, the first of which is catalyzed by FAD dependent pyridoxic acid 4-dehydrogenase enzyme and he second by a NAD depedent-5-formaly-3-hydroxyl-2-methylpyridine-4-carboxylic acid dehydrogenase².

Then, the acid is decarboxylated to form 2-methyl-3-hydroxypyridine-5-carboxylic acid under the catalytic action of an enzyme that depends on the presence of magnesium ions². The product then undergoes an oxidative opening of the ring (2-(N-acetamidomethylese) succinic acid). A FAD dependent 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase catalyzes the succinic acid to form acetate, carbon dioxide, ammonia and succinic semialdehyde, which completes the pathway².

On the other hand, the B pathway has only five steps. First, pyridoxine is oxidized under the catalytic action of FAD-dependent pyridoxine-5-dehydrogenase enzyme to form isopyridoxal, which is then oxidized to form 5-pyridoxolactone. This product is further oxidized to form 5-pyridoxic acid under the enzyme 5’-lactonase action². The 5-pyridoxic acid undergoes a process of oxidative ring opening under the catalytic action of FAD dependent 5-pyridoxic acid oxegenase, resulting into 2-hydroxymethyl-(N-acetomidomethylene) succinic acid. The final step involves the hydrolysis of the succinct acid to form acetate, carbon dioxide and ammonia².