Liquid chromatographic studies of vitamin B6 metabolism in man

The effects of increased intake of pyridoxine hydrochloride on plasma vitamin B6 metabolism within therapeutic limits (up to 800 mg/day) were investigated. Maximum plasma concentrations of pyridoxal phosphate were attained at relatively low intakes of pyridoxine hydrochloride. Two metabolism thought to be unidentified forms of vitamin B6 were present in subjects taking more than 200 mg of pyridoxine hydrochloride per day as have recently been described. We investigated the possibility that these were isomeric forms of vitamin B6. However, 'Peak 2' metabolite was shown to be probably 4-pyridoxolactone. The metabolism of isopyridoxal has not previously been investigated in man. We demonstrated that it is an active vitamer of the B6 complex in humans. The main fluorescent metabolite of isopyridoxal present in plasma and urine had a similar retention time to 'Peak 1' metabolite. Isopyridoxal was incapable of being directly phosphorylated in rat liver extract and it is therefore unlikely that peak 1 is isopyridoxal phosphate. Its nature remains unknown.