Transport and metabolism of pyridoxine in rat liver

Evidence, obtained with in situ perfused rat liver, indicated that pyridoxine is taken up from the perfusate by a non-concentrative process, followed by metabolic trapping. These conclusions were reached on the basis of the fact that at low concentrations (0.125 μM), the ³H of ³H]pyridoxine accumulated against a concentration gradient, but high concentrations (333 μM) of pyridoxine or 4-deoxypyridoxine prevented this apparent concentrative uptake. Under no conditions did the tissue water : perfusate concentration ratio of ³H]pyridoxine exceed unity.The perfused liver rapidly converted the labeled pyridoxine to pyridoxine phosphate, pyridoxal phosphate and pyridoxamine phosphate and released a substantial amount of pyridoxal and some pyridoxal phosphate into the perfusate. Since muscle and erythrocytes failed to oxidize pyridoxine phosphate to pyridoxal phosphate, it is suggested that the liver plays a major role in oxidizing dietary pyridoxine and pyridoxamine as their phosphate esters to supply pyridoxal phosphate which then reaches to other organs chiefly as circulating pyridoxal.