Pyridoxine (vitamin B6: here) is a group of related chemicals that play an important role as coenzymes in energy metabolism in humans. Dietary pyridoxine is absorbed to enterocytes of the gut and then phosphorylated to pyridoxal 5’-phosphate, the active coenzyme form of the vitamin. Pyridoxal 5’-phosphate then enters circulation where it perform its cofactor role. Plasma levels of pyridoxal 5’-phosphate are a reliable marker of vitamin B6 status, because they accurately reflect tissue levels of the vitamin. Alcoholics often have low vitamin B6 status as measured by depressed plasma levels of pyridoxal 5’-phosphate, even when liver damage is not present. The reason for the low vitamin B6 status of alcoholics has been extensively investigated and studies show that poor dietary intake is a key factor. However, indoubtedly alteration to the absorption, storage and metabolism of the vitamin also plays a part. For example, a reduction in the rate of absorption through the effects of concomitant alcohol ingestion may be a key factor determining subsequent plasma levels of the vitamin.
Experiments in rats suggest that ethanol is able to inhibit the alkaline phosphatase enzyme responsible for intestinal hydrolysis of pyridoxal 5’-phosphate1. This is significant because this step is necessary in the absorption of the vitamin, whereby only non-phosphorylated pyridoxine can be absorbed passively to the enterocytes of the gut. In the experiments, ethanol was able to inhibit the absorption of phosphorylated pyridoxine in a dose dependent manner with in vitro experiments showing a 48% inhibition of pyridoxal 5’-phosphate hydrolysis at pH 7.4 (close to the pH of the duodenum). In in vivo experiments in the rats, a 4% weight for volume ethanol solution was able to inhibit the disappearance of pyridoxal 5’-phosphate by 55%. When the authors tested other alcohols (methanol and n-butanol) they found similar inhibitory effects on the hydrolysis of pyridoxal 5’-phosphate. These results suggest that if vitamin B6 is ingested in the pyridoxal 5’-phosphate form, absorption is greatly diminished in the present of ethanol, which may be one way in which alcoholics become deficient in the vitamin.
