Distribution of pyridoxal-5-phosphate in human blood between the cells and the plasma: effect of oral administration of pyridoxine on the ratio in Down's and hyperactive patients.

Abstract The distribution of pyridoxal-5-phosphate (PLP) in blood between the cells and the plasma in control subjects and in hyperactive and Down's patients has been studied. The data show an almost equal distribution of PLP between the cells (mostly in the red cells) and plasma, with about equal concentrations in the two fractions in all the three groups. The distribution pattern in patients receiving pharmacological doses of pyridoxine has also been investigated. These patients were from a study on the possible therapeutic value of prolonged administration of pharmacological doses of the vitamin. The PLP content of blood rose sharply during the treatment. The distribution of PLP, however, differed markedly over the base line pattern, with most of PLP now being localized in the red cells. The red cell PLP may belong to a different pool with a slower turnover rate, and may be involved in the regulation of plasma PLP content. The data also suggest that plasma PLP may be a better indicator of the PLP status in such individuals. While the functional significance of such high concentrations of PLP in the red cells is not clear at this time, it seems likely that it would have a significant effect on the metabolism of red cells and perhaps of other cell types in blood.