Regulation of vitamin B6 metabolism in human red cells

The effects of pyridoxine and pyridoxal 5-phosphate (PLP) administration on pyridoxine kinase (PnK) and asparate aminotransferase (EGOT), a PLP-dependent enzyme, were studied in human red cells separated into young and old populations by density centrifugation. After a delay of 48 hr, both pyridoxine and PLP increase EGOT activity in mature red cells by activating preformed GOT apoenzyme. In addition, in young erythroid cells, pyridoxine therapy induces synthesis of PnK, while both pyridoxine and PLP induce synthesis of GOT apoprotein. Thus, PLP stimulates EGOT induction without a change in PnK activity, suggesting that PLP enters erythroid precursor cells without prior dephosphorylation. However, with both pyridoxine and PLP, the full induction of enzyme activities reflect the gradual replacement of circulating red cells by newly formed cells with higher enzyme levels. Therefore, the use of EGOT as a measure of vitamin B6 nutritional status requires recognition of the complexities of intracellular enzyme regulation.