Regulation of vitamin B6 metabolism in human red cells

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Abstract
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 48 h pyridoxine and PLP increased EGOT activity in mature red cells by activating preformed GOT [glutamic oxaloacetic transaminase] apoenzyme. In young erythroid cells pyridoxine therapy induces synthesis of PnK while pyridoxine and PLP induced synthesis of GOT apoprotein. PLP stimulates EGOT induction without a change in PnK activity; PLP apparently enters erythroid precursor cells without prior dephosphorylation. With pyridoxine and PLP the full induction of enzyme activities reflects the gradual replacement of circulating red cells by newly formed cells with higher enzyme levels. The use of EGOT as a measure of vitamin B6 nutritional status requires recognition of the complexities of intracellular enzyme regulation.