A simple, sensitive and reproducible assay for pyridoxal 5 '-phosphate and4-pyridoxic acid in human plasma

We describe a procedure for the measurement of pyridoxal 5'-phosphate and o f 4-pyridoxic acid in human plasma samples. It is based on the conversion o f pyridoxal 5'-phosphate to 4-pyridoxic acid 5'-phosphate by cyanide in alk aline medium, followed by a high pressure liquid chromatographic separation , with fluorescence detection at acid pH. The assay is robust, sensitive, l inear over a wide range, reproducible, and simple to perform. Samples store d at -80 degrees C are stable. Satisfactory agreement was obtained with res ults from the tyrosine decarboxylase-based assay for pyridoxal 5'-phosphate , in two other laboratories. Plasma samples from a National Survey of older British people were analyzed, and reference intervals for plasma pyridoxal 5'-phosphate intervals were derived. From the lower 2.5 percentile of the reference group, taken as the lower cut-off of the normal range, ca. 20% of elderly men and 11% of elderly women in the UK showed evidence of biochemi cal deficiency. (C) 1999 Elsevier Science B.V. All rights reserved.