A convenient lactic dehydrogenase-coupled assay for determining pyridoxal 5'-phosphate in plasma.

An assay for determining the concentration of pyridoxal Y-phosphate in plasma from 0.4 ml whole blood is reported. The assay consists of incubating deproteinized plasma with o-setine apodehydratase from Escherichia cob’ in 0.5 M N-2-hydroxyethylpipetazine-N’-3-propesulfonic acid, pH 7.8, at 37°C for 15 mitt, and then determining the o-serine dehydratase activity of an aliquot of the incubation mixture. A lactic dehydrogenase-coupled assay (at 25°C) was used to measure the rate of enzymicahy catalyzed conversion of D-serine to pyruvate, wherein depletion of NADH was followed continuously at 340 nm. The concentration of pyridoxal Y-phosphate in the plasma samule was estimated from the enzymic activity which is a linear function of the amount of pyridoxal 5’-phosphate present in the assay.