An assay method for the determination of pyridoxal 5'-phosphate (PLP) in plasma is presented. The procedure is rapid and requires only a small volume. The method includes PLP-dependent enzymatic decarboxylation of L-tyrosine to tyramine (enzyme used: L-tyrosine apodecarboxylase (EC 4.1.1.25) from Streptococcus faecalis) and measurement of tyramine by high-performance liquid chromatography with electrochemical detection. This technique confers specificity, rapidity, and low-cost measurement. Human plasma PLP from 30 normal healthy adults had a mean value of 65.10 +/- 4.70 nM. Sensitivity of the reaction was 1.3 nM and the coefficient of variation of the method (30 repeated assays of sample with a value of 60 nM) was 1.9%.
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