ABSTRACT
The metabolically active form is pyridoxal-5′-phosphate (PLP). PLP acts as a cofactor in a large number of amino acid transformations, most notable of which is transamination. In general, PLP enzymes act through Schiff base aldimine intermediates with the formation of a resonance stabilized carbanion. Vitamin B6-dependent enzymes are categorized in Table 9.2. The primary route for PLP catabolism is through oxidation to 4-pyridoxic acid (4-PA), which is excreted in the urine. Pyridoxal (PL) and 4-PA are the primary urinary excretion forms. Concentrations of 4-PA in the urine respond rapidly to dietary intake and are not considered to be a good indicator of status.5 As recently reviewed by Gibson5, the three most widely used biochemical tests to assess vitamin B6 status for the human are the activation coefficient for the erythrocyte aspartate aminotransferase, plasma PLP concentration, and urinary excretion of 4-PA. The plasma PLP concentration is a direct measure of the coenzyme form of vitamin B6 and is considered the best measure of status. Inadequacy has been defined as levels less than 20 nmol L−1.6 Methods to measure plasma B6 levels include the tyrosine apodecarboxylase procedure and liquid chromatography (LC) methods (Section 9.4). Radiometric methods include the conversion of [1-14C] tyrosine to 14CO2 by apo-tyrosine decarboxylase and the vitamin B63H radioenzymatic assay based on the PLP-dependent tyrosine decarboxylase. These methods are used in clinical laboratories for measurement of PLP in plasma.7-9 Han et al.10 introduced an enzymatic PLP assay that uses the apo form of the PLP-dependent recombinant enzyme, homocysteine-α-γ-lyase (rHCYase), which does not require radioisotopes.
