Enzymatic Determination of Pyridoxal Phosphate in Plasma by Decarboxylation of L-Tyrosine-¹⁴ C (U) and a Comparison with the Tryptophan Load Test

Abstract

A method using the tyrosine decarboxylase for determining the pyridoxal phosphate concentration in biological samples, such as plasma, is presented. The method was found to be specific and sensitive, which makes it possible to analyze samples containing low concentrations of pyridoxal phosphate. A significant correlation was found when the pyridoxal-phosphate concentration in plasma was compared with the excreted amount of xanthurenic acid at the tryptophan-load test. Certain cases were noticed with low xanthurenic-acid values at low pyridoxal-phosphate concentrations and vice versa and these discrepancies are discussed.

Key Words:

• Enzyme tests
• pyridoxal phosphate
• pyridoxin
• tryptophan
• tyramine
• tyrosine
• tyrosine decarboxylase
• vitamin B complex
• xanthurenates