Studies on Inhibition of 5-Hydroxy Tryptophan Decarboxylase by Phenylalanine Metabolites. ∗

Summary

Administration of high phenylalanine diets to weanling rats resulted in serum phenylalanine levels of 3 × 10⁻³M. At these concentrations, the decrease of kidney 5 HTP decarboxylase can be overcome by supplementation with excess pyridoxal phosphate. However, if incubation was carried out with 3 × 10⁻²M inhibitor, the inhibitor effect can no longer be reversed by supplementation with cofactor. The inhibition of phenylalanine and its derivatives, phenylpyruvic, phenyllactic, and phenylacetic acid, is competitive and substrate-dependent at pH 8. The K_m for 5 HTP decarboxylase is estimated to be 5.12 × 10⁻⁴.