The importance of transamination and decarboxylation in phenylalanine metabolism in vivo in the rat

The extent of hydroxylation, transamination, and decarboxylation in the metabolism of excess phenylalanine in vivo has been examined by measuring the amount of radioactivity from [14C]phenylalanine that is converted to 14CO2 and urinary metabolites. Transamination and direct decarboxylation represent only 6% of total phenylalanine metabolism. The major aromatic metabolites in the urine after phenylalanine loading are phenylacetylglycine, phenylacetic acid, phenylpyruvate, and phenylalanine. A small but significant portion (1.5%) of phenylalanine is degraded to nonaromatic compounds. The maximum phenylalanine oxidation in vivo is approximately 75%24 h at saturating concentrations of phenylalanine; thus, the major route of degradation of phenylalanine in the rat, even when intake is high, is via formation of acetoacetic acid and fumaric acid.