Metabolic derangement
Phenylalanine is an essential amino acid provided by the various proteins contained in the diet. Normally, a small amount is used for protein synthesis. The remainder is hydroxylated to tyrosine, which is used for synthesis of protein and several compounds or is degraded to produce energy. The hydroxylation of phenylalanine to tyrosine requires 2 enzymes, phenylalanine hydroxylase and dihydropterine reductase, and 2 cofactors, BH4 and NADH. BH4 is synthesized de novo from guanosine triphosphate in a series of steps. BH4 is regenerated from quinonoid dihydrobiopterin by the dihydropteridine reductase.
Based on plasma phenylalanine levels and residual phenylalanine hydroxylase activity in liver, 3 different inherited phenotypes of HPA due to phenylalanine hydroxylase deficiency have been described: classical or typical PKU, atypical PKU, and permanent mild HPA (Fig. 1).