In Vivo Phenylalanine Hydroxylation in Phenylketonuria Patients

In phenylketonuria (PKU), the enzyme phenylalanine hy-droxylase is deficient, resulting in a decreased conversion of phenylalanine (Phe) into tyrosine (Tyr). The severity of the disease is expressed as the tolerance for Phe at 5 yr of age. In PKU patients it is assumed that the decreased conversion of Phe into Tyr is directly correlated with the tolerance for Phe. We investigated this correlation by an in vivo stable isotope study. The in vivo residual hydroxylation was quan-titated using a primed continuous infusion of