PHENYLKETONURIA - THE IN-VIVO HYDROXYLATION RATE OF PHENYLALANINE INTO TYROSINE IS DECREASED

In phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is defi cient, resulting in a decreased conversion of phenylalanine (Phe) into tyrosine (Tyr). The severity of the disease is expressed as the toler ance for Phe at 5 yr of age. In PKU patients it is assumed that the de creased conversion of Phe into Tyr is directly correlated with the tol erance for Phe. We investigated this correlation by an in vivo stable isotope study. The in vivo residual hydroxylation was quantitated usin g a primed continuous infusion of L-[ring-H-2(5)]Phe and L-[1-C-13]Tyr and the determination of the isotopic enrichments of L-[ring-H-2(5)]P he, L-[ring-H-2(4)]Tyr, and L-[1-C-13]Tyr in plasma. Previous reports by Thompson and coworkers (Thompson, G.N., and D. Halliday. 1990. J. C lin. Invest. 86:317-322; Thompson, G.N., J.H. Waiter, J.V. Leonard, an d D. Halliday. 1990, Metabolism. 39:799-807; Treacy, E., J.J. Pitt, K. Seller, G.N. Thompson, S. Ramus, and R.G.H. Cotton. 1996. J. Inherite d Metab. Dis. 19:595-602), applying the same technique, showed normal in vivo hydroxylation rates of Phe in almost all PKU patients. Therefo re, our study was divided up in two parts, First, the method was re-ev aluated. Second, the correlation between the in vivo hydroxylation of Phe and the tolerance for Phe was tested in seven classical PKU patien ts. Very low (0.13-0.95 mu mol/kg per hour) and normal (4.11 and 6.33 mu mol/kg per hour) conversion rates were found in patients and contro ls, respectively. Performing the infusion study twice in the same pati ent and wash-out studies of the labels at the end of the experiment in a patient and control showed that the method is applicable in PKU pat ients and gives consistent data. No significant correlation was observ ed between the in vivo hydroxylation rates and the tolerances. The res ults of this study, therefore, showed that within the group of patient s with classical PKU, the tolerance does not depend on the in vivo hyd roxylation.