Intracellular phenylalanine and tyrosine concentration in homozygotes and heterozygotes for phenylketonuria (PKU) and hyperphenylalaninemia compared with normals.

Assuming adequate technique, determinations of intracellular phenylalanine and tyrosine concentrations in lymphocytes are very reproducible. The concentrations found in this study (1981) in five homozygotes and five obligate heterozygotes for PKU and seven normals, are identical with the corresponding concentrations found in 1979 in 13 homo- and 19 obligate heterozygotes for PKU and 26 normals. The intracellular concentrations in six homo- and five heterozygotes for hyper-Phe, as determined in the present study, are intermediate between the concentrations found in PKUs and normals in the present and the former study. As in PKUs, there is no difference between homo- and heterozygotes for hyper-Phe. The hypothesis of an intracellular threshold concentration for phenylalanine triggering the production of a toxic metabolite, could explain the severe brain damage observed in untreated PKU-homozygotes, the slight damage in well-treated PKU-homozygotes and in PKU-heterozygotes, and the absence of damage in hyper-Phe homozygotes (and heterozygotes). Also the difference in brain function between homozygotes for both conditions (PKU-treated), can be understood in spite of comparably elevated extracellular phenylalanine concentrations in young patients.