[Phenylalanine hydroxylase activity in the liver as a parameter for distinguishing various forms of hyperphenylalaninemias (author's transl)].

The activity of phenylalanine hydroxylase in the liver without the cofactor 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropterin is similar in cases of classical PKU (14 cases), mild hyperphenylalaninemias (2 cases), heterozygotes (1 case) and normal controls (18 cases). The addition of cofactor increases the activity 30-40-fold in normal controls and 2-5 fold in mild hyperphenylalaninemias or heterozygotes. In classical PKU the addition of cofactor is without effect or causes even a diminution of activity. Therefore, for classification of the various forms of hyperphenylalaninemias the effect of cofactor on enzyme activity is important. For decisions from this findings to be made as to whether treatment should or should not be used further investigations are needed.