[Hyperphenylalaninaemia with normal phenylalanine-hydroxylase activity and a deficiency of tetrahydrobiopterin and dihydropteridine reductase].

Two cases of hyperphenylalaninemia with a normal activity of phenylalanine hydroxylase are described. No activity of DHP reductase was found in the first case, having very high biopterin levels in basal conditions and after intravenous perfusion of phenylalanine. In the other case, the DHP reductase activity is normal but plasma and urinary levels of the reduced forms of biopterin are largely lowered and do not increase during the phenylalanine load. Early substitutive treatment with L-dopa and 5-HTP in one of the cases avoided the development of the "progressive neurological illness unresponsive to dietary treatment" characterizing two variants. This raises the question of a liver biopsy in order to assay the hydroxylation enzyme activities when screening hyperphenylalaninemia whatever the type.