Sensitive in vivo assay of the phenylalanine hydroxylating system with a small intravenous dose of heptadeutero L-phenylalanine using high pressure liquid chromatography and capillary gas chromatography/mass fragmentography.

A method is described which allows the in vivo determination of the phenylalanine hydroxylating system in atypical and classical phenylketonuria. Phenylalanine-d7 is administered i.v. (0.030 g/kg body weight) within 10 min. Tyrosine-d6 in plasma is measured from 30 to 240 min post load by using a computerized capillary gas chromatography/mass fragmentography system. In two patients with hyperphenylalaninemia, the residual activity of the phenylalanine hydroxylating system was 15.7 and 3.7% of the normal, in two phenylketonurics 1.5 and 0.3% respectively. The in vivo figures correspond well to the in vitro assay of the residual activity of the phenylalanine hydroxylase in needle liver biopsy material.