Biopterin derivatives in normal and phenylketonuric patients after oral loads of L-phenylalanine, L-tyrosine, and L-tryptophan.

Plasma biopterin derivatives studied in 10 normal and 21 phenylketonuric children showed a significantly high concentration in the latter group. Biopterin derivatives correlated with plasma phenylalanine concentration, but in normal adults given an oral phenylalanine load the rate of increase with phenylalanine differed from that in phenylketonuric patients. A patient with hyperphenylalaninaemia, not due to phenylketonuria, had an abnormal biopterin derivatives response to phenylalanine distinct from that of patients with classical phenylketonuria. This may be a useful investigation to differentiate some variants of phenylketonuria.