The labelling of urinary acids after oral doses of deuterated L-phenylalanine and L-tyrosine in normal subjects. Quantitative studies with implications for the deuterated phenylalanine load test in phenylketonuria.

Oral doses of L-[2H5]-phenylalanine (25 mg/kg) and L-[2H2]-tyrosine (12.5 mg/kg) were given separately to three normal subjects and together to a fourth. Blood samples were analysed for deuterium labelled phenylalanine and tyrosine, and urine for labelled o-and p-hydroxyphenylacetic, p-hydroxyphenyllactic and p-hydroxymandelic acids. The labelling patterns of the urinary metabolites indicated that the para-compounds all originated in both hepatic and extra-hepatic tissues. The plasma tyrosine did not appear to be in equilibrium with the tyrosine in the liver. It is concluded that a simple quantitative relationship between the labelling of these metabolites and the synthesis of labelled tyrosine from labelled phenylalanine in liver is unlikely.