Serum tyrosine within the first hour after an oral load of phenylalanine

Twenty-seven heterozygotes for phenylketonuria (PKU) and sixteen normal homozygotes were loaded with an amount of L-phenylalanine per body mass = 0.6 mmol/kg. Serum tyrosine concentration increased significantly within 15 min after the intake and the increase was rectilinear within the first 30 min. The initial rate of increase in serum tyrosine in heterozygotes was 0.47 mumol/l/min (range 0.20-0.98 mumol/l/min) and in normal homozygotes 1.2 mumol/l/min (range 0.80-1.9 mumol/l/min). The median serum tyrosine concentration increased within the first hour after an oral phenylalanine load (0.6 mmol/kg) of twelve infants with persistent hyperphenylalaninaemia (HPA), whereas serum tyrosine showed a decrease in forty infants with classical PKU. In ninetten infants with mild PKU serum tyrosine remained unchanged within the first hour after the load and then declined.