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

Abstract

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 μmol/1/min (range 0.20–0.98 μmiol/1/min) and in normal homozygotes 1.2 μmol/1/min (range 0.80-1.9 μmiol/1/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 nineteen infants with mild PKU serum tyrosine remained unchanged within the first hour after the load and then declined.