TYROSINE SUPPLEMENTATION IN THE TREATMENT OF MATERNAL PHENYLKETONURIA

Lower than average concentrations of tyrosine could be a factor in the fetal damage produced by maternal phenylketonuria (PKU). Dietary supp lementation with L-tyrosine has been inconsistent in these reported pr egnancies. Consequently, we studied the response to supplementation wi th L-tyrosine in five maternal PKU pregnancies. Before supplementation the mean plasma tyrosine concentration during midpregnancy was only 3 4 mu mol/L compared with the expected value of 45 mu mol/L in the norm al population. A single dose of 1.3 g L-tyrosine was sufficient to rai se plasma tyrosine concentrations well above the recommended minimum c oncentration of 45 mu mol/L. The response was rapid and sustained over a 3-h study period in each subject and was associated with a markedly increased ratio of tyrosine to large neutral amino acids. These resul ts indicate that the plasma tyrosine concentration can be increased to normal or above-normal values in maternal PKU pregnancies for a perio d of greater than or equal to 3 h by supplementation of the diet with L-tyrosine. The high ratio of tyrosine to large neutral amino acids in dicates that tyrosine might readily cross the placenta and provide suf ficient tyrosine to the fetus to support normal protein and catecholam ine synthesis in what otherwise might be a tyrosine-poor environment.