Lower than average concentrations of
tyrosine could be
a factor in the fetal damage produced by
maternal phenylketonuria (PKU). Dietary supplementation with
L-tyrosine has been inconsistent in these reported pregnancies. Consequently, we studied the response to supplementation with
L-tyrosine in five maternal PKU pregnancies. Before supplementation the mean plasma
tyrosine concentration during midpregnancy was only 34 micromol/L compared with the expected value of 45 micromol/L in the normal population. A single dose of 1.3 g
L-tyrosine was sufficient to raise plasma
tyrosine concentrations well above the recommended minimum concentration of 45 micromol/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 results indicate that the plasma
tyrosine concentration can be increased to normal or above-normal values in maternal PKU pregnancies for a period of > or = 3 h by supplementation of the diet with
L-tyrosine. The high ratio of
tyrosine to large
neutral amino acids indicates that
tyrosine might readily cross the placenta and provide sufficient
tyrosine to the fetus to support normal
protein and
catecholamine synthesis in what otherwise might be a
tyrosine-poor environment.