Plasma phenylalanine and tyrosine levels during the day in normal female controls and female obligate phenylketonuria heterozygotes.

The plasma phenylalanine and tyrosine levels of 8 female obligate phenylketonuria heterozygotes and 8 female controls have been measured over a 7-hour period during the day. No significant differences were observed in the plasma tyrosine levels. It is concluded that the supply of tyrosine to the growing fetal brain is not necessarily impaired by the decreased ability to convert phenylalanine to tyrosine of the phenylketonuria heterozygous mother.