Cord-blood tyrosine levels in the full-term phenylketonuric fetus and the "justification hypothesis".

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Abstract
The justification hypothesis attributes mental retardation in phenylketonuria (PKU) to an inability of the heterozygous mother to deliver an appropriate amount of tyrosine to the PKU fetus who, in turn, is unable to correct for this deficiency because of its genetic constitution. This hypothesis was tested by measuring concentrations of tyrosine and phenylalanine in cord blood obtained at delivery from 9 infants with PKU and 5 infants with persistent (non-PKU) hyperphenylalaninemia (PHP). For each of these specimens there were four control cord-blood specimens from infants born on the same day and, generally, in the same hospital. PKU and PHP groups were similar with respect to cord-blood tyrosine and phenylalanine values. There was no biologically significant deficiency of tyrosine in cord blood of the pooled PKU and PHP groups (54 .+-. 10 .mu.M, mean .+-. SD) compared with controls (61 .+-. 16 .mu.M, P = 0.13). Phenylalanine in cord blood of the pooled PKU and PHP groups was significantly increased (144 .+-. 30 .mu.M, mean .+-. SD) compared with controls (128 .+-. 24, P = 0.004). The magnitude of the differences in cord-blood tyrosine and phenylalanine between control and PKU subjects are so small that it is unlikely that they have any consequences for physical and mental development. The justification hypothesis, as it pertains to blood tyrosine at term, is not upheld.