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

Gas chromatographic-mass spectrometric method for quantitation of phenylalanine and tyrosine in serum

A validated gas chromatographic-mass spectrometric method for quantitation of phenylalanine and tyrosine in serum is described. Quantitation of phenylalanine and tyrosine with a non-labelled non-endogenous internal standard, L-2-chlorophenylalanine, compared favourably with isotope dilution mass spectrometric quantitation. The 95% reference ranges for phenylalanine. tyrosine and the phenylalanine-tyrosine molar ratio in neonate cord blood serum were determined by isotope dilution mass spectrometry and were found to be 77.1-144.7, 33.3-109.3 micromol/l and 1.1-3.0, respectively.

Treatment of phenylalanine hydroxylase deficiency

In phenylalanine hydroxylase deficiency detected by screening treatment in early life, both age at start of treatment and phenylalanine control during treatment are the major determinants of eventual psychological status. The influence of phenylalanine control declines with age but executive performance is influenced by hyperphenylalaninaemia at all ages. In a few subjects neurological deterioration has been reported years after relaxing or stopping treatment. MRI changes in brain white matter are present in most subjects no longer on a strict diet. These changes are usually reversible and closely related to phenylalanine status at the time of investigation. Whether or not the changes point to a specific vulnerability of white matter remains uncertain, although MRI changes were particularly prominent in subjects with neurological disability and may be irreversible in such subjects. Policies on treatment have to take account of these findings.

Birth weight and pathogenesis in phenylketonuria

The birthweights of an ethnically homogeneous sample of infants with phenylketonuria, their unaffected siblings, and control infants were compared after adjusting for the effects of: mother's age, mother's date of birth, mother's height and obstetric history, the length of gestation, the infant's sex, the place and date of birth. There were no significant differences between the infants with phenylketonuria and their unaffected siblings either in adjusted or unadjusted birthweights. Control infants had slightly, but statistically significant, greater adjusted and unadjusted birthweights than the combined phenylketonuria and unaffected sibling groups. This effect of the phenylketonuria gene is a previously unreported finding but unlikely to be related to the pathogenesis of phenylketonuria. Our results do not provide support for the "justification" hypothesis that the mental and neurological defects in phenylketonuria result from prenatal tyrosine deprivation which would be reflected in lower birthweights.

Parenteral nutrition in a premature infant with phenylketonuria

Parenteral nutrition (PN) is now an important facet of the management of the extremely premature infant. However, its effects on those with inherited metabolic disease have not been well documented. We report an infant with classical phenylketonuria (PKU) who had unusually high serum phenylalanine at 12 days of life (5200 mumol; greater than 3.2 SD above our mean for PKU at 10-15 days of age) while on parenteral nutrition, despite a relatively high tolerance for phenylalanine on oral feeds at 3-4 months of life (97-128 mg/kg/day; normal for PKU: less than 90 mg/kg/day). Identification of PKU was somewhat delayed in this child because of failure to recognize that parenteral nutrition provides a phenylalanine load equivalent to or greater than the routine oral formula feeding. Despite the high levels of phenylalanine in the first 2 wk of life, mental and motor development are normal at 16 months of age. This case, the first such reported, suggests the parenteral nutrition in the premature PKU infant is relatively safe, but draws attention to the possible need for phenylalanine-free amino acid infusates for those who require long-term treatment.