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Tyrosine deficiency and retardation.

ABSTRACT To add to the confusion about phenylketonuria (PKU), there is a new wrinkle. According to a report published in March of this year in the Proceedings of the National Academy of Sciences (75:1562), deficiency of the nonessential amino acid tyrosine may be just as responsible for the mental retardation in PKU as high levels of phenylalanine.A study by Samuel Bessman, MD, Malcolm Williamson, MD, and Richard Koch, MD, of the University of Southern California School of Medicine, attempted to correlate the mean IQ of siblings of known PKU children (67% of PKU siblings are heterozygous for PKU and 33% are normal, according to Mendelian predictions) with their ability to convert phenylalanine to tyrosine. Results showed a 10 point mean IQ difference between those siblings least able and those most able to convert phenylalanine to tyrosine.According to the investigators, since the mothers of PKU children are always heterozygous for