Hyperphenylalaninemia

Hyperphenylalaninemia is a metabolic condition in which the blood holds mildly or strongly raised levels of the amino acid phenylalanine. Phenylketonuria is its severe form, and newborns are routinely screened for it with the heel prick Guthrie test. Typical levels are about 2 to 6 milligrams per deciliter, while untreated cases can reach 20, and measurable intelligence deficits often appear as levels near 10. It is a recessive hereditary disorder in which the body cannot convert phenylalanine to tyrosine because the enzyme phenylalanine hydroxylase is wholly or partly missing. Newborns are treated with a diet very low in phenylalanine, which allows normal mental development, and a rarer malignant form comes from defects in recycling the cofactor tetrahydrobiopterin.