An enzymatic method for the determination of plasma phenylalanine (Phe) and tyrosine (Tyr) in newborns and phenylketonurics is described. The method uses yeast phenylalanine ammonia-lyase to catalyze the conversion of l-Phe to transcinnamic acid and l-Tyr to p-coumaric acid, which are then determined by differential spectrophotometry at 290 and 315 nm. This method, requiring only one drop of blood, is simple, quantitative, and time efficient, and it has been adopted for the management of phenylketonuria (PKU). It can be used for monitoring plasma Phe and Tyr levels of PKU patients during dietary treatment, for diagnosis of classical PKU through the Phe-loading test, and for detection of PKU heterozygotes. A total of 350 plasma samples collected from 31 PKU patients over a 4 year period were analyzed and yielded results similar to those obtained by automated amino acid analysis. In an exploratory study, 1084 newborn infants were screened for PKU by our enzymatic method and the Guthrie test. In this group we detected 5 infants with Phe levels greater than 8 mg/dl plasma and 2 infants with Tyr levels greater than 8 mg/dl plasma, while the Guthrie test on the same subjects gave no positives.
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