A QUANTITATIVE MICROMETHOD FOR THE DETERMINATION OF PHENYLALANINE AND TYROSINE IN BLOOD AND ITS APPLICATION IN THE DIAGNOSIS OF PHENYLKETONURIA IN INFANTS

Abstract

A simple enzymatic spectrophotometric method has been described which permits the quantitative determination of phenylalanine and tyrosine in 0.10 ml of whole blood. For elevated levels of phenylalanine and tyrosine, the method is rapid, specific, and precise. The method should be of value in establishing the diagnosis of phenylketonuria in infants and in following the blood levels of phenylalanine of those on the low phenylalanine diet. Analysis of heel-prick samples of premature and term infants has shown that normal infants have a broader range of blood phenylalanine values than adults, and that the levels of tyrosine may be as high as 20 mg/100 ml in older infants and premature babies. The fact that transient elevation of blood phenylalanine may occur in normal newborn infants must be appreciated in making a diagnosis of phenylketonuria in early infancy.