Micromethods for Measuring Phenylalanine and Tyrosine in Serum.

Spectrophotofluorometricmicromethodsfor the determination of phenylalanineand tyrosine on 25 ,J. of serum are described. These methods were applied to detect phenylketonuriaamong homozygotesand heterozygotesand in the newborn population. The data presentedagreewith those previouslyreported. (1) Phenylketonurics have markedly elevated serum phenylalanineand lower serum tyrosinethan the controls. (2) The heterozygoteshave a higher fasting serum phenylalanineand, after a standard oral phenylalanine test, show a higher and more prolongedrise of serum phenylalaninethan the controls. There was a lesserincreaseof serum tyrosineafter phenylalanineloading in the heterozygotesthan the controls. (3) Premature infants of low birth weight havehigher serum phenylalanineand tyrosinelevels than normal birth weight infants, presumablydue to enzyme immaturity. Simultaneousdeterminationsof serum phenylalanineand tyrosine will differentiate newborn infants who are suspectedto be phenylketonuricsfrom the homozygotes (phenylketonurics).