An Enzymatic Spectropiiotometric Metiiod for the Determination of Phenylalanine 1n Blood

T HERE is a need for a simple rapid method to measure phenylalanine in serum or plasma, not only as an aid in the diagnosis of phenylketonuria, but also in the evaluation of the effectiveness of a diet low in phenylalanine in the treatment of patients with this condition. The method most commonly used to measure phenylalanine in plasma is that of Udenfriend and Cooper,* in which plasma is deproteinized and treated with Xtreptococcus faecalis t,o decarboxylat,c r,-phenylalanine to phenylethylamine. The latter is then determined colorimetrically after reaction with methyl orange. Although this method has been used successfully by many workers,2-6 it requires relatively large amount,s of plasma and takes several hours to perform each set of analyses. Furthermore, extremel) careful t,echnique is necessary to avoid an appreciable and variable blank from methyl orange. Another method for the determination of serum phenylalanine is that of Berry,7 by paper chromatography. This met,hod, although simple, has limitations in quantitative analysis inherent in any method which depends upon estimat,ion of the intensity of colored spots on paper chromatograms.