The enzymatic conversion of phenylalanine to tyrosine.

The enzymatic oxidation of phenylalanine to tyrosine has been studied as part of an investigation of the intermediates and enzyme systems involved in the biosynthesis of epinephrine. Womack and Rose (1) indicated that phenylalanine was converted to tyrosine in viva by showing that tyrosine is not essential when sufficient phenylalanine is available. Definite proof was provided by Moss and Schoenheimer (2), who demonstrated that isotopically labeled phenylalanine given to rats appeared both as labeled phenylalanine and labeled tyrosine in the body proteins. Information concerning the tissue catalysts responsible for the oxidation of phenylalanine to tyrosine is meager. Embden and Baldes (3) perfused livers with phenylalanine and showed an increase in tyrosine. Bernheim and Bernheim (4) demonstrated that surviving liver slices, on incubation with phenylalanine, produced an increased amount of phenolic material which showed many of the properties of tyrosine. The experiments reported in the present paper demonstrate that liver slices and homogenates can convert phenylalanine to tyrosine. The properties and requirements of a soluble system that is capable of catalyzing the reaction are described. Materials-Phenylalanine and tyrosine were obtained from the Nutritional Biochemicals Corporation. There was less than 0.1 per cent of the D isomer in the L-phenylalanine and about 0.2 per cent of the L isomer in the n-phenylalanine.’ These determinations were made by the method of Meister, Levintow, Kingsley, and Greenstein (5). 3-C14-DL-Phenylalanine was obtained from Tracerlab, Inc. Samples of m-cyclohexylglycine, cyclohexyl-DL-alanine, and cyclohexyl-nn-aminobutyric acid were obtained from Dr. Alton Meister of the National Cancer Institute, the /?-2-thienylalanine from the Nutritional Biochemicals Corporation (Lot 3045), and p-hydroxycinnamic acid and p-hydroxymandelic acid from Dr. Emery M. Gall of the University of California, Department of Bio-