Although the formation of tyrosine from phenylalanine had been demonstrated in perfused dog liver as early as 1913 (l), the nature of the enzyme system involved appears to have been largely neglected during the subsequent 40 years. Indeed, if the reaction was discussed at all in textbooks of biochemistry written prior to 1950, it was depicted as shown in Fig. 1. The mechanism of the reaction was so obscure that a simple balanced equation could not be formulated without invoking the imaginary species, one-half an oxygen molecule.