Certain aspects of tyrosine metabolism in the young. I. The development of the tyrosine oxidizing system in human liver.

The major pathway for tyrosine catabolism in the animal is the non-oxidative transamination of tyrosine to p-hydroxyphenylpyruvate and its subsequent oxidation to homogentisate. The homogentisate is oxidized to fumarylacetoacetate (maleylacetoacetate), which is finally hydrolyzed to fumarate (malate) and acetoacetate. The enzyme system responsible for this catabolism, tentatively called the tyrosine oxidizing system, is soluble and a partial purification can be accomplished with high speed centrifugation of homogenates of liver (1) and kidney (2). When these preparations are used it has been observed (3) that one mole of tyrosine will be completely oxidized to one mole of acetoacetate and one mole of fumarate, with the utilization of two moles of oxygen. In order to obtain maximal tyrosine transaminase activity it is necessary to add a-ketoglutarate and pyridoxal phosphate in vitro. The oxidation of the formed p-hydroxyphenylpyruvate requires ascorbic acid (1, 4, 5) as a cofactor or an equivalent oxidation-reduction substitute such as dichlorophenolindophenol (6, 7). Clinically, it was shown (8, 9) that premature infants maintained on a diet high in protein without added ascorbic acid excreted p-hydroxyphenylpyruvic acid in their urine. Similarly, p-hydroxyphenylpyruvic acid was excreted in the urine of scorbutic adults (10, 11). The addition of ascorbic acid to the diet of both groups resulted in the disappearance of this substance from the urine. It was also observed that when large doses of pteroylglutamic acid were given to scorbutic guinea pigs (12), monkeys (13), and prema-