A study of the cofactors required by the tyrosine oxidase system of liver.

Recently considerable work has been reported on the enzyme system in mammalian liver which metabolizes tyrosine to fumarate and acetoacetate. This conversion has been shown to consist of several steps, involving first a transamination of tyrosine to p-hydroxyphenylpyruvic acid, and subsequent oxidation of this intermediate to 2,5-dihydroxyphenylpyruvic acid, then to homogentisic acid, and finally to fumaryl acetoacetate, which is hydrolyzed to fumarate and acetoacetate (l-5). Additional intermediate steps have not yet been elucidated, although they probably exist. Ascorbic acid has been implicated as a possible cofactor for at least one step in tyrosine oxidation, although its exact point of action has not been demonstrated (6-8). With the exception of the expected cofactor for the transaminase step, pyridoxal phosphate, cofactors for the other possible enzymatic steps have not been indicated. Therefore, the authors felt it would be important to study the tyrosine oxidase system to attempt to locate the site of action of ascorbic acid as well as to uncover the existence of any additional cofactors that might be involved in the system. For these purposes our investigations have been initiated with a substance known to destroy reduced ascorbic acid, 2,6-dichlorophenol indophenol (DCPP), used as a tool. The studies carried out to explain some apparently anomalous results in these experiments have led to new and interesting facts concerning the site of ascorbic acid action and the implication of perhaps two new cofactors for the tyrosine oxidase system.