The Nature and Mechanism of the Tryptophan Pyrrolase (Peroxidase-Oxidase) Reaction of Pseudomonas and of Rat Liver

Abstract

The oxidation reaction was catalyzed by an enzyme that had the following properties: (a) utilization of one molecule of oxygen in a single step, (b) inhibition by catalase, and (c) specific reversal of catalase inhibition by hydrogen peroxide from a donor reaction. Other properties of the enzyme, including cyanide and light reversible CO inhibitions, suggested that it was an iron porphyrin enzyme occurring in both the ferrous and ferric states (3). On the basis of the above properties it was assumed that there was an intermediate formation and utilization of peroxide in the reaction. The reaction was therefore formulated as a coupled oxidation (4), probably catalyzed by a single enzyme, and the enzyme was called the “tryptophan peroxidase-oxidase.” Further investigation of the reaction mechanism is reported here with both the liver enzyme and a highly purified bacterial enzyme which was first obtained by Hayaishi and Stanier (5) in crude cell extracts from tryptophan-grown Pseudomonas sp. with 100 times the specific activity of liver extracts. The bacterial enzyme shares all of the properties of the animal enzyme. A preliminary report of these results has appeared (6). Peroxide was not directly involved in the oxidation reaction of either enzyme, but was necessary to activate it. The enzyme was identified as an iron-porphyrin protein which was reduced to its active ferrous form by peroxide. In conjunction with the demonstration that gaseous oxygen was incorporated into the product of this reaction (7), the present results indicate that the reaction catalyzed is an oxygenation, a direct oxygen transfer to tryptophan, and it does not involve peroxidation. For this reason we will refer to the enzyme by the original name, tryptophan pyrrolase.