Tryptophan pyrrolase of liver. II. The activating reactions in crude preparations from rat liver.

The activation of the apoenzyme of tryptophan pyrrolase in preparations from hydrocortisone-induced rats was shown to consist of two separate, sequential steps, only the second of which is necessary to activate preparations from tryptophan-induced rats. These reactions are as follows. (a) Conjugation of the apoenzyme to the oxidized holoenzyme with hematin supplied by added methemoglobin, a reaction which requires the presence of L-tryptophan or certain analogues and which is inhibited by thiol reagents as well as by globin. (b) Reduction of the oxidized holoenzyme, a reaction which is promoted by L-tryptophan specifically and by ascorbate. It is reversed by oxidation in air in the absence of L-tryptophan. Conjugation is the more rapid of the two reactions, especially at lower temperatures. The over-all activation is, therefore, limited by the rate of reduction. The sites on the apoenzyme and the holoenzyme that react with L-tryptophan for conjugation and catalysis, respectively, are different. They are distinguished by the wider specificity and higher affinity of the site on the apoenzyme involved in conjugation.