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Tryptophan Pyrrolase of Liver

II. THE ACTIVATING REACTIONS IN CRUDE PREPARATIONS FROM RAT LIVER

From the ¹ From the Department of Biological Chemistry, Harvard Medical School, and the Cancer Research Institute, New England Deaconess Hospital, Boston, Massachusetts 02215

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.

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