The role of superoxide and hydroperoxide in the reductive activation of tryptophan-2,3-dioxygenase.

Activation of aerobically aged rat liver and Pseudomonas acidouorans tryptophan-2,3-dioxygenase (EC 1.13.1.12) by reducing agents was studied in an attempt to define the chemical nature of the species which achieves the reduction. With the use of two enzymes, superoxide dismutase and catalase, reductive activation by sodium ascorbate, sodium borohydride, hydroperoxide, and dithiothreitol was shown to be caused by superoxide, OsT, and hydroperoxide, H202, as well as by direct reduction, i.e. direct transfer of electrons from the added compound to the enzyme. The ability of catalase to prevent the activation of tryptophan oxygenase by tryptophan indicates this activation involves processes similar to the reducing agents. A study of the effect of binding of nonproductive tryptophan analogues to the catalytic or allosteric sites of the pseudomonad enzyme showed the involvement of the catalytic site in activation by sodium ascorbate. Activation by tryptophan was shown to require a substrate or catalysis. The nonspecific nature of reductive activation of tryptophan oxygenase supports the view that oxidative inactivation of the enzyme is an artifact of handling and purification.