A Tryptophan Pyrrole-Ring Cleavage Enzyme in the Most Primitive Eukaryote
Two types of dioxygenases that catalyze the oxidative cleavage of the pyrrole ring of tryptophan by insertion of molecular oxygen to yield N-formylkynurenine have been reported (Feigelson and Brady, 1974). One is tryptophan 2,3-dioxygenase (TDO), and the other is indoleamine 2,3-dioxygenase (IDO). Although protoheme IX is a sole prosthetic group for both dioxygenases (Feigelson and Brady, 1974; Hirata and Hayaishi, 1975; Ishimura et al., 1980), these have proved to be distinct enzymes. TDO is a tetrameric protein (M.W. 120,000–167,000) metabolizing L-tryptophan specifically (Feigelson and Brady, 1974). This dioxygenase requires nonspecific reductants such as L-ascorbic acid for its activation in vitro (Feigelson and Brady, 1974). IDO is a monomeric protein (M.W. 41,000) (Shimizu et al., 1978) exhibiting a wide substrate specificity for various indoleamine derivatives including L-and D-tryptophan and serotonin (Ishimura et al., 1970). This enzyme can be also activated by a wide variety of reductants. In addition, methylene blue (or toluidine blue) is absolutely required as an electron mediator from a reductant to the ferric enzyme for its activation (Yamamoto and Hayaishi, 1967).
KeywordsMethylene Blue Protein Spot Pyrrole Ring Electron Mediator Wide Substrate Specificity
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