The Search for Copper in L-Tryptophan 2,3-Dioxygenases
L-Tryptophan 2,3-dioxygenase (EC 18.104.22.168) catalyzes the formation of N-formylkynurenine from L-tryptophan and O2 (Figure 1) (see references 1, 2 for reviews). The enzymes from L-tryptophan induced Pseudomonas acidovorans (ATCC 11299b) and from L-tryptophan and cortisone induced rat liver have been purified to homogeneity (3,4). Both proteins are allosteric enzymes composed of tetramers of subunits of equal size, having a total molecular weight of 122,000 for the bacterial enzyme and 167,000 for the hepatic enzyme. Before tryptophan oxygenase was purified to homogeneity it was established as a heme protein, based on inhibitor and spectral studies (5) and the demonstration of a restoration of catalytic activity to hepatic apo-tryptophan oxygenase by the addition of ferri protoporphyrin IX (6–8). Both the bacterial and hepatic enzymes contain two g-atoms copper per mole enzyme, as functioning cofactors. The possible presence of copper in these enzymes has been a source of dispute amongst several laboratories for many years. In this paper I will present historically the evidence for copper being a cofactor of tryptophan oxygenase and will attempt to rebut the evidence for copper not being a cofactor.
KeywordsReductive Activation Atomic Absorption Spectroscopy Homogeneous Preparation Atomic Absorption Spectro Allosteric Enzyme
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