Mitochondrial thioredoxin-glutathione reductase was purified from larval Taenia crassiceps (cysticerci). The preparation showed NADPH-dependent reductase activity with either thioredoxin or GSSG, and was able to perform thiol/disulfide exchange reactions. At specific activities were mU mg-1 and mU mg-1 with thioredoxin and GSSG, respectively. Apparent values were μM, μM and μM for thioredoxin, GSSG and NADPH, respectively. Thioredoxin from eukaryotic sources was accepted as substrate. The enzyme reduced H2O2 in a NADPH-dependent manner, although with low catalytic efficiency. In the presence of thioredoxin, mitochondrial TGR showed a thioredoxin peroxidase-like activity. All disulfide reductase activities were inhibited by auranofin, suggesting mTGR is dependent on selenocysteine. The reductase activity with GSSG showed a higher dependence on temperature as compared with the DTNB reductase activity. The variation of the GSSG- and DTNB reductase activities on pH was dependent on the disulfide substrate. Like the cytosolic isoform, mTGR showed a hysteretic kinetic behavior at moderate or high GSSG concentrations, but it was less sensitive to calcium. The enzyme was able to protect glutamine synthetase from oxidative inactivation, suggesting that mTGR is competent to contend with oxidative stress. 1. Introduction The mitochondrion is a cell organelle where much of the reactive oxygen species (ROS) are produced, mainly as collateral reactions of the respiratory complexes [1]. An excessive increase in the concentration of such species will result in severe oxidative stress [2]. However, cells have the ability to cope with the endogenously generated ROS through both enzymatic and nonenzymatic defense systems [3]. Additionally, there are other antioxidant protection systems with the ability to revert the damage produced by ROS. Such is the case of the reversible thiol-disulfide exchange reactions, which are involved in the maintenance of a proper redox environment in cells. Outstanding in this sense are the glutathione and thioredoxin systems, with a broad distribution in the living world and playing a variety of physiological functions [4, 5]. In both cases, the proper operation as antioxidants depends on the reduced state of the molecule. For the majority of the organisms, independent and specific NADPH-dependent disulfide reductases are present. Thus, while glutathione reductase (GR) is involved in the reduction of GSSG, thioredoxin reductase (TrxR) reduces oxidized thioredoxin. Both enzymes are homodimeric flavoproteins members of the disulfide
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