Antioxidant enzymes in the digestive gland of the common mussel Mytilus edulis

Abstract

Antioxidant enzymes function to remove deleterious reactive oxygen species, including the superoxide anion radical and H2O2. Subcellular distributions and optimal and other properties of catalase (EC. 1.11.1.6), superoxide dismutase (SOD; EC. 1.15.1.1), selenium-dependent glutathione peroxidase (Se-GPX; EC. 1.11.1.9) and total glutathione peroxidase (GPX) activities were determined in the digestive gland of the common musselMytilus edulis L. by spectrophotometric and cytochemical/electron microscopic (catalase) techniques. Assay conditions for Se-GPX and total GPX activities were determined which optimized the difference between the non-enzymic and enzymic rates of reaction. General peroxidase activity (guaiacol as substrate) (EC. 1.11.1.7) was not detectable in any subcellular fraction. Catalase was largely, if not totally, peroxisomal, whereas SOD and GPX activities were mainly cytosolic. Distinct mitochondrial (Mn-SOD) and cytosolic (CuZn-SOD) SOD forms were indicated. Catalase properties were consistent with a catalase, rather than a catalase-peroxidase. The pH-dependence and temperature-dependence of GPX activity were different with H2O2 or CHP as substrate, and these and other observations indicate the existence of a distinct Se-GPX. Under saturating or optimal (GPX) assay conditions, the apparent Michaelis constantsKm (mM) were: catalase, 48 to 68 (substrate, H2O2); Se-GPX, 0.11 (H2O2) and 2.0 (glutathione); and total GPX, 2.2 (eumene hydroperoxide) and 1.2 (glutathione). Calculated catalase activity was 2 to 4 orders of magnitude greater than Se-GPX activity over an [H2O2] of 1 to 1000 μM. The results are discussed in relation to theoretical calculations of in vivo oxyradical production and phylogenetic differences in antioxidant enzyme activities.