Micromethods in single muscle fibers. 2. Determination of glutathione reductase and glutathione peroxidase

This paper extends the previous study for systems which control intracellular oxidative events in muscle and describes procedures suitable to assay glutathione peroxidase (GSHPx), glutathione reductase (GR), and total glutathione (GSH + GSSG) after fiber typing of individual muscle fibers. In human skeletal muscle, both GR and GSHPx activities were relatively low when compared to those of other tissue. No difference was found among fiber types (I, IIA, and IIB) with regard to GR activity, but in contrast GSHPx activity was significantly lower in type IIB fibers than in the other types. These results suggest that type IIB fibers may have a reduced ability to cope with hydroperoxides generated during oxidative stress, which, in turn, could lead to increased damage to membrane structures by lipid peroxidation or oxidation of sensitive intracellular thiol (-SH) enzymes by hydrogen peroxide. The K_m of skeletal muscle GR for GSSG was 27 μm and for NADPH was 22 μm. If one assumes ∼95% of total glutathione is present in the reduced state, then GSSG concentration would be of the order of 0.3 mmol/kg and under these conditions skeletal muscle GR would be efficient in all muscle fiber types.