Disturbances of the Ca++ transport enzyme system in membranes of the sarcoplasmic reticulum caused by hydroperoxides of phospholipids and of fatty acids

Oxygen free radicals induced release of lysosomal enzymes in vitro

The effect of oxygen free radicals, generated by xanthine and xanthine oxidase, was studied on the release of lysosomal hydrolase from rat liver lysosomes in vitro. A lysosomal enriched subcellular fraction was prepared, using differential centrifugation technique, from the homogenate of rat liver. The biochemical purity of the lysosomal fraction was established by using the markers of different cellular organelles. Oxygen free radicals were generated in vitro by the addition of xanthine and xanthine oxidase. The release of lysosomal hydrolase (beta-glucuronidase) from the lysosomal fraction was measured. There was a 3 to 4 fold increase in the release of beta-glucuronidase activity in the presence of xanthine and xanthine oxidase when compared to that in the absence of xanthine and xanthine oxidase. In the presence of superoxide dismutase (SOD), a scavenger of oxygen free radicals, the xanthine and xanthine oxidase system was unable to induce the release of beta-glucuronidase activity from the lysosomes. Sonication (2 bursts for 15 sec each) and Lubrol (2 mg/10 mg lysosomal protein) treatment, which are known to cause membrane disruption, also induced the release of beta-glucuronidase from lysosomal fraction. This release of beta-glucuronidase by sonication and lubrol treatment was not prevented by SOD. These data indicate that lysosomal disruption is a consequence of oxygen free radicals, generated by xanthine and xanthine oxidase.

Effect of high-dose of methylprednisolone on tourniquet ischaemia

High doses of corticosteroids have been found to have beneficial effects in various shock states. It has been well recognized that ischaemia is one of the important features in shock states. This prompted us to investigate the effect of high-dose methylprednisolone on tourniquet-induced ischaemia using mongrel dogs. After inflation of tourniquets to 600 mmHg on each thigh of the hind legs, one leg received an intravenous infusion of methyl-prednisolone, 3 mg.kg-1 dissolved in 20 ml of autologous blood. The other leg received the same amount of blood only, as a control. During two hours of tourniquet time and until 30 min after tourniquet deflation, venous blood was sampled five times from both hind legs for measurements of blood gas tensions (PvO2, PvCO2) and pH, lactic acid, creatinine kinase (CK), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH). During tourniquet ischaemia, PvO2 and pH dropped and PvCO2, lactic acid, CK, AST and LDH rose steadily and significantly in both groups of legs, indicating respiratory and metabolic acidosis, and muscle cell damage. However, those changes were significantly smaller in the methylprednisolone-treated legs. The beneficial effect of methylprednisolone could be attributed to its vasodilatory effect, cellular membrane stabilization and direct metabolic effect on skeletal muscle cells. Although the tourniquet-induced ischaemia in our study is slightly different from the clinical paradigm, the results suggest that high-dose methylprednisolone may provide a beneficial effect during tourniquet ischaemia.

Oxidative stress impairs the function of sarcoplasmic reticulum by oxidation of sulfhydryl groups in the Ca2+-ATPase

Sarcoplasmic reticulum (SR) microsomes were oxidized by exposure to peroxydisulfate, hydrogen peroxide, or iron/ascorbate or by extended storage. The decline in Ca2+-ATPase activity, Ca2+ transport, and the increase in Ca2+ permeability which occurred under these conditions did not appear to result from lipid oxidation because these functional changes were not correlated with the amount of thiobarbituric acid-reactive lipid. Consistent with this interpretation, lipid antioxidants did not prevent the decline in SR function. This suggests that inhibition was independent of lipid oxidation. Instead, oxidation directly inhibited the Ca2+-ATPase. The decline in enzyme activity may be due to oxidation of SH groups, as suggested by the ability of reducing agents to prevent inhibition, the decline in sulfhydryl content of oxidized SR, and the ability of sulfhydryl-binding agents to inhibit Ca2+-ATPase. Inhibition was not primarily due to crosslinking of the Ca2+-ATPase, because sodium dodecyl sulfate-polyacrylamide gels of normal and oxidized SR showed that the area of the Ca2+-ATPase band was not correlated with the Ca2+-ATPase activity. Inhibition of the Ca2+-ATPase by oxidative stress is relevant to models of cellular dysfunction in which toxicity is caused by a rise in intracellular calcium.

Characterization of free radical-mediated damage of canine cardiac sarcoplasmic reticulum

In vitro generation of free radicals by xanthine oxidase acting on xanthine as substrate depressed steady-state calcium uptake by canine cardiac sarcoplasmic reticulum vesicles. The effect of free radicals on the calcium-dependent ATPase activity of the SR vesicles was pH dependent. At pH 7.0, ATPase activity was decreased, but at pH 6.4 it was unchanged. Exposure to free radicals increased the passive permeability of the vesicles to calcium. This increase was inhibited by superoxide dismutase (SOD) at pH 7.0, and SOD plus mannitol at pH 6.4. The increased permeability per se was insufficient to explain the effects of free radicals on ATPase activity, since the calcium ionophore A23187 was unable to mimick these effects. Direct measurement of the number and turnover of the pump units indicated that the number of units was unchanged but turnover was decreased by free radicals at pH 7.0. The overall data suggest at least two mechanisms of free radical damage, one associated with an increase in passive permeability and another associated with an as yet undefined change in some specific steps of the ATPase reaction.