Free radicals can disturb the intracellular homeostasis by either modification of essential free sulfhydryl groups or by inducing lipid peroxidation. The damage provoked by oxidation of sulfhydryl groups might be reversible but the damage induced by the process of lipid peroxidation is probably not reversible. The main protective constituents of the cell are thiols and vitamin E. Thiols, especially glutathione, protect the cytosol while vitamin E protects the lipid membranes against free radicals. In the scavenging of free radicals in the lipid membrane, vitamin E becomes oxidized. However continuous recycling of vitamin E by a reductase, with the cytosolic thiol glutathione as cofactor, will keep the vitamin E levels high enough to protect against lipid peroxidation. In the recycling glutathione is oxidized. Dihydrolipoic acid cannot provide directly reducing equivalents for the recycling of vitamin E by the free radical reductase. However indirectly, via the reduction of oxidized glutathione, dihydrolipoic acid can mediate the regeneration of vitamin E. One of the secondary mechanisms that mediates free radical induced damage is the rise in intracellular free Ca'f-concentration caused by inactivation of the endoplasmic reticulum Caz++-ATPase. The Caz+-ATPase can be inactivated either by sulfhydryl alkylation or by lipid peroxidation. The authors used the thiol-alkylating agent N-ethylmaleimide, cystamine and ebselen. Dithiothreitol reversed the inhibition caused by all the three agents, while dihydrolipoic acid reversed the inhibition caused by ebselen. Glutathione was not able to reverse the effects of the sulfhydryl reactive agents. The reactivation of the microsomal CaZ+-ATPase by dihydrolipoic acid, may -besides the reduction of oxidized glutathionecontribute to the protective effect of dihydrolipoic acid on lipid peroxidation.
I n t r o d u c t i o n
Free radicals are formed in vivo even under normal conditions. Although cells are equipped with an elaborate defense system to protect themselves against the devastating effects of free radicals, they are pushed out of balance under conditions which are associated with an excessive radical production such as inflammation, aging, reperfusion injury', 2.
Free radicals can perturb the (intra-)cellular homeostasis in two ways namely (i) oxidation, alkylation or arylation of proteins, particularly of essential free sulfhydryl groups of enzymes, (ii) perturbation of biomembranes by inducing the process of lipid peroxidation.
Modification of essential free protein-sulfhydryl groups usually leads to diminished enzyme-activity, although some enzymes show increased activity after alkylation of sulfhydryl groups3-5. It is very difficult to separate events which occur through modification of free sulfhydryl groups from changes caused by lipid peroxidation which are associated with numerous (patho-)physiological conditions. On the one hand, products formed during lipid peroxidation, like aldehydes, are able to react with sulfhydryl groups. On the other hand, enzymes involved in the protection against lipid peroxidation contain essential sulfhydryl groups, and alkylation of these sulfhydryl groups may indirectly induce lipid peroxidation.