Effects of aurothioglucose on iron-induced rat liver microsomal lipid peroxidation

Aurothioglucose (ATG), an inhibitor of selenium-dependent glutathione peroxidase activity, at a concentration of 100 μM, strongly increases lipid peroxidation of rat liver microsomes exposed to either ferrous ion (10 μM) or the combination of ferric ion (10 μM) and ascorbic acid (500 μM), in the presence of reduced glutathione (GSH, 800 μM). This effect was not achieved using heat-inactivated microsomes and was dependent on the presence of GSH. ATG did not affect the lag period associated with ascorbic acid/ferric ion-induced microsomal lipid peroxidation (previously attributed to an undefined GSH-dependent microsomal agent), but did increase the rate of peroxidation subsequent to the lag period. The potent GSH-dependent inhibition of microsomal lipid peroxidation by cytosol (10% of total volume) was completely reversed by ATG (100 μM). ATG similarly reversed an inhibition of phosphatidyl-choline hydroperoxide-dependent liposomal peroxidation that has been attributed to phospholipid hydroperoxide glutathione peroxidase (PHGPX), an enzyme distinct from the classical glutathione that cannot utilize intact phospholipids. ATG inhibited, in addition to the classifical selenium-dependent glutathione peroxidase, both cytosolic and microsomal (basal and N-ethyl maleimide-stimulated) glutathione S-transferase activities with greater than 80% inhibition achieved at 100 μM ATG. ATG, at concentrations up to 250 μM, did not inhibit PHGPX activity measured by the coupled-enzyme method in the presence of Triton X-100 (0.1%). These data demonstrate the potential of ATG to increase toxicity of lipid peroxidative stimuli by inhibition of microsomal and cytosolic defense mechanisms. Although ATG did not inhibit Triton-enhanced PHGPX activity, overall evidence points toward inhibition of this enzyme as the mechanism for ATG-augmented lipid peroxidation and supports the conclusion that PHGPX plays a major role in the cellular defense mechanism.