Assessment of the participation of sulfhydryl proteins in the glutathione peroxidase mimicry of diphenyl diselenide in the presence of thiol alkylating agent

Diphenyl diselenide (DPDS) is an organoselenium which has garnered profound interest due to its reported antioxidant viz-a-viz glutathione peroxidase (GPx) mimetic activity which is ultimately speculated to rely on oxidation of free thiols or protein thiols critical to the activities of some sulfhydryl proteins or enzymes. However, this hypothesis has not been fully established. This study was therefore carried out to investigate the antioxidant effect as well the possible involvement of proteins thiols in the in vitro GPx mimicry of DPDS. The effect of DPDS on Fe²⁺ (10 µM) and H₂O₂ (1mM)-induced lipid peroxidation and the possible involvement of thiols of cerebral sodium pump (Na⁺/K⁺-ATPase) and hepatic delta-aminolevulinic acid dehydratase (δ-ALAD) in the GPx mimicry of DPDS were evaluated in the cerebral and hepatic tissue homogenates of rat in the presence of thiol alkylating agent, iodoacetamide (IA, 2 mM). The results revealed that DPDS exerted marked (p < 0.05) inhibitory effect on Thiobarbituric acid reactive species (TBARS) production induced by Fe²⁺ and H₂O₂ in the rats hepatic and cerebral tissues and this effect was significant (p < 0.05) when compared with the control. However, IA profoundly (p < 0.05) counteracted the inhibitory action of DPDS on the TBARS production process. Furthermore, results also showed that DPDS inhibited the sulfhydryl enzymes, cerebral Na⁺/K⁺-ATPase and hepatic δ-ALAD as well as TBARS production in the same reaction system. Finally, we further established the involvement of thiols in the DPDS inhibition of sulfhydryl enzymes by assaying for pump and δ-ALAD activities in the presence of exogenous thiols [dithiothreitol (DTT) and glutathione (GSH)]. Interestingly DPDS did not inhibit the activity of these enzymes when pre-incubated or post-incubated with DTT or GSH indicating that DPDS switched from proteins/enzymes thiols to the oxidation of exogenous thiols. It is therefore apparent that the GPx mimicry/antioxidant action of DPDS is related to the loss of enzymes' activities. Consequently, we conclude that the in vitro GPx mimicry/ antioxidant mechanism of DPDS is largely dependent on the oxidation of essential thiols of sulfydryl proteins and that DPDS could be an effective therapeutic candidate for oxidative stress-mediated conditions in which endogenous GSH level is depleted.