Evidence for Electron Transfer Reactions Involved in the Cu2+-dependent Thiol Activation of Fat Cell Glucose Utilization

Previous studies indicated that the ability of thiols to mimic insulin action on fat cell glucose oxidation and lipolysis was dependent on Cu2f.Evidence now presented indicates that HzOz formed by the reaction of thiols, Cu2+, and 02 mediates the effects of these agents on isolated fat cells.Oxidation of thiols to the respective disulfides in the presence of Cu2f accompanied the stimulatory effects on glucose metabolism.Diphenyl-1 , lo-phenanthroline, which binds tightly to Cuf but not Cu 2+, blocks copper-catalyzed thiol oxidation and the stimulatory effect on fat cell glucose utilization, indicating that reduction of Cu2+ is an obligatory step for thiol action.The thiol-Cu*+ effect on fat cells is readily reversible since stimulated rates of glucose oxidation returned to control levels following addition of the chelator.Catalase inhibited the stimulatory effect of thiols but not insulin on fat cell glucose oxidation, whereas HzOz mimicked the action of thiols and insulin on this process.As little as 10 pM HzOz stimulated glucose utilization, whereas 1 mM HzOz was maximally effective.Higher concentrations were less effective.The stimulatory effect of various mercaptoethanol and Cu2f concentrations on labeled CO2 production from n-lJ4Cglucose by fat cells paralleled the peroxide formed under these conditions.Other oxidants such as MnOp and diamide were also effective in enhancing fat cell glucose oxidation.The stimulatory effect of all three oxidants, as well as insulin, was inhibited by phlorizin and cytochalasin B, known glucose transport inhibitors.Fat cells enhanced the decomposition of added Hz02 or that generated by cysteine, Cu2+, and OZ.These data are consistent with the concept that an electron transfer reaction between a fat cell component or components and oxidants results in stimulated glucose transport or metabolism or both.