Ferritin-dependent inactivation of microsomal glucose-6-phosphatase.

Glucose-6-phosphatase (G6Pase) is a microsomal enzyme which is very sensitive to inactivation by lipid peroxidation. Experiments were carried out to evaluate whether ferritin, which is the major storage form of iron within cells, could catalyze inactivation of G6Pase and to determine the mechanism responsible for this effect of ferritin. Incubation of microsomes with NADPH in the absence of ferritin led to decreased activity of G6Pase. Ferritin stimulated this inactivation of G6Pase in a time- and concentration-dependent manner. Ferritin did not stimulate G6Pase inactivation when NADH replaced NADPH as the microsomal reductant. Superoxide dismutase but not catalase or DMSO prevented the ferritin-stimulated inactivation of G6Pase suggesting a role for superoxide, but not H2O2 or hydroxyl radical, in the overall mechanism. Trolox, at concentrations which prevent lipid peroxidation, also prevented the ferritin-catalyzed inactivation of G6Pase. Inhibition of G6Pase by ferritin was further enhanced in the presence of ATP but was inhibited in the presence of EDTA or desferrioxamine; ferric-ATP stimulates, whereas ferric-EDTA inhibits microsomal lipid peroxidation. The redox cycling agent paraquat increased the ability of ferritin to inactivate G6Pase by a reaction prevented by superoxide dismutase, trolox, EDTA, and desferrioxamine, but not by catalase or DMSO. Ferritin stimulated microsomal light emission, a reaction reflecting lipid peroxidation, with time and concentration dependence, and sensitivity to scavengers (trolox, superoxide dismutase), iron chelators and paraquat, identical to the inactivation of G6Pase. These results indicate that one possible toxicological consequence of ferritin-catalyzed lipid peroxidation is inhibition of microsomal enzymes such as G6Pase.