On the role of lipid peroxidation and protein-bound aldehydes in the haloalkane-induced inactivation of microsomal glucose 6 phosphatase

The inactivation of liver microsomal glucose 6 phosphatase induced either by Fe2+ or by haloalkanes (CCI4, CBrCl3) was investigated in NADPH-microsomes systems. In the case of haloalkanes, EDTA was included in the incubation mixtures, so to exclude participation of free Fe2+ in the ensuing lipid peroxidation. Microsomal glucose 6 phosphatase activity was measured along with the release of malonic dialdehyde and the appearance of carbonyl products bound to microsomal protein, taken as indices of the peroxidative process. Fe2+ was added to NADPH-microsomes at different concentrations, one (6 μM) resulting in an extent of lipid peroxidation comparable with that induced by haloalkanes, the other (60 μM) representing a situation of excess Fe2+, leading to massive lipid peroxidation. Inhibition of glucose 6 phosphatase caused by 6 μM Fe2+ was comparable to that induced by haloalkanes in EDTA-microsomes systems, which supports the view that lipid peroxidation - rather than covalent binding of free radical metabolites - represents the main event leading to the inactivation of glucose 6 phosphatase caused by haloalkanes. The production of 4-hydroxynonenal - the known toxic product of lipid peroxidation - was also studied. A remarkable accumulation of 4-hydroxynonenal was observed in the microsomal membranes after peroxidation induced by 6 μM Fe2+ or haloalkanes, as compared to the incubation medium. In addition, experiments carried out with CCl4 and CBrCl3 in vivo suggested the possible existence of a cytosolic detoxification system able to remove lipid-derived carbonyls bound to microsomal protein.