Lipid peroxidation of rat-liver microsomes. Its effect on the microsomal membrane and some membrane-bound microsomal enzymes.

Abstract

Rat liver microsomes were peroxidized in vitro and chemical, physical and morphological changes in the membrane were related to the effects on certain membrane‐bound microsomal enzymes.With increasing peroxidation, microsomal phospholipids revealed decreasing concentrations of 20:4 and 22:6 fatty acids. At a high level of peroxidation, where alterations in permeability and ultrastructural changes such as breakage and deformation of microsomal vesicles were apparent, there was also a decrease in the concentration of 18:2 fatty acids.Glucose‐6‐phosphatase, cytochrome P‐450 and uridine‐5′‐diphosphate glucuronyltransferase (UDP glucuronyltransferase) (three microsomal enzymes known to be highly dependent on the integrity of the membrane for their function) responded differently to lipid peroxidation. Glucose‐6‐phosphatase showed an initial decrease in activity as well as in Km for glucose 6‐phosphate, followed by a restoration to control values at an intermediate level of lipid peroxidation. At a high level of peroxidation, both kinetic parameters were again markedly decreased. Cytochrome P‐450 concentration, on the other hand, revealed a continuous decrease with proceeding lipid peroxidation, which was only partly accounted for by a conversion to cytochrome P‐420. The loss of cytochrome P‐450 was paralleled by decreased rates of aminopyrine demethylation and 3,4‐benzpyrene hydroxylation. Glucuronyltransferase activity, finally, was activated at low, but again normalized at high, levels of lipid peroxidation.It thus seems that although electron‐microscopically observable changes in the microsomal membrane occurs only late in lipid peroxidation, chemical, physical and enzyme alterations are early phenomena during this process. The continuous loss of cytochrome P‐450 and the final decrease in glucose‐6‐phosphatase late during peroxidation probably reflect damage to membrane lipids of vital importance for the integrity and function of these enzymes whereas the activation of UDP glucuronyltransferase early during peroxidation most likely can be ascribed to increased membrane permeability.