Molecular Pharmacology, Vol 17, 408-414, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Microsomal Lipid Peroxidation

II. Stimulation by Carbon Tetrachloride

¹ Department of Radiation Biology and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642

Carbon tetrachloride initiated lipid peroxidation in isolated rat liver microsomes in the absence of free metal ions. In contrast to the nonenzymatic process stimulated by ferrous iron, CCl₄-induced peroxidation showed an absolute requirement for NADPH and appeared dependent on the integrity of cytochrome P-450 as reflected by measurement of aminopyrine demethylase and aniline hydroxylase activities. The peroxidation subsided after only 10% of the total peroxidizable polyunsaturated fatty acids had reacted, at which point the cytochrome P-450 activities were 85-95% inhibited, whereas NADPH-cytochrome P-450 reductase activity was not significantly affected. Ferrous iron, in contrast, caused peroxidation of 100% of the available polyunsaturated fatty acids and produced decreases in cytochrome P-450 activities as well as a loss of susceptibility to CCl₄-induced peroxidation that approximately paralleled the extent of peroxidation. Thus, CCl₄-induced peroxidation appeared to be 10 times more potent in inhibiting cytochrome P-450 activities than the peroxidation caused by iron. Boiled liver microsomes or liposomes prepared from extracted liver lipid underwent extensive peroxidation in the presence of untreated microsomes when ferrous iron was the initiating species. In contrast, the CCl₄-induced peroxidative response was not affected by the addition of these exogenous forms of lipid substrate and thus the initiating species appeared to be confined to the active microsomes. No detectable peroxidation was induced by CCl₄ in microsomes from brain, kidney, or lung, and microsomal aminopyrine demethylase and aniline hydroxylase activities were more than 10-fold lower in these tissues compared to liver. These results are consistent with activation of CCl₄ by cytochrome P-450 to a reactive short-lived radical which initiates peroxidation in the immediate vicinity of the cytochrome and thereby inhibits enzyme activity either by destruction of essential lipids or by direct attack on the enzyme by reactive intermediates of the peroxidative process. Loss of cytochrome P-450 activity then results in cessation of the CCl₄-induced peroxidative response prior to more extensive reaction of membrane polyunsaturated lipids.