Protection against carbon tetrachloride hepatotoxicity by 5,10-dihydroindeno[1,2-b]indole, a potent inhibitor of lipid peroxidation.

The influence of 5,10-dihydroindeno[1,2-b]indole (indenoindole) on carbon tetrachloride (CCl4)-mediated hepatotoxicity and lipid peroxidation were examined. Indenoindole (25 mg/kg body weight) ameliorated the increase in liver enzymes appearing in the plasma 24 hr after CCl4 administration, with about a 63% reduction for alanine transaminase, 56% for ornithine transcarbamylase and 84% for alkaline phosphatase. Indenoindole also partially prevented, in a dose-dependent fashion, the decrease in hepatic cytochromes P-450, total tissue reducing equivalents and hepatic ascorbate levels resulting 4 hr after CCl4 administration. In a homogeneous chemical system consisting of purified soybean phospholipid substrate in chlorobenzene, azobisisobutyronitrile-initiated lipid peroxidation was inhibited by indeno-indole, with 50% inhibition occurring at about 17 microM. Inhibition by indenoindole of iron-ascorbate-initiated lipid peroxidation in aqueous buffer containing phospholipid vesicles was about tenfold more efficient, with 50% inhibition occurring at about 1.5 microM. Presumably, this was due to the increased concentration of indenoindole in the membrane of the phospholipid vesicle. The efficiency of inhibition of lipid peroxidation was in the order of indenoindole = butylated hydroxytoluene (BHT) greater than alpha-tocopherol much greater than indole greater than indene. These 50% inhibition values of lipid peroxidation for these compounds were similar in an assay system composed of NADPH-fortified mouse-liver microsomes initiated with CCl4. For indenoindole, the 50% inhibition value (1.3 microM) was more than two orders of magnitude less than the spectral binding constant for indenoindole to mouse-liver cytochrome P-450 (Kd = 236 microM), implying that the partial inhibition of metabolic activation of CCl4 was not responsible for the inhibition of lipid peroxidation observed with indenoindole in this system. It appears that indenoindole may trap reactive radicals and inhibit lipid peroxidation in vitro. Regardless of whether inhibition is at the level of scavenging CCl4 metabolite radicals, or lipid radicals in membranes, radical trapping provides a plausible mechanism by which this compound inhibited CCl4 hepatotoxicity.