The hepatotoxicity of CCl4 is mediated through its initial reduction by cytochrome P-450 to the CCl3· radical. This radical then damages important metabolic systems such as the ATP-dependent microsomal Ca2+ pump. Previous studies from our laboratory on isolated microsomes have shown that NADPH in the absence of toxic agents inhibits this pump. We have now found in in vitro incubations that CCi4 (0.5-2.5 mM) enhanced the NADPH-dependent inhibition of Ca2+ uptake from 28% without CCl4 to a maximum of 68%. These concentrations are in the range found in the livers and blood of lethally intoxicated animals (Dambrauskas, T., and Cornish, H.H. (1970) Toxicol. Appl. Pharmacol. 17, 83-97; Long, R.M., and Moore, L. (1988) Toxicol. Appl. Pharmacol. 92, 295-306) and are toxic to cultured hepatocytes (Long, R.M., and Moore, L. (1988) Toxicol. Appl. Pharmacol. 92, 295-306). The inhibition of Ca2+ uptake was due both to a decrease in the Ca2+-dependent ATPase and to an enhanced release of Ca2+ from the microsomes. The NADPH-dependent CCl4 inhibition was greater under N2 and was totally prevented by CO. GSH (1-10 mM) added during the incubation with CCl4 prevented the inhibition. This protection was also seen when the incubations were performed under nitrogen. When samples were preincubated with CCl4, the CCl4 metabolism was stopped, and then the Ca2+ uptake was determined; GSH reversed the CCl4 inhibition of CA2+ uptake. This reversal showed saturation kinetics for GSH with two K(m) values of 0.315 and 93 μM when both the preincubation and the Ca2+ uptake were performed under air, and 0.512 and 31 μM when both were performed under nitrogen. Cysteine did not prevent the NADPH-dependent CCl4 inhibition of Ca2+ uptake. CCl4 increased lipid peroxidation in air, but no lipid peroxidation was seen under nitrogen. Lipid peroxidation was only modestly reversed by GSH. GSH did not remove 14C bound to samples preincubated with the 14CCl4. Although EDTA (100 μM) decreased the CCl4 inhibition, the metal-complexing agents deferoxamine (100 μM) and diethyldithiocarbamate (100 μM) had no effect on the inhibition of the pump. Similarly, the reactive oxygen scavengers catalase (65 μg/ml), superoxide dismutase (15 μg/ml), mannitol (10 mM), and dimethyl sulfoxide (50 mM) also had no effect. Our results suggest that the initial toxicity of CCI4 for the Ca2+ pump results from the metabolism of CCl4 to the CCl3· radical. This radical then directly oxidizes the Ca2+ pump, leading to decreased Ca2+ uptake. Finally, GSH would appear to reduce protein disulfides enzymatically back to the sulfhydryls and to reactivate the pump.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - Jun 12 1990|