Evaluation of a role for phosgene production in the hepatotoxic mechanism of action of carbon tetrachloride and bromotrichloromethane

During aerobic incubations of rat liver microsomes containing a NADPH generating system, 1.54% of added carbon tetrachloride ( 14CCl 4), and 3.05% of added Bromotrichloromethane (CBrCl 3) could be recovered as the 2-oxothiazolidene-4-carboxylic acid derivative of phosgene. Actual nanomolar quantities of phosgene formed were very small in comparison to input concentrations of phosgene necessary to depress microsomal cytochrome P-450 and glucose-6-phosphatase. Cysteine had no statistically significant effect on covalent binding of 14CCl 4 metabolites to either microsomal lipids or proteins. Furthermore, presence of cysteine had no protective effect against loss of cytochrome P-450, glucose-6-phosphatase, or the capacity of microsomes to sequester calcium ions, all of which losses occur in vitro as a result of the metabolism of either CCl 4 or CBrCl 3. The low level of phosgene production, the lack of any effect of cysteine on the degree of covalent binding of CCl 4 metabolites, the failure of cysteine to afford any protection against CCl 4- or CBrCl 3-dependent loss of microsomal enzyme activity, and the relative ineffectiveness of phosgene itself as a microsomal poison argue against the possibility that formation of phosgene plays a significant role in the liver injury resulting from CCl 4 or CBrCl 3 intoxication. Our experiments, however, do not rule out the possibility that some phosgene production and subsequent toxicological action may occur in a hydrophobic microenvironment of the endoplasmic reticulum.