Bioactivation of halogenated hydrocarbons

In vivo protection by protein A of hepatic microsomal mixed function oxidase system of CCl4-administered rats

The in vivo protection by protein A of hepatic mixed function oxidase system of carbon tetrachloride (CCl4) administered rats, has been investigated in the present communication. Aryl hydrocarbon hydroxylase activity was decreased by 63% in CCl4 administered rats while in protein A + CCl4 administered rats the decrease was in the range of 22-25% (group IV-V). The aryl hydrocarbon hydroxylase activity in protein A + CCl4 administered rats showed significant increase in group IV (P less than 0.005) and group V (P less than 0.001) in comparison to CCl4 alone (group II). Similarly, aniline hydroxylase and aminopyrene N-demethylase were decreased, by 75 and 84% respectively in CCl4 administered rats and 31% and 54-64%, respectively in protein A + CCl4 administered rats (groups IV and V). The aniline hydroxylase activity was also found enhanced in protein A + CCl4 administered group IV and V (P less than 0.001). In addition the aminopyrene N-demethylase also showed significant increase in its activity in group IV (P less than 0.001) and group V (P less than 0.001) in comparison to CCl4 alone. In accordance with these data, serum glutamic oxaloacetic transaminase and glutamic pyruvic transaminase exhibited significantly less increase in their activity in animals receiving protein A and CCl4 than those treated with CCl4 alone. Protein A alone was found to have no effect on any of these enzymes. Our results indicate that protein A protects CCl4 induced injury as judged by the biochemical alterations and suggests that it may be useful in providing an excellent system for the investigation on the regeneration of the hepatic enzyme activity following toxic insult of CCl4.

Reductive metabolism of 1,1,1,2-tetrachloroethane and related chloroethanes by rat liver microsomes

The susceptibility of polychlorinated ethanes to reductive metabolism was evaluated by measuring the amount of each compound consumed during anaerobic incubations with rat live microsomes; 1,1,1,2-tetrachloroethane, pentachloroethane and hexachloroethane were metabolized extensively, 1,1,1,2-tetrachloroethane and the trichloroethanes were metabolized very slowly and the dichloroethanes were not metabolized at a detectable rate. The electron affinity of the chloroethanes was determined by measuring electrochemical half-wave reduction potentials. Chloroethanes with an E1/2 of - 1.35 V or less negative were reduced readily in microsomes while those with an E1/2 equal to or more negative than -1.90 V were not good substrates for enzymatic reduction. Metabolites produced from 1,1,1,2-tetrachloroethane in vitro were 1,1-dichloroethylene (DCE) and 1,1,2-trichloroethane (TCEA) and the ratio DCE/TCEA was about 25:1. These conversions were NADPH-dependent and were inhibited by air, CO and metyrapone. In the presence of SKF 525-A, DCE formation was inhibited by 47%. Microsomes from untreated or beta-naphthoflavone-treated rats were 70-90% less active than microsomes from phenobarbital-treated rats. The Km was 0.50 mM and the Vmax was 66 nmol min-1 mg-1 protein for DCE formation. The results are consistent with the proposal that 1,1,1,2-tetrachloroethane is reduced by hepatic cytochrome(s) P-450 to a free radical intermediate which, for the most part, remains closely associated with the enzyme, is reduced further and undergoes beta-elimination of a chloride ion to form DCE. The occurrence of this reductive pathway in vivo was demonstrated by the quantitation of DCE and TCEA in blood from rats treated with 1,1,1,2-tetrachloroethane.

Isopropanol enhancement of cytochrome P-450-dependent monooxygenase activities and its effects on carbon tetrachloride intoxication

Acute or chronic treatment of rats with isopropanol caused a significant increase in hepatic cytochrome P-450 content and a two- to threefold increase in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but no significant change in ethylmorphine N-demethylase or benzo(a)pyrene hydroxylase activity. In rats treated with isopropanol and challenged with CCl4, liver toxicity of CCl4 was characteristically potentiated, as assessed by elevation of serum glutamic-pyruvic transaminase (SGPT) levels. Isopropanol pretreatment also potentiated CCl4-induced damage to the hepatic monooxygenase system. In addition to a decrease in cytochrome P-450, rats treated with isopropanol and challenged with CCl4 showed a nonspecific decrease not only in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but also in ethylmorphine N-demethylase, benzo(a)pyrene hydroxylase, and NADPH-cytochrome c reductase activities. These results were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized microsomes. The electrophoretic results showed that isopropanol pretreatment markedly potentiated the CCl4-caused destruction of cytochrome P-450 hemeproteins. The data strongly suggest that isopropanol increases one or more forms of cytochrome P-450 which selectively enhance the metabolism of CCl4 to an active metabolite. This active metabolite then causes a nonselective damage to the microsomal mixed-function oxidase system.