Carbon tetrachloride metabolic activation

The metabolism of carbon tetrachloride proceeds via cytochrome P-450-dependent dehalogenation (Sipes et al. 1977). The first step involves cleavage of one carbon-chlorine bond to yield Cl- and a trichloromethyl free radical, which is then oxidized to the unstable intermediate trichloromethanol, the precursor of phosgene. Hydrolytic dechlorination of phosgene yields C02 and HC1 (Shah et al. 1979). Although there are similarities in the metabolism of chloroform and carbon tetrachloride, metabolic activation of chloroform produces primarily phosgene, whereas the level of phosgene production from... 

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

Albano, E., Lott, K.A.K., Slater, T.F., Stier, A., Symons, M.C.R.and Tomasi, A. (1982). Spin trapping studies on the free radical products formed by metabolic activation of carbon tetrachloride in rat liver microsomal fractions, isolated hepato-cytes and in vivo in the rat. Biochem. J. 204, 593-603. 

A single dose oral LDso value of approximately 13,000 mg/kg was reported for mice, and 14 daily doses of 625 mg/kg were lethal for 6 of 20 exposed male mice (Hayes et al. 1986). In rats fed carbon tetrachloride in stock diets or protein-free diets, LDso values of 10,200 or 23,400 mg/kg, respectively were reported (McLean and McLean 1966). The authors attributed the difference in sensitivity in animals in this study to protein depletion which has reportedly afforded protection against carbon tetrachloride toxicity. This may result from protein depletion-induced reduction in cytochrome P-450 synthesis, with a consequent diminished metabolic activation of carbon tetrachloride to toxic metabolites. In other studies using rats, an LDso value of approximately 7,500 mg/kg was reported (Pound et al. 1973), while 17/20 animals were killed within 14 days of a single oral gavage exposure to 8,000 mg/kg (Thakore and Mehendale 1991). Doses as low as 400 mg/kg have resulted in the death of cats (Chandler and Chopra 1926). 

In animals, acute oral exposure to doses of 4,000 mg/kg has been observed to cause respiratory edema, atelectasis and hemorrhage (Gould and Smuckler 1971). This is accompanied by marked disruption of subcellular structure in most pulmonary cell types, including granular pneumocytes, capillary endothelial cells and Clara cells (Boyd et al. 1980 Gould and Smuckler 1971 Hollinger 1982). It has been shown that Clara cells were most severely injured because they are the most active in metabolic activation of carbon tetrachloride. Injury to capillary endothelial cells is dose-... 

Many other cases of carbon tetrachloride-induced hepatic and/or renal injury associated with ethanol ingestion have been described in the medical literature (Durden and Chipman 1967 Guild et al. 1958 Jennings 1955 Lamson et al. 1928 Markham 1967 Tracey and Sherlock 1968). These clinical reports establish that occasional or frequent ingestion of alcoholic beverages can increase the danger from relatively moderate carbon tetrachloride exposure. As ethanol is known to induce microsomal mixed-function oxidase activity in man (Rubin and Lieber 1968), the mechanism of potentiation may involve ethanol-induced enhancement of the metabolic activation of carbon tetrachloride. 

Haloalkanes. Certain haloalkanes and haloalkane-containing mixtures have been demonstrated to potentiate carbon tetrachloride hepatotoxicity. Pretreatment of rats with trichloroethylene (TCE) enhanced carbon tetrachloride-induced hepatotoxicity, and a mixture of nontoxic doses of TCE and carbon tetrachloride elicited moderate to severe liver injury (Pessayre et al. 1982). The researchers believed that the interaction was mediated by TCE itself rather than its metabolites. TCE can also potentiate hepatic damage produced by low (10 ppm) concentrations of carbon tetrachloride in ethanol pretreated rats (Ikatsu and Nakajima 1992). Acetone was a more potent potentiator of carbon tetrachloride hepatotoxicity than was TCE, and acetone pretreatment also enhanced the hepatotoxic response of rats to a TCE-carbon tetrachloride mixture (Charbonneau et al. 1986). The potentiating action of acetone may involve not only increased metabolic activation of TCE and/or carbon tetrachloride, but also possible alteration of the integrity of organelle membranes. Carbon tetrachloride-induced liver necrosis and lipid peroxidation in the rat has been reported to be potentiated by 1,2- dichloroethane in an interaction that does not involve depletion of reduced liver glutathione, and that is prevented by vitamin E (Aragno et al. 1992). 

Administration of hyperbaric oxygen following exposure to carbon tetrachloride improved survival from 31 to 96% in rats (Ellenhorn and Barceloux 1988). Hyperbaric oxygen has also been used in treating overdoses of carbon tetrachloride in humans and may correct regional tissue hypoxia and damage, as well as inhibit the P-450-dependent reductive dehalogentation of carbon tetrachloride to the metabolically active acute trichloromethyl radical in the liver. However, the effectiveness of this method has not been established in humans (Burkhart et al. 1991 Ellenhorn and Barceloux 1988). 

Boyd MR, Statham CN, Longo NS. 1980. The pulmonary Clara cell as a target for toxic chemicals requiring metabolic activation studies with carbon tetrachloride. J Pharmacol Exp Ther212 109-114. 

Brittebo EB, Brandt I. 1989. Metabolic activation of carbon tetrachloride by the cervico-vaginal eqithelium in rodents. Pharmacol Toxicol 65 336-342. 

Ingall A, Lott KAK, Slater TF. 1978. Metabolic activation of carbon tetrachloride to a free-radical product study using a spin trap. Biochem Soc Trans 6 962-964. 

Pronzato MA, Domenicotti C, Biasi F, et al. 1990. Inactivation of hepatocyte protein kinase C by carbon tetrachloride involvement of drug s metabolic activation and prooxidant effect. Biochem Biophys Res Commun 171 1353-1360. 

The converse is true of drugs requiring metabolic activation for toxicity. For example, paracetamol is less hepatotoxic to newborn than to adult mice, as less is metabolically activated in the neonate. This is due to the lower levels of cytochromes P-450 in neonatal liver (Fig. 5.30). Also involved in this is the hepatic level of glutathione, which is required for detoxication. Although levels of this tripeptide are reduced at birth, development is sufficiently in advance of cytochrome P-450 levels to ensure adequate detoxication (Fig. 5.30). The same effect has been observed with the hepatotoxin bromobenzene. (For further details of paracetamol and bromobenzene see chap. 7.) Similarly, carbon tetrachloride is not hepatotoxic in newborn rats as metabolic activation is required for this toxic effect, and the metabolic capability is low in the neonatal rat. 

The cytochrome P-450 destroyed may be a specific isoenzyme, as is the case with carbon tetrachloride and allylisopropylacetamide (Table 5.28). Indeed, with carbon tetrachloride the isoenzyme destroyed is the one, which is responsible for the metabolic activation (CYP1A2). With allylisopropylacetamide, it is the phenobarbital-inducible form of the enzyme, which is preferentially destroyed as can be seen from Table 5.25. It seems that it is the heme moiety, which is destroyed by the formation of covalent adducts between the reactive metabolite, such as the trichloromethyl radical formed from carbon tetrachloride (see chap. 7), and the porphyrin ring. 

The hepatocytes, or parenchymal cells, represent about 80% of the liver by volume and are the major source of metabolic activity. However, this metabolic activity varies depending on the location of the hepatocyte. Thus, zone 1 hepatocytes are more aerobic and therefore are particularly equipped for pathways such as the p-oxidation of fats, and they also have more GSH and GSH peroxidase. These hepatocytes also contain alcohol dehydrogenase and are able to metabolize allyl alcohol to the toxic metabolite acrolein, which causes necrosis in zone 1. Conversely, zone 3 hepatocytes have a higher level of cytochromes P-450 and NADPH cytochrome P-450 reductase, and lipid synthesis is higher in this area. This may explain why zone 3 is most often damaged, and lipid accumulation is a common response (see "Carbon Tetrachloride," for instance, chap. 7). 

---