Chloroform 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... 

Pohl, L.R., Martin, J.L., and George, J.W. 1980a. Mechanism of metabolic activation of chloroform by rat liver microsomes. Biochem. Pharmacol. 29 3271—3276. 

The metabolism of chloroform is well understood. Approximately 50% of an oral dose of 0.5 grams of chloroform was metabolized to carbon dioxide in humans (Fry et al. 1972). Metabolism was dose-dependent, decreasing with higher exposure. A first-pass effect was observed after oral exposure (Chiou 1975). Approximately 38% of the dose was converted in the liver, and < 17% was exhaled unchanged from the lungs before reaching the systemic circulation. On the basis of pharmacokinetic results obtained in rats and mice exposed to chloroform by inhalation, and of enzymatic studies in human tissues in vitro, in vivo metabolic rate constants (V, 3,C =15.7 mg/hour/kg, = 0.448 mg/L) were defined for humans (Corley et al. 1990). The metabolic activation of chloroform to its toxic intermediate, phosgene, was slower in humans than in rodents. 

Letteron P, Degott C, Labbe G, et al. 1987. Methoxsalen decreases the metabolic activation and prevents the hepatotoxicity and nephrotoxicity of chloroform in mice. Toxicol Appl Pharmacol 91 266-273. 

Metabolic activation. Although the kidney does not contain as much cytochromes P-450 as the liver, there is sufficient activity to be responsible for metabolic activation, and other oxidative enzymes such as those of the prostaglandin synthetase system are also present. Such metabolic activation may underlie the renal toxicity of chloroform and paracetamol (see chap. 7). Other enzymes such as C-S lyase and GSH transferase may also be involved in the activation of compounds such as hexachlorobutadiene (see chap. 7). In some cases, hepatic metabolism may be involved followed by transport to the kidney and subsequent toxicity. 

The mechanism is believed to involve metabolic activation in the kidney itself. Thus, when radiolabeled chloroform was given to mice, in the kidney, the radiolabel was localized in... 

Chloroform is an anesthetic and solvent, which may be nephrotoxic and hepato toxic. It requires metabolic activation by cytochrome P-450, and male mice are more susceptible to the nephrotoxicity than females, which are more likely to suffer hepatic damage. The renal damage, proximal tubular necrosis, is accompanied by fatty infiltration. The metabolic activation, which may take place in the kidney, produces phosgene, which is reactive and can bind to critical proteins. 

Chloroform causes kidney damage (proximal tubular necrosis) and liver damage (hepatic necrosis). The mechanism is believed to involve metabolic activation of the chloroform in the kidney to produce phosgene, which is probably responsible for the toxicity. Both liver and kidney damage may be modulated by treating animals with enzyme inducers and therefore it seems likely that the liver damage is also mediated by a reactive metabolite. 

Figure 8.3 Metabolism of tetrachloromethane. Upon metabolic activation a CCI3 radical is formed. This radical extracts protons from unsaturated fatty acids to form a free fatty-acid radical. This leads to diene conjugates. At the same time, O2 forms a hydroperoxide with the C radical. Upon its decomposition, malondialdehyde and other disintegration products are formed. In contrast, the CC13 radical is converted to chloroform, which undergoes further oxidative metabolism. (Reprinted from H. M. Bolt and J. T. Borlak, in Toxicology, pp. 645-657, copyright 1999, with permission from Elsevier.)...

Kidneys have relatively low xenobiotic-metabolizing enzyme activities, and chemically induced nephrotoxicity has been assumed to be produced by toxic intermediates generated in the liver and transported to the kidney. If a single hepatic metabolite of chloroform produced both kidney and liver injury, species, strain, and sex differences in susceptibility to chloroform nephro- and hepatotoxicity should be similar. However, species, strain and sex differences in susceptibility to chloroform nephrotoxicity are not consistent with those of chloroform hepatotoxicity. In addition, several modulators of tissue xenobiotic-metabolizing activities alter... 

The mechanism is believed to involve metabolic activation in the kidney itself. Thus, when radiolabelled chloroform was given to mice, in the kidney the radiolabel was localized in the tubular cells which were necrotic. Certain microsomal enzyme inducers such as 3-methylcholanthrene decreased the nephrotoxicity but not hepatotoxicity of chloroform, and phenobarbital pretreatment had no effect on nephrotoxicity but increased hepatotoxicity. Pretreatment with polybrominated biphenyls, however, increased toxicity to both target organs and also increased mixed function oxidase activity in both. In vitro studies have shown that microsomal enzymemediated metabolism of chloroform to C02 occurs in... 

In the Bacillus subtilis rec assay with strains H17 rec and M45 rec, no mutagenic activity of an ethanol extract of cinnamon was observed. Petroleum ether and chloroform extracts exhibited mutagenic activity that was not seen after metabolic activation by S9 mix (Ungsurungsie et al. 1984). 

No mutagenic effects of ethyl acetate, methanol, or aqueous extracts of saffron were observed in the Salmonella/micro-some assay using S. typhimurium strains TA98 and TAIOO with or without metabolic activation (Yamamoto et al. 1982). No mutagenic effects of a 2 1 chloroform-methanol extract of saffron were observed in pig kidney cells or in trophoblastic placenta cells treated at concentrations of 100 mg/plate (Rockwell and Raw 1979). 

No mutagenic activity of saline, aqueous, or chloroform extracts of the aerial parts of stinging nettle was observed in Salmonella typhimurium strains TA98 or TAIOO with or without metabolic activation (Basaran et al. 1996). The same... 

The mechanism proposed for the metabolic activation of chloroform is outlined in Fig. 21. On the basis of in vitro studies, chloroform was found to be oxidized by microsomal enzymes (cytochrome P-450 monooxygenase) to produce trichloromethanol (377). This intermediate can spontaneously eliminate HCl to yield phosgene, which is capable of reacting with cellular mac-... 

Fig. 21. Proposed metabolic activation of chloroform and carbon tetrachloride.

A homolytic cleavage of the CCI3—Cl bond has been proposed as a mechanism for the metabolic activation of carbon tetrachloride (Fig. 21). While the formation of methylene chloride via a CHCI2 radical has not been observed during the metabolism of chloroform, hexachloroethane (CCI3CCI3) has been detected in the tissues of rabbits exposed to carbon tetrachloride 181). 

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