3-methylcholanthrene metabolites

Cerniglia CE, RH Dodge, DT Gibson (1982d) Fungal oxidation of 3-methylcholanthrene formation of proximate carcinogenic metabolites of 3-methylcholanthrene. Chem-Biol Interactions 38 161-173. 

Table I. Optical Purity of the Dihydrodiol Metabolites Formed in the Metabolism of Benzo[a]pyrene by Liver Microsomes from Untreated, Phenobarbital (PB)-, 3-Methylcholanthrene (3MC)-, and Polychlorinated Biphenyls (PCBs, Aroclor 1254)-Treated Rats...

It was recently reported that. >97% of BaP 4,5-epoxide metabolically formed from the metabolism of BaP in a reconstituted enzyme system containing purified cytochrome P-450c (P-448) is the 4S,5R enantiomer (24). The epoxide was determined by formation, separation and quantification of the diastereomeric trans-addition products of glutathione. Recently a BaP 4,5-epoxide was isolated from a metabolite mixture obtained from the metabolism of BaP by liver microsomes from 3-methylcholanthrene-treated Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene oxide, and was found to contain a 4S,5R/4R,5S enantiomer ratio of 94 6 (Chiu et. al., unpublished results). However, the content of the 4S,5R enantiomer was <60% when liver microsomes from untreated and phenobarbital-treated rats were used as the enzyme sources. Because BaP 4R,5S-epoxide is also hydrated predominantly to 4R,5R-dihydro-... 

Methods for the synthesis of the biologically active dihydrodiol and diol epoxide metabolites of both carcinogenic and noncarcinogenic polycyclic aromatic hydrocarbons are reviewed. Four general synthetic routes to the trans-dihydrodiol precursors of the bay region anti and syn diol epoxide derivatives have been developed. Syntheses of the oxidized metabolites of the following hydrocarbons via these methods are described benzo(a)pyrene, benz(a)anthracene, benzo-(e)pyrene, dibenz(a,h)anthracene, triphenylene, phen-anthrene, anthracene, chrysene, benzo(c)phenanthrene, dibenzo(a,i)pyrene, dibenzo(a,h)pyrene, 7-methyl-benz(a)anthracene, 7,12-dimethylbenz(a)anthracene, 3-methylcholanthrene, 5-methylchrysene, fluoranthene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)-fluoranthene, and dibenzo(a,e)fluoranthene. 

Fluoranthenes. With the exception of 3-methylcholanthrene, much less work has been undertaken on nonalternant PAHs. Several recent studies have reported on the major metabolites and mutagenicity of various fluoranthenes (181-185), but little is known about the DNA adduct which they form. Some studies on dibenzo[a,e]fluoranthene showed that several adducts are formed by microsomal incubations (185) and additional studies will be required to provide complete structural elucidation of the products formed. 

Recently, the mechanism of 6-nitro-BaP ring hydroxylation has been elucidated by using 3-deutero-6-nitro-BaP (144). When incubated with 3-methylcholanthrene-induced rat liver microsomes, this deuterated analogue yielded the same metabolite profile previously observed with 6-nitro-BaP. Spectroscopic analysis of 3-hydroxy-6-nitro-BaP and 6-nitro-BaP-3,9-hydroquinone indicated that 30% of the deuterium label had migrated to carbon 2, presumably via an NIH shift. Therefore, it appears that 6-nitro-BaP-2,3-oxide is a common intermediate for these two metabolites. 

As well as detoxication via reaction with GSH, the reactive 3,4-epoxide can be removed by hydration to form the dihydrodiol, a reaction that is catalyzed by epoxide hydrolase (also known as epoxide hydratase). This enzyme is induced by pretreatment of animals with the polycyclic hydrocarbon 3-methylcholanthrene, as can be seen from the increased excretion of 4-bromophenyldihydrodiol (Table 7.5). This induction of a detoxication pathway offers a partial explanation for the decreased hepatotoxicity of bromobenzene observed in such animals. A further explanation, also apparent from the urinary metabolites, is the induction of the form of cytochrome P-450 that catalyzes the formation of the 2,3-epoxide. This potentially reactive metabolite readily rearranges to 2-bromophenol, and hence there is increased excretion of 2-bromophenol in these pretreated animals (Table 7.5). 

Table 7.5 Effect of 3-Methylcholanthrene (3-MC) Pretreatment on the Urinary Metabolites of Bromobenzene in Rats...

Liver necrosis will occur in experimental animals treated with 3-methylcholanthrene. It seems likely that the metabolic activation takes place in situ rather than the reactive metabolite being transported from the liver. 

Bromobenzene is toxic to the liver. It produces two reactive metabolites. Which one is thought to be responsible for the hepa to toxicity and why Are there any routes of detoxication, and if so, what are they What effect would treating with the enzyme inducer 3-methylcholanthrene have ... 

The metabolite of bromobenzene that is believed to be responsible for the hepatic necrosis is bromobenzene 3,4-oxide. This reacts with liver cell protein, which causes cell death. The reactive metabolite can be detoxified by conjugation with glutathione or be detoxified by metabolism to a dihydrodiol by epoxide hydrolase. Pretreatment of animals with the enzyme inducer 3-methylcholanthrene decreases the toxicity. This is because it increases metabolism to the 2,3-oxide. This reactive metabolite is not as toxic as the 3,4-bromobenzene oxide readily undergoing rearrangement to 2-bromophenol. 3-Methylcholanthrene also induces epoxide hydrolase and so increases detoxication. 

These reactive metabolites can bind covalently to cellular macromolecules such as nucleic acids, proteins, cofactors, lipids, and polysaccharides, thereby changing their biologic properties. The liver is particularly vulnerable to toxicity produced by reactive metabolites because it is the major site of xenobiotic metabolism. Most activation reactions are catalyzed by the cytochrome P450 enzymes, and agents that induce these enzymes, such as phenobarbital and 3-methylcholanthrene, often increase toxicity. Conversely, inhibitors of cytochrome P450, such as SKF-525A and piperonyl butoxide, frequently decrease toxicity. 

Figure 9.122 HPLC chromatogram of acetanilide and its metabolites generated by hepatic microsomes from mice induced with 3-methylcholanthrene (left) and control (right) mice. Approximately 0.33 mL fractions were collected. Arrow indicates formation of 2-OH acetanilide. (From Guenthner et al., 1979.)...

Rats were treated with vehicle (control), phenobarbital (PB), or 3-methylcholanthrene (3-MC). Cytochrome P450, lipid, and reductase fractions were prepared and reconstituted. The reductase and lipid fractions were prepared from PB-treated rats. No hydroxylation activity was detected when hemoprotein was omitted from the reaction mixture. In Experiment 1, benzo[a]pyrene metabolism was measured by formation of fluorescent phenolic metabolites, and benzphetamine metabolism was measured by the rate of benzphetamine-dependent NADPH oxidation. In Experiment 2, the metabolism of pentobarbital, benzo[a]pyrene, and chlorcyclizine was measured by product formation. Experiment 1 was taken from Ref. (53) and Experiment 2 was taken from Ref. (55). 

Figure 1 Transcriptional regulation of the rat GSTA2 and NQOl genes by bifunctional and mono functional inducers. The bifunctional inducers and the dioxin TCDD bind to and activate the AhR, which then translocates into the nucleus and associates with ARNT to activate transcription through the XRE. The bifunctional inducers can also activate transcription through the ARE via a separate pathway following their biotransformation into reactive metabolites that have characteristics of the monofunctional inducers. The monofunctional inducers can only act through the ARE-mediatedpathway. 3-MC, 3-methylcholanthrene B(a)P, benzo(a)pyrene TCDD, 2,3,7,8-tetrachlorodibenzo-/>-dioxin.

The carcinogenicity of dimethylnitrosamine and 4-nitrosomorpholine was reduced by deuterium substitution for hydrogen on carbon atoms alpha to the amino nitrogen.81 82 Consistent with the hypothesis that alpha-carbon oxidation is required for reactive metabolite formation from nitrosamines, there is a substantial primary deuterium isotope effect (ku/kg = 3.8) on the rate of dimethylnitrosamine N-demethylation. 3 Specific deuteration of 3-methylcholanthrene, a potent polycyclic hydrocarbon carcinogen, showed that oxidation of the 1-carbon atom is critical in the tumor-initiating process in mouse skin. ... 

TABLE 7.4 Effect of 3-methylcholanthrene (3-MC) pretreatment on the urinary metabolites of bromobenzene in rats... 

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