Bromobenzene, metabolism

The epoxide molecule is very active and can bind chemically to certain liver cell molecules and cause damage and even death to the cell (Path A). But an alternative reaction path (Path B) can also operate. If the amount of bromo benzene that enters the cell is low enough, Path B (which actually creates several metabolites) dominates and little or no cell damage occurs because the metabolic products are relatively non-toxic and are readily excreted from the body. But as soon as the capacity of the cell to detoxify is overcome because of excessive concentrations of bromobenzene, the dangerous Path A begins to operate and cell damage ensues. 

The epoxide of bromobenzene is one such toxic intermediate, and this example is discussed in more detail in chapter 7. In the case of some carcinogenic poly cyclic hydrocarbons such as benzo[a]pyrene, however, it seems that the dihydrodiol products are in turn further metabolized to epoxide-diols, the ultimate carcinogens (see chap. 7, Figs. 7.2 and 7.3). 

Another example of a glutathione conjugate responsible for toxicity is the industrial chemical hexachlorobutadiene discussed in chapter 7. The diglutathione conjugate of bromobenzene is believed to be involved in the nephrotoxicity after further metabolic activation (chap. 7, Fig. 7.31). 

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. 

Bromobenzene is a toxic industrial solvent that causes centrilobular hepatic necrosis in experimental animals. It may also cause renal damage and bronchiolar necrosis. The study of the mechanism underlying the hepa to toxicity of bromobenzene has been of particular importance in leading to a greater understanding of the role of GSH and metabolic activation in toxicity. 

Figure 7.22 Metabolism of bromobenzene. The bromobenzene 2,3-oxide and 3,4-oxide may undergo chemical rearrangement to the 2- and 4-bromophenol, respectively. Bromobenzene 3,4-oxide may also be conjugated with glutathione, and in its absence react with tissue proteins. An alternative detoxication pathway is hydration to the 3,4-dihydrodiol via epoxide hydrolase.

Thus, bromobenzene hepato toxicity is probably the result of metabolic activation to a reactive metabolite, which covalently binds to protein and other macromolecules and other cellular molecules. It may also stimulate lipid peroxidation, and biochemical effects, such as the inhibition of SH-containing enzymes, may also play a part. 

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. 

The formation of 3-halophenols in the metabolism of chlorobenzene, bromobenzene, and fluorobenzene215 cannot be explained on the basis of arene oxides as intermediates. These metabolites may represent examples of a direct hydroxylation of the ring. Besides, the magnitude of the isotopic effects observed during the metabolic formation of such meta-substituted phenols... 

Figure 14.6 Metabolism of bromobenzene. (From P. E. Levi, A Textbook of Modem Toxicology, 2nd ed., Appleton and Lange, 1997.)...

Bromobenzene, displays a dose response for toxicity that illustrates the threshold for an observable toxic effect. The rate of bromobenzene metabolism is dependent on the level of specific cytochrome P450s (Figure 18.7). The concentration of GSH... 

No information was located regarding toxic interactions of 1,1-dichloroethane with other xenobiotics. Evidence exists to indicate that 1,1- dichloroethane is detoxified by glutathione (Colacci et al. 1985). Thus, it is likely that other substances that deplete glutathione stores such as other chlorinated hydrocarbons (e.g. 1,1-dichloroethane and 1,2-dichloroethane), acetaminophen, and bromobenzene may enhance the toxicity of 1,1- dichloroethane. Substances that alter the activity of the microsomal enzymes that are responsible for the metabolism of 1,1 -dichloroethane may also affect the toxicity of this chemical. For example, it has been shown that ethanol increases the metabolism of 1,1 -dichloroethane in vitro (Sato et al. 1980). 

A pattern of liver necrosis similar to that caused by bromobenzene is observed in patients who ingest massive doses of acetaminophen (Table 16.2). This toxic reaction also has been produced experimentally in mice and rats and is thought to occur in two phases. An initial metabolic phase in which acetaminophen is converted to a reactive iminoquinone metabolite is followed by an oxidation phase in which an abrupt increase in mitochondrial permeability, termed mitochondrial permeability transition (MPT), leads to the release of superoxide and the generation of oxidizing nitrogen and peroxide species that result in hepatocellular necrosis (13, 14). 

FIGURE 16.5 Metabolism of bromobenzene (1) to a chemically reactive epoxide (arene oxide) metabolite (2) that can then either bind covalently to nearby macro-molecules, be scavenged by glutathione (GSH) (4) and be further metabolized 7), or be converted nonenzymatically or by epoxide hydrolase to stable hydroxylated metabolites 5, 8). 

Aromatic chemicals are metabolized into unstable arene-oxides, which, as epoxides, are comparable to potentially equivalent electrophilic carbocations. These metabolites react easily with thiol groups derived from proteins, leading, for example, to hepatotoxicity. Bromobenzene seems to target a large group of functionally diverse hepatic proteins, as demonstrated recently in a proteomic analysis. The chemical is oxidized (Figure 33.10) into a 3,4-epoxide, which... 

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