Bromobenzene metabolites

Isolated proximal tubules have been utilized to study the mechanisms of nephrotoxicity induced by antibiotics (Sina et al., 1985, 1986), radiocontrast dyes (Humes et al., 1987), metals (Rylander et al., 1985), anoxia (Weinberg, 1985 Weinberg et al., 1987), cellular oxidants (Messana et al., 1988), cysteine conjugates (Rylander et al., 1985 Schnellman et al., 1987 Zhang and Stevens, 1989), and a variety of nephrotoxic bromobenzene metabolites (Schnellman and Mandel, 1986 Schnellman et al., 1987). 

For example, consider bromobenzene as a cause of liver necrosis. Identification of metabolites formed from bromobenzene and their relationship to observed necrosis has been investigated, and a reactive intermediate of bromobenzene was implicated. Pretreatment with inducers and inhibitors of bromobenzene metabolites were used experimentally. This is represented by a general relationship shown in Figure 1 ( 3). In the case of bromobenzene, the reactive intermediate is 3,4-bromobenzene oxide. These studies require the efforts of biochemists and toxicologists, and the interdisciplinary nature of the investigation is readily seen. The joint effort promotes understanding. 

Schnellman, R.G. and Mandel, L.J. (1986). Cellular toxicity of bromobenzene and bromo-benzene metabolites to rabbit proximal tubules The role and mechanism of 2-bromohy-droquinone. J. Pharmacol. Exp. Then 237 456 161. 

Toxicity can occur because, unfortunately, some metabolites are, unlike benzoic acid, more toxic than the chemical that enters the body. Enzymes can cause certain changes in molecular arrangements that introduce groupings of atoms that can interact with components of cells in highly damaging ways. The industrial chemical bromobenzene can be converted in the liver to a metabolite called bromobenzene epoxide, as depicted in the diagram. 

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. 

Such intermediates may also travel to adjacent cells. These intermediates can be detected as they are covalently bound to cellular constituents. Examples include the reactive metabolite produced from paracetamol (NAPQUI) and bromobenzene epoxide (see chap. 7). 

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...

However, administration of primary phenolic metabolites does not cause hepatotoxicity. At least seven GSH conjugates have been identified as metabolites of bromobenzene and its primary phenolic metabolites. 

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. 

In addition to being hepatotoxic, bromobenzene is also nephrotoxic because of the production of reactive polyphenolic GSH conjugates, covalent binding to protein, and the production of ROS. 2-Bromophenol and 2-bromohydroquinone are both nephrotoxic metabolites of bromobenzene. Quinones are both oxidants and electrophiles, undergoing both one and two electron reduction and reaction with sulfydryl groups such as GSH and... 

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. 

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... 

Versatile epoxyquinol intermediates were conveniently accessed via metabolites derived from the enzymatic dihydroxylation of bromobenzene [172]. The chemoen-zymatic approach led to shortened total syntheses of several epoxyquinols derived... 

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... 

Koen, Y. M., Gogichaeva, N. V., Alterman, M. A., Hanzlik, R. P. A proteomic analysis of bromobenzene reactive metabolite targets in rat liver cytosol in vivo. Chem. Res. Toxicol. 2007, 20, 511-519. 

Koen, Y.M. Hanzlik, R.P. Identification of Seven Proteins in the Endoplasmic Reticulum as Targets for Reactive Metabolites of Bromobenzene, Chem. Res. Toxicol. 15, 699-706 (2001). 

Primary events. As already mentioned many compounds are toxic following metabolism to reactive metabolites. These reactive metabolites may then initiate one or more primary events. For example, paracetamol and bromobenzene-induced liver damage results from metabolic activation, discussed in more detail in Chapter 7. In other cases metabolic activation is not necessary, and the parent compound or a stable metabolite initiates the primary event. For example, cyanide is cytotoxic as a result of inhibition of crucial enzymes and carbon monoxide deprives the cell of oxygen (see Chapter 7 for more details). 

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