Bromobenzene epoxides

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. 

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). 

To a mixture of 100 ml of THF and 0.10 mol of the epoxide (note 1) was added 0.5 g Of copper(I) bromide. A solution of phenylmagnesium bromide (prepared from 0.18 mol of bromobenzene, see Chapter II, Exp. 5) in 130 ml of THF was added drop-wise in 20 min at 20-30°C. After an additional 30 min the black reaction mixture was hydrolysed with a solution of 2 g of NaCN or KCN and 20 g of ammonium chloride in 150 ml of water. The aqueous layer was extracted three times with diethyl ether. The combined organic solutions were washed with water and dried over magnesium sulfate. The residue obtained after concentration of the solution in a water-pump vacuum was distilled through a short column, giving the allenic alcohol, b.p. 100°C/0.2 mmHg, n. 1.5705, in 75% yield. 

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. 

Glutathione conjugation Glutathione (GSH) GSH-S-transferase (cytosol, microsomes) Epoxides, arene oxides, nitro groups, hydroxylamines Acetaminophen, ethacrynic acid, bromobenzene... 

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). 

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.

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). 

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. 

On treatment with sodium bis (2-methoxyethoxy)aluminium hydride (Scheme 16), 122 gave the aldehyde 123, which was converted by standard synthetic operations, into the methyl-O-benzylester 124. Acid hydrolysis of 124 followed by allylic epoxidation of the diol furnished the p-oxirane 125. On heating 125 in water containing sodium benzoate a remarkable chain of chemical events took place that resulted in the generation of (+)-pancratistatin (94) in 2% overall yield starting from bromobenzene. 

Epoxidation/hydroxylation Aldrin, benzo(a)pyrene, aflatoxin, bromobenzene... 

Bromobenzene is a toxic industrial solvent that is known to produce centrilobular hepatic necrosis through the formation of reactive epoxides. Figure 14.6 summarizes... 

Slaughter DE, Hanzlik RP (1991) Identification of epoxide- and quinone-derived bromobenzene adducts to protein sulfur nucleophiles. Chem Res Toxicol 4 349-359... 

Epoxidation and hydroxylation A-Dealkylation O-Dealkylation -Dealkylation -Oxidation A-Oxidation P-Oxidation Desulfuration Dehalogenation Nitro reduction Azo reduction Cytochrome P450 (CYP) Aflatoxin, aldrin, benzo[a]pyrene, bromobenzene, naphthalene Ethylmorphine, atrazine, dimethylnitrocarbamate, dimethylaniline p-Nitroanisole, chlorfenvinphos, codeine Methylmercaptan Thiobenzamide, phorate, endosulfan, methiocarb, chlorpromazine 2-Acetylaminofluorene Diethylphenylphosphine Parathion, fonofos, carbon disulfide CCLt, CllCb Nitrobenzene O-Aminoazotoluene Flavin-Containing Monooxygenase (FMO)... 

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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