Hepatotoxicity Of Bromobenzene Research Articles

Decreased acute hepatotoxicity of carbon tetrachloride and bromobenzene by cholestyramine in the rat

Numerous factors are known to increase or decrease drug-induced liver injury. The aim of this study was to test the effect of cholestyramine. Cholestyramine, and anion exchange resin binding in the gut substances taken up and metabolized by the liver such as bile salts, vitamins, endotoxins, etc., could indirectly modify drug-induced toxicity. Two groups of animals were studied: cholestyramine-fed and pair-fed controls. Five days after feeding, carbon tetrachloride or corn oil was injected intraperitoneously. Liver function and histology were normal after corn oil injection in both groups. One day after carbon tetrachloride injection liver weight/body weight ratio was lower in the cholestyramine-fed than in the pair-fed group (4.0 ± 0.4 mean ± SD vs. 4.4 ± 0.3, p < 0.05). Alanine and aspartate aminotransferases were lower (618 ± 782 IU and 242 ± 147 IU vs. 8245 ± 8189 and 1966 ± 1524 IU, p < 0.001), as was necrosis (p < 0.05). Steatosis and inflammatory reaction were similar in both groups. Two and four days later there were no significant differences between the two groups, because necrosis was no longer a major feature in the pair-fed group. Similar experiments were performed with bromobenzene. Here too cholestyramine prevents necrosis but to a much lesser extent. These results confirm that steatosis and necrosis are independent toxic effects of carbon tetrachloride. Cholestyramine, a widely used drug in cholestasis, could provide a potentially clinically important hepatocellular resistance to toxicity from environmental agents.

Induction of hepatic heme oxygenase activity by bromobenzene

Hepatic heme oxygenase, an enzyme which converts heme to carbon monoxide and bile pigment in vitro, is inducible by heme but also by large “toxic” doses of such nonheme substances as hormones, endotoxin, and heavy metal ions. When we gave rats a single hepatotoxic dose of allyl alcohol, ethionine, acetaminophen, furosemide, or endotoxin, hepatic heme oxygenase activity rose modestly (two- to fivefold) after 20 h. In contrast, administration of bromobenzene (5 mmol/kg) induced heme oxygenase in the liver an average of 15-fold after 20 h but was without effect on the enzyme in the kidney or spleen. The change in heme oxygenase was accompanied by a loss in cytochrome P-450 concentration and, in rats labeled with 5-δ-amino[ 14C]levulinic acid, an increased rate of degradation of hepatic [ 14C]heme to 14CO. Induction of heme oxygenase by bromobenzene was blocked by cycloheximide, an inhibitor of protein synthesis, but not by actinomycin D, an inhibitor of RNA synthesis. This suggests that bromobenzene stimulates de novo enzyme synthesis at the step of translation. Subtoxic doses of bromobenzene (less than 1 mmol/kg) gave proportionately greater induction of heme oxygenase. Furthermore, induction of the enzyme remained unaffected when bromobenzene hepatotoxicity was blocked by pretreatment of rats with SKF-525A, 3-methylcholanthrene, or cysteine (which supplements liver sulfhydryl content), or when hepatotoxicity was enhanced by pretreatment with phenobarbital or with diethylmaleate (which depletes hepatic glutathione). These data suggest that with induction of heme oxygenase by bromobenzene, neither liver cell necrosis nor alteration in hepatic sulfhydryl metabolism is indispensible. The latter characteristic differs from induction of the enzyme by metal ions in which depletion of sulfhydryl-containing constituents has been thought to be essential. We conclude that bromobenzene is a novel inducer of heme oxygenase activity in the liver, differing from other nonheme substances in potency and specificity for the liver, and in utilizing mechanism(s) which require neither production of hepatotoxicity, depletion of hepatic glutathione, nor sensitivity to actinomycin D.

Effect of fasting on metabolite-mediated hepatotoxicity in the rat

Acetaminophen and bromobenzene are transformed in the liver into chemically reactive metabolites that may either bind to glutathione and be detoxified or bind to hepatic proteins and produce liver cell necrosis. Fasting for 42 hr (a) decreased hepatic glutathione concentration, (b) increased the amount of chemically reactive metabolite irreversibly bound to hepatic proteins after administration of 3H-acetaminophen or 14C-bromobenzene, and (c) increased the hepatotoxicity of acetaminophen or bromobenzene. In rats fasted for various lengths of time, there was an inverse relationship between the concentration of glutathione in the liver and the activity of serum glutamic pyruvic transaminases after administration of acetaminophen or bromobenzene. In vitro, there was an inverse relationship between the concentration of glutathione in the incubate and the amount of chemically reactive metabolite bound to microsomal proteins after incubation of 3H-acetaminophen or 14C-bromobenzene with hepatic microsomes. It is concluded that fasting may decrease the inactivation of chemically reactive metabolites by glutathione, increase their binding to hepatic proteins, and enhance the hepatotoxicity of drugs transformed into chemically reactive metabolites that are detoxified by binding to glutathione.

Effect of substituents on in vitro metabolism and covalent binding of substituted bromobenzenes

The relative hepatotoxicity of o-bromobenzonitrile, bromobenzene, o-bromotoluene, and o-bromobenzotrifluoride decreases in the order mentioned. In this study, the nature of the substrate-enzyme interaction was investigated by determining spectral binding constants and by following the disappearance of several substituted bromobenzenes in vitro in the presence of the hepatic monooxygenase system. The disappearance rates generally parallel substrate electronic character; all the compounds display spectral binding constants of similar magnitude, and differences are not obviously attributable either to substituent solubility or electronic properties. In addition, the extent to which metabolites of these materials become covalently bound to microsomal proteins following incubation in vitro was determined. Despite an apparently low rate of metabolism, the very toxic o-bromobenzonitrile was covalently bound to a greater extent than bromobenzene. Surprisingly, the less toxic o-bromotoluene was the most efficiently covalently bound substrate studied. Whereas the extent of covalent binding of bromobenzene is increased ninefold by pretreating the animals with phenobarbital, only a 2.4-fold enhancement of o-bromobenzonitrile binding was observed. Glutathione reduced the covalent binding observed for all substrates in concentrations as low as 0.01 m m (except o-bromobenzotrifluoride, which required 0.1 m m). The in vivo metabolism of o-bromotoluene was also studied, and it was determined that the major urinary metabolites are phenols and mercapturic acids, products of the arene oxide metabolic pathway. There does not emerge a simple correlation between macromolecular covalent binding and toxicity among these compounds.

Development and maturation of drug-metabolizing enzymes

One of the main factors responsible for the difference in reactivity to bio-active compounds between newborn and adult individuals is the low activity of the drug-metabolizing enzymes during the perinatal period. The rate of development of these enzymes varies among other things with species, sex, and, apparently, the xenobiotic substrate involved. Various regulatory mechanisms acting upon the development are reviewed. A comparative study on the hepatotoxicity of bromobenzene in neonatal and adult rats is included to illustrate that an immature oxidative capacity may be considered effective protection from toxic compounds acting through the formation of reactive epoxides.

Synergistic effects of phorone on the hepatotoxicity of bromobenzene and paracetamol in mice.

Administration of phorone (diisopropylidene acetone), an industrial solvent, to mice caused a rapid depletion of hepatic glutathione, which is due to enzymatic conjugation of phorone with glutathione, mediated by cytoplasmic enzymes of the liver. Whereas phorone, even in high doses, did not show hepatotoxic effects by itself, combined administration of phorone with a subtoxic dose of either paracetamol or bromobenzene strongly enhanced hepatotoxicity of the latter compounds as was judged from a rise in serum transaminase activities. These findings are compatible with the concept of a dose threshold for biologically reactive intermediate compounds which are bioinactivated through glutathione conjugation.