Bromobenzene: a Hepatotoxin

Liver damage can be induced by drugs or chemicals resulting from overdose or workplace exposure. Bromobenzene is an efficient model to perform studies on hepatotoxicity. After biotransformation into liver,it causes production of reactive oxygen species (ROS). Organs that have been found to be affected due to bromobenzene induced toxicity are liver, kidney and lungs. In the following review, mechanisms and various genes involved in the bromobenzene induced hepatotoxicity have been discussed. Also, the toxicokinetics, involving absorption, metabolism and elimination of bromobenzene from the body have been reviewed in this article. Various herbal extracts and chemicals have been found to exhibit protective effect against bromobenzene induced liver injury proving their potential in use as hepatoprotective agents. Article History:-----------------------Date of Submission: 22-04-2013 Date of Acceptance: 06-05-2013 Conflict of Interest: NIL Source of Support: NONE R e v ie w P a p e r *Corresponding author, Mailing address: Evan Prince Sabina Email: [email protected] Int. J. Drug Dev. & Res., April-June 2013, 5 (2): 66-73 Covered in Scopus & Embase, Elsevier 66 and bind to various macromolecules [3,4] thus resulting in DNA damage and ultimately cell death[5,6]. Modern medicines that are used for the prevention and cure of liver disorders have adverse side effects and are costlier. Therefore, phytocompounds which are relatively cheaper and easily available and have few or no side effects seem to be highly attractive. This creates an urge to evaluate the therapeutic potential of some natural products and for this chemically induced hepatotoxic models are being used. The chemicals used for inducing hepatotoxicity include Acetaminophen [7,8], Carbon tetrachloride[9,10], Galactosamine [11], Antituberculosis Drugs [12],Bromobenzene [13,14] ,etc. Recently it has been shown that bromobenzene induced hepatotoxicity represents a very good model for the study of lipid peroxidation, since in this experimental condition the level of detectable lipid peroxidation in the liver is by far greater than that of the others. Bromobenzene(Fig. 1) is an aryl halide and is used for organic synthesis, as an additive to motor oils, as a flame retardant, in manufacture of various drugs and chemicals and as a crystallizing solvent. Release of bromobenzene to the environment may occur during its production or its use, resulting in its exposure to living beings. Bromobenzene being a hydrophobic molecule is subjected to biotransformation in the liver, where it elicits toxicity and causes necrosis. The purpose of this review is to understand the mechanism of Bromobenzene hepatotoxicity and its metabolism in liver. Fig.1 Structure of Bromobenzene Chemical and Physical Properties of Bromobenzene Bromobenzene (BB) is a heavy, colourless liquid with a pungent odour [15]. Its Synonyms include monobromobenzene and phenyl bromide [16]. It has a molecular weight of 157.01 Da and chemical formula is C6H5Br. It has a high Boiling Point of 156.0°C and a very low melting point -30.6°C. It is very slightly soluble in water; its water solubility being 4.46 × 102 mg/L at 30°C and is miscible with chloroform, benzene, and petroleum hydrocarbons. The soil sorption constant or Koc of Bromobenzene is 150. Concentration of Bromobenzene more than or equal to 1 mg/m3 = 0.15 ppm, 1 ppm = 6.53 mg/m3 (17Verschueren, 2001) are considered to air pollution factors. Models of bromobenzene(BB) induced liver injury To study mechanisms of Bromobenzene toxicity, mouse or rats in vivo or their primary hepatocytes are most frequently used. The LD50 of BB was reported to be approximately 20 mmol/kg body weight for male Crl:Cd rats (Haskell Laboratory for Toxicology and Industrial Medicine, 1981, unreviewed). In most studies a dosage of 0.45 g/kg or 10mM or 460 mg/kg of body weight has been used. It can be administered alongwith 0.1m of corn oil or 0.1m of coconut oil. Route of administration for bromobenzene can be intraperitoneally [18], intragastric intubation[19,20,21] or oral gavage [23]. Usually animals are fasted for 24 h before and 19 h after Bromobenzene administration as fasting allows lower doses of Bromobenzene to be used and results in less variation of the injury but non-fasted animals can also be used [22]. Liver injury develops after 24 at a dosage of 2mmol/kg body weight, as pronounced centrilobular necrosis can be observed [22]. Metabolic Action of Bromobenzene During the metabolism of bromobenzene(Fig.2) in the liver, it gets converted into either the 3, 4-oxide R e v ie w P a p e r Mahima Vedi et al: Bromobenzene: A Hepatotoxin Int. J. Drug Dev. & Res., April-June 2013, 5 (2): 66-73 Covered in Scopus & Embase, Elsevier 67 derivative or the 2, 3-oxide derivative catalysed primarily by cytochrome isozymes (CYP 450 1A2, 1B1, etc). 3,4-oxide derived from bromobenzene binds with glutathione-S-transferase (GST), liver fatty acid binding protein (L-FABP), and carbonic anhydrase. [24]. According to Lau and Zannoni [25] BB 2, 3-oxide is more stable than other alternative forms and it covalently binds to haemoglobin. These oxides are among the most reactive BB-metabolites and thus their hydrolysis is important in the hepatic detoxification. Also, both the 3, 4-oxide and 2, 3oxide derivatives are converted to their corresponding dihydrodiols by epoxide hydrolase(EH) which is upregulated and increase in mEH messenger RNA (mRNA) levels by BB is observed thus proving its role; the hydrolysis of epoxide BB intermediates, enabling their excretion. The mEH is also induced by some other xenobiotics like phenobarbital, trans-stilbene oxide, and Aroclor 1254 [26]. Then bromophenols (2-, 3-, and 4bromophenol) from the oxide derivatives are formed and many different pathways are proposed for this [27,14,28].According to Lertratanangkoon et al.[27] 2, 3oxide derivative has a relatively short biological halflife so spontaneous rearrangement can be the primary pathway for conversion to 2-bromophenol. Also, it has been suggested that 2-bromophenol and 3-bromophenol may also be produced by nonenzymatic dehydration of the 2, 3-dihydrodiol [27,28]. The bromophenol metabolites can be subsequently converted to their respective bromocatechols (2-, 3-, or 4-bromocatechol) by Cytochrome isozymes and redox cycling of 2and 4-bromocatechol and conjugation by glutathione S-transferase produces 2bromo-3-(glutathione-S-yL) hydroquinone and 6glutathione-S-yL-4-bromocatechol, respectively [14]. At high doses of bromobenzene, the levels of glutathione (GSH) are reduced as a result of conjugation to the metabolites and thus the intracellular protection against reactive oxygen species (ROS) and hazardous xenobiotic metabolites is lost. This may lead to a number of secondary events that damage the cell, like lipid peroxidation[29], ATP depletion [30,31] mitochondrial dysfunction, energy imbalance, and altered intracellular calcium levels. Gopi and Setty[20] found that in mitochondria there is a block in transfer of electrons through the electron transport chain at complex I. Complex I is a major site for the production of ROS and hence can be expected to have a greater damage under heavy oxidative stress. Level of ATP synthase beta subunit was found to decrease in study conducted by Heijne et al. [32]. Heijne et al. [32,22,34] observed increased production of more than 20 liver proteins (including γglutamylcysteine synthetase, rate limiting enzyme thus important in glutathione biosynthesis) Also, genes typically involved in oxidative stress such as Heme Oxygenase-1, Peroxiredoxin 1, Metallothionein, Ferritin were found to be induced and changes in the transcriptional expression of numerous genes involved in drug metabolism (epoxide hydrolase, aldehyde dehydrogenase, etc), cellular response to reduced glutathione levels, the acute phase response (Metallothionein, Vitronectin, etc) and intracellular signalling, following intraperitoneal administration of Bromobenzene to rats using transcriptomics and proteomics approach was noted. Some other studies such as those conducted by Waters et al. [34], Minami et al. [35]; Stierum et al. [36] have utilized toxicogenomics to characterize the relationship between Bromobenzene hepatotoxicity and hepatic gene expression profile. R e v ie w P a p e r Mahima Vedi et al: Bromobenzene: A Hepatotoxin Int. J. Drug Dev. & Res., April-June 2013, 5 (2): 66-73 Covered in Scopus & Embase, Elsevier 68 Fig.2 Metabolism of Bromobenzene