ANTIOXIDANT AND HEPATOPROTECTIVE EFFECTS OF THYMOQUINONE AGAINST CARBON TETRA- CHLORIDE-INDUCED HEPATOTOXICITY IN ISOLATED RAT HEPATOCYTES Osama S. El-Tawil * and Said Z. Moussa ** J. Egypt. Soc. Toxicol. (Vol. 34: 33-41 Jan. 2006) WWW.estoxicology.org * Toxicology and Forensic Medicine Department, Faculty of Veterinary Medicine, Cairo University, Cairo, Egypt. Email: osamaeltawil@yahoo.com ** Biochemistry Department, Faculty of Veterinary Medicine, Cairo University, Cairo, Egypt ABSTRACT Herbs are known to play a vital role in the management of various liver diseases. Thymoquinone (TQ) is the bioactive constituent of Nigella sativa seed. The present work was planned to evaluate the potential hepatoprotective effects of TQ against the cytotoxic effects and the oxidative stress induced by carbon tetrachloride (CCl 4 ) in isolated primary rat hepatocytes. Cytotoxicity was determined by assessing cell viability and leakage of cytosolic enzymes, such as lactate dehydrogenase (LDH), alanine transaminase (ALT) and aspartate transaminase (AST). Oxidative stress was assessed by determining reduced glutathione (GSH) level and lipid peroxidation as indicated by thiobarbituric acid reactive substances (TBARS) production. Exposure of isolated rat hepatocytes to CCl 4 (5mM) caused cytotoxicity and oxidative injury, manifested by loss of cell viability and significant increase in LDH, ALT and AST leakages. As well as, CCl 4 caused progressive depletion of intracellular GSH content and significant enhancement of TBARS accumulation. Preincubation of hepatocytes with either TQ (1mM) or silymarin (5 mM) which is a known hepatoprotective agent, ameliorated the hepatotoxicity and oxidative stress induced by CCl 4 , as indicated by significant improve in cell viability, significant decrease in LDH, ALT and AST leakages, significant prevent GSH depletion and significant decrease in TBARS formation as compared to CCl 4 alone-treated cells. The present results indicate that CCl 4 has a potential cytotoxic effect in rat heptocytes; and TQ can afford a significant protection against CCl 4 –induced hepatotoxicity. Keywords : Thymoquinone; Carbon Tetrachloride; Hepatotoxicity; Isolated Rat Hepatocytes. INTRODUCTION Liver diseases remain one of the serious health problems. The high prevalence of bilharziasis and viral hepatitis in Egypt resides behind the wide occurrence of liver disorders specially hepatic fibrosis and cirrhosis (el- Zayadi et al. , 1992). Liver injury caused by toxic chemicals and certain drugs has been recognized as a toxicological problem. Carbon tetrachloride (CCl 4 ), a clear, heavy, and nonflammable liquid is most widely used for experimental induction of hepatic cirrhosis (Bahcecioglu et al. , 1999). It is known to be hepatotoxic as well as nephrotoxic to humans (Abraham et al. , 1999). Furthermore, it has been identified as a probable human carcinogen based on evidence of tumors in animals (Thrall et al. , 2000). In years past, CCl 4 was widely used as a dry cleaning solvent until it was recognized as a potent hepatotoxin and carcinogen. Recently, it is primarily used as an organic solvent (Kovacic et al. , 2002). In the absence of reliable liver protective drugs in medical practices, herbs play a role in the management of various liver disorders. A number of plants have been documented to exhibit hepatoprotective properties (Handa et al. , 1986).The black seed ( Nigella sativa, Ranunculaceae family), also known as Black Caraway Seed and “the Blessed Seed”, is an annual herb that grows in countries bordering the Mediterranean Sea. Although black seed is not a significant component of the human diet, it is regarded in the Middle East as part of an overall holistic approach to health and is thus incorporated into diets and everyday lifestyles. Numerous studies have shown that the seeds and oil of this plant are characterized by a very low degree of toxicity (Ali and Blunden, 2003). TQ is the bioactive constituent of the volatile oil of black seed (54%) and was first extracted by El-Dakhakhany (1963). Previous studies suggested that TQ or Nigella sativa oil might have a variety of pharmacological actions such as