Vol 66, No. 4;Apr 2016 74 Jokull Journal Response of enzymatic antioxidant defense of Nile tilapia liver and white muscle to temperature Mohamed A M Hegazi*, Mona M Hegazi, and Sally Salaah Eldin Department of Zoology, Faculty of Science, Tanta University, Tanta, Egypt *Correspondence: E-mail m_a_m_hegazi@hotmail.com Tel.: +201224440506 Abstract The effect of acclimation to the temperatures 15, or 26°C for four weeks on the antioxidant enzymes of liver and white muscle of Nile tilapia juveniles (16±2.6 g) was studied. Liver is a central organ in xenobiotic metabolism and muscle tissues, which comprise about 60-65% of fish weight, and their enzyme activity generally corresponds the whole-animal metabolism (Hazel and Prosser, 1974, Physiol. Rev., 54, 620-677). The studied enzymes were antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), and glutathione reductase (GR), γ-glutamylcysteine synthetase (γ-GCS) and γ-Glutamyl transferase (γ-GT) related to glutathione synthesis in liver and white muscles. The activity of liver and white muscle enzymes increased significantly in fish acclimated to 26°C in comparison with that acclimated at 15°C. Keywords: antioxidant enzymes, glutathione related enzymes, γ-glutamylcysteine synthetase, γ-Glutamyl transferase. 1. Introduction Under normal circumstances, it has been indicated that 0.1% of the total oxygen consumed in the aerobic respiration is converted in the mitochondria into reactive oxygen species (ROS) (Fridovich, 2004). ROS also created by the microsomal systems of the endoplasmic reticulum and by a variety of enzymatic oxidase reactions (Abele, and Puntarulo, 2004). High concentrations of ROS can be important intermediaries of harm to cell structures, nucleic acids, lipids, and proteins (Valko et al., 2006). Excessive ROS (superoxide anion (O2 − ), hydrogen peroxide (H2O2), and hydroxyl radical, ( OH) productions are controlled by the cellular antioxidant defense systems. Like mammals, fish possesses well-developed antioxidant enzyme defense systems. However, the ectothermic fish posses a lower antioxidant defenses capacity than endothermic animals as birds and mammals, and the active fish species usually has a higher antioxidant enzymes activity in comparison with sluggish species (Wilhelm-Filho, 2007). The antioxidant enzymes are superoxide dismutase (SOD) which catalyzes the superoxide anion to molecular oxygen and hydrogen peroxide, catalase (CAT) which transforms hydrogen peroxide to molecular oxygen and water, glutathione peroxidase (GPx) which transforms hydrogen peroxide into water, glutathione S-transferase (GST) which detoxifies a variety of reactive intermediates and products of oxidative stress, and glutathione reductase (GR) which catalyzes the reduction of oxidized glutathione (GSSG) into reduced glutathione (GSH) (Halliwell, and Gutteridge, 2007). GSH is a substrate of GPx and GST and considered the first scavenger line of defense against ROS in cell. GSH is the most abundant intracellular non-protein thiol, and is present in the millimolar range in most cells (Storey, 1996). Most fish species are ectothermic, which means they do not and can not control their internal body warmth and have a body temperature that follows their environmental