Seasonal changes in antioxidant defence system of liver and gills of Salmo trutta caspius, Salmo trutta labrax and Salmo trutta macrostigma N. M. ARAS, A. BAYIR*, A. N. SIRKECIOGLU, M. BAYIR, E. AKSAKAL AND H. I. HALILOGLU Department of Fishery Science, Faculty of Agriculture, Atat¨ urk University, 25240, Erzurum, Turkey (Received 4 February 2008, Accepted 18 November 2008) Seasonal changes in antioxidant enzyme activities (superoxide dismutase, SOD, EC 1.15.1.1; catalase, CAT, EC 1.11.1.16; glutathione peroxidase, GPx, EC 1.11.1.9; glutathione reductase, GR, EC 1.6.4.2; glucose-6-phosphate dehydrogenase, G6PD, EC 1.1.1.49 and glutathione S- transferase, GST, EC 1.5.1.18) and lipid peroxidation (LPO) levels of livers and gills of female Caspian trout Salmo trutta caspius, Black Sea trout Salmo trutta labrax and mountain trout Salmo trutta macrostigma were investigated. SOD, CAT, GPx, G6PD and GST activities were higher in liver compared to gills of all sub-species; concomitantly, the GR activity was also higher in the livers of S. t. caspius and S. t. labrax, but the reverse was seen in S. t. macrostigma. LPO levels were higher in the gills compared to the liver of all sub-species. There was no general trend in the seasonal changes in the gill antioxidant enzyme (AE) activities or LPO levels. Higher AE activities, however, were found in the liver of each sub-species during autumn, and this coincided with an increase in the gonado-somatic index. # 2009 The Authors Journal compilation # 2009 The Fisheries Society of the British Isles Key words: antioxidant enzymes; gonado-somatic index; oxidative stress; reactive oxygen species; Salmo trutta sp.; seasonal variations. INTRODUCTION Free radicals and other reactive oxygen species (ROS) are generated by all aer- obic cells (Winston & Di Giulio, 1991), and if the cell is unable to neutralize ROS activity then acute damage to the most major macromolecules such as DNA, proteins and lipids occurs (Berger, 2005). To minimize the negative ef- fects of ROS, fishes like other vertebrates possess an antioxidant defence sys- tem (ADS), which utilizes enzymatic and non-enzymatic mechanism (Jos et al., 2005). Key enzymatic players in ADS are superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.16), glutathione peroxidase (GPx, EC 1.11.1.9), glutathione reductase (GR, EC 1.6.4.2), glucose-6-phosphate dehy- drogenase (G6PD, EC 1.1.1.49) and glutathione S-transferase (GST, EC 1.5.1.18) (Halliwell & Gutteridge, 1999; Bayir, 2005). *Author to whom correspondence should be addressed. Tel.: þ90 442 2311079; fax: þ90 442 2360958; email: abayir@atauni.edu.tr Journal of Fish Biology (2009) 74, 842–856 doi:10.1111/j.1095-8649.2008.02164.x, available online at http://www.blackwell-synergy.com 842 # 2009 The Authors Journal compilation # 2009 The Fisheries Society of the British Isles