177 Molecular and Cellular Biochemistry 170: 177–185, 1997. © 1997 Kluwer Academic Publishers. Printed in the Netherlands. Antioxidant systems and anoxia tolerance in a freshwater turtle Trachemys scripta elegans W.G. Willmore and K.B. Storey Institute of Biochemistry and Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada Received 20 August 1996; accepted 12 November 1996 Abstract The effects of anoxic submergence (20 h at 5°C) and subsequent 24 h aerobic recovery on the antioxidant systems of six or- gans were examined in freshwater turtles, Trachemys scripta elegans. Both xanthine oxidase and xanthine dehydrogenase were detected in turtle tissues with xanthine oxidase composing 36–75% of the total activity. Turtle organs displayed high constitu- tive activities of catalase (CAT), superoxide dismutase (SOD), and alkyl hydroperoxide reductase (AHR). Measurements of lipid peroxidation damage products (conjugated dienes, lipid hydroperoxides, thiobarbituric acid reactive substances) showed minimal changes during anoxia or recovery suggesting that natural anoxic-aerobic transitions occur without the free radical damage that is seen during ischemia-reperfusion in mammals. Anoxia exposure led to selected decreases in enzyme activities in organs, consistent with a reduced potential for oxidative damage during anoxia: SOD decreased in liver by 30%, CAT de- creased in heart by 31%, CAT and total glutathione peroxidase (GPOX) decreased in kidney (by 68 and 41%), and CAT and SOD decreased in brain (by 80 and 15%). AHR, however, increased 2 and 3.5 fold during anoxia in heart and kidney respec- tively. Most anoxia-induced changes were reversed during aerobic recovery although brain enzyme activities remained sup- pressed. Some specific changes occurred during the recovery period: SOD increased from controls in heart by 45%, AHR increased to 200 and 168% of control values in red and white muscle respectively, and total GPOX decreased from controls in heart and white muscle by 75 and 77% respectively. The results show that biochemical adaptation for natural anoxia tolerance in turtles includes well-developed antioxidant defenses that minimize or prevent damage by reactive oxygen species during the reoxygenation of organs after anoxic submergence. (Mol Cell Biochem 170: 177–185, 1997) Key words: metabolic rate depression, oxidative stress, freshwater turtles, antioxidant enzymes, lipid peroxidation Abbreviations : AHR – alkyl hydroperoxide reductase; CAT – catalase; GPOX – glutathione peroxidase; ROS – reactive oxy- gen species; SOD – superoxide dismutase; XDH – xanthine dehydrogenase; XO – xanthine oxidase; TBARS – thiobarbituric acid reactive substances Introduction Since the advent of O 2 -based aerobic metabolism, living or- ganisms have required antioxidant defenses in order to com- bat the damaging effects of reactive oxygen species (ROS) [15]. ROS include the superoxide anion radical (O 2 – ), hydro- gen peroxide (H 2 O 2 ), and the hydroxyl radical (OH·). Of these, OH· is the most reactive and destructive. Many cellu- lar metabolic activities produce ROS including the enzymatic breakdown of small biological compounds [28], the micro- somal cytochromes P450 and b5, microsomal flavoprotein reductases and superoxide leakage from the electron trans- port chain [17, 29]. Other sources of O 2 – and H 2 O 2 can in- clude enzymes, both cytosolic and peroxisomal. Of these, xanthine oxidase is purported to be a key source of toxic O 2 – in acute reoxygenation injury of anoxic tissues [34, 39, 40]. ROS attack many biological macromolecules including mem- branes (causing peroxidation of lipids), cellular proteins and Address for offprints: K.B. Storey, Institute of Biochemistry and Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario,