Molecular adaptation to anoxia and recovery from anoxia in the freshwater turtle Trachemys scripta elegans
The responses to stresses associated with oxygen extremes, namely anoxia and oxidative stress associated with recovery from anoxia, were studied in an organism that must tolerate both extremes. The model organism used was the turtle Trachemys scripta elegans that can survive complete anoxia for up to 18 weeks at lower temperatures. Metabolic adaptations that support survival of oxygen extremes in this organism that were examined included: (a) tissue-specific changes in the maximal activities of enzymes of intermediary metabolism, (b) an assessment of the metabolite and enzymatic antioxidant defenses utilized to combat the oxidative stress encountered upon recovery from anoxia, (c) adaptations in specific enzymatic antioxidant defenses and (d) adaptations to anoxia and recovery from anoxia of a specific protease. Of the key enzymes of intermediary metabolism, 14 instances of reduced and only 3 instances of increased enzyme activities were observed in tissues during anoxia as compared to controls. Brain had changes related to both metabolic rate depression and neurotransmitter release. Maximal activities which increased during anoxia included G6PDH in heart, MDH in white muscle and CPT in kidney. Anoxia stress and aerobic recovery produced relatively few changes in organ antioxidant enzyme activities and levels of lipid peroxidation products. Changes included tissue specific decreases in antioxidant enzymatic activities during anoxia, particularly SOD and CAT. The levels of enzymatic antioxidant activities were high in turtles in comparison with other vertebrates and non-vertebrates. Turtles maintained high levels of total glutathione in tissues in comparison to other ectotherms. Tissue-specific changes in the maximal activities of the glutathione-related enzymes occurred, the most dramatic being a decrease in gamma-GTPase during anoxia to 2% of control values. Turtle liver contained one homodimeric alpha class and a unique heterodimeric alpha-like GST. Turtle liver GR revealed a high affinity for GSSG. No new isoforms of either GST or GR were formed during anoxia. MPC displayed an increase in peptidylglutamyl-peptide bond hydrolyzing activity during recovery from anoxia, possibly as a result of oxidative damage to proteins in this tissue. Overall, T. S. elegans maintains continually high defenses against the stresses of anoxia and oxidative stress upon recovery from anoxia.