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Antioxidant defense
- Reptile freeze tolerance: Metabolism and gene expression

Good antioxidant defenses have been identified as an important component of freezing survival among freeze tolerant frogs [54] and [81] and the addition of antioxidants is known to improve hypothermic and freezing preservation of cells and tissues in cryomedical applications [8]. Antioxidant defense is critical in situations where oxygen availability varies widely and rapidly. For example, situations of ischemia (interrupted blood flow) in mammals result in metabolic damage due to ATP limitation when oxygen-based metabolism is interrupted but the reintroduction of oxygen after ischemia is equally damaging [8], [51] and [76]. Reperfusion injuries occurring when oxygen is reintroduced result from a burst of reactive oxygen species (ROS) generation that can overwhelm existing antioxidant defenses. Organisms that endure frequent anoxic or ischemic episodes in nature are typically prepared with high constitutive activities of antioxidant enzymes (e.g., superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase, and peroxiredoxin), proteins (e.g., thioredoxin), and metabolites (e.g. ascorbate, glutathione) [50] and [51]. Adult freshwater turtles (T. s. elegans) that are excellent facultative anaerobes and able to survive for as long as 3 months in deoxygenated water at low temperature [53] have the highest antioxidant enzyme activities among cold-blooded vertebrates that have been examined [92] and [93]. Freeze tolerant frogs (R. sylvatica) that undergo cycles of ischemia/reperfusion with each freeze/thaw event also exhibit high antioxidant enzyme activities, much higher than activities in the same organs of freeze intolerant leopard frogs (Rana pipiens) [54]. The antioxidant enzyme, γ-glutamyltranspeptidase, also increased significantly (by not, vert, similar2.5-fold) during freezing in liver of both R. sylvatica and C. picta [43] and [44] and catalase activity increased strongly in liver of hatchlings of several turtle species in response to either anoxia or freezing exposure [30]. Interestingly, the magnitude of the catalase response by four out of five species was similar under both stresses but the effect was most pronounced in species with low freeze tolerance [30]. This could suggest that constitutive activities in liver of freeze tolerant species were already largely sufficient to deal with oxidative stress during freezing/anoxic excursions. Indeed, inducible defenses are more commonly seen in species that deal with infrequent situations of anoxia/ischemia exposure [51]. For example, freezing exposure of garter snakes (T. sirtalis), resulted in significant increases in the activities of catalase and glutathione peroxidase in skeletal muscle whereas anoxia exposure strongly increased superoxide dismutase in liver [48].

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