## 4 Discussion
This result is similar to the present study, although the exposure times are different, showing that the RBCs of the turtles are more resistant. We found that loggerhead turtle RBCs can survive at 4 \({}^{\circ}\)C for about eight weeks when stored with plasma. The determination of the LC\({}_{50}\) allowed for selecting the appropriate concentrations of MeHg to be used in the bioassay (0, 1, and 5 mg L\({}^{-1}\)), so that the majority of RBCs remained alive until the end of the bioassay, as corroborated by cell viability close to 100%. Studies on oxidative damage to wild animals from an ecological perspective have been conducted over the past 20 years [68], but knowledge on oxidative stress in sea turtles is scarce [1; 55; 69; 70]. Further, the toxic level of Hg and others metals and their short- and long-term adverse effects are still unknown for sea turtles [1; 55; 71]. RBCs have an antioxidant defense system that is composed of the enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) and molecules, such as reduced glutathione (GSH), tierrodoxins, alphatocopherol, beta caroten, and vitamin C [72]. The exposure of RBCs to methylmercury induces oxidative stress, which destabilizes the lipid bilayer of the cell membrane and causes morphological and physiological changes in the cell [63; 73]. RBCs increase or decrease the expression levels of catalase, superoxide dismutase, and glutathione enzymes in order to counteract stress. These enzymes have been monitored in sea turtles in blood and liver tissues [74; 75] to determine the correlation between the bioaccumulation of metals and oxidative stress and, in this way, understand the possible mechanisms that sea turtles use to balance themselves. There are no reports of in vitro bioassays of RBCs of sea turtles that were exposed to MeHg where oxidative stress is evaluated. In this study, where loggerhead RBCs exposed in concentrations of 5 mg L\({}^{-1}\) of MeHg, cells presented lipid peroxidation and reduction in the enzymatic activity of SOD and GST after 6 h of exposure. It seems that high concentrations of MeHg inhibit the expression of enzymes and, therefore, their activity [76; 77; 78]. These results are similar to those that were reported by [PERSON] et al. [76] in brain, liver, and kidney tissues of fish that were exposed to 5 and 10 mg L\({}^{-1}\) MeHg. Greater susceptibility of brain tissue to oxidative stress was reported, which was represented by increased levels of lipid peroxidation (MDA) and ROS formation, and parallel reduction in the activity of SOD and GST enzymes in brain cells with increasing MeHg concentration (up to 10 mg L\({}^{-1}\) MeHg). The increase of the enzymes Cu, Zn-SOD, and Mn-SOD or \(\gamma\)-glutamylcysteine synthetase is the first adaptive response that cells present when exposed to mercury compounds [79]. However,studies in mice that were exposed to Hg and MeHg have shown that doses of these compounds reduce glutation and SOD levels, respectively, in liver and brain tissues [80; 81]. These experimental results agree with the results shown in this study, and they would indicate that loggerhead turtle RBCs are as susceptible to oxidative stress that is caused by MeHg exposure as the brain cells of the fish [76], mouse, and insect neuronal and liver cells [79], since these same trends were observed for the activity of the SOD and GST enzymes, and lipid peroxidation. The SOD and GST enzymes make up the first line of cellular defense against oxidative stress, neutralizing oxygen and nitrogen reactive species and preventing lipid peroxidation [20]. When the levels of these enzymes and other non-enzymatic antioxidants, such as cysteine, which participates in the uptake and release of mercury [79], are not intracellularly sufficient or their production levels reach maximum ceilings, their activity is reduced and metals and free radicals that could not be extracellularly expelled cause the degradation of PUFA membrane and, therefore, lipid peroxidation [60]. MeHg is known to generate lipid peroxidation [82].