## 4 Discussion
Studies have shown that long-lived species have erythrocyte cell membranes that are more resistant to lipid peroxidation, which might vary with the age of the individual and, therefore, be a direct predictor of survival [83]. In the present study, it was observed that the mean levels of MDA increased at 6 and 12 h of exposure, even though observed differences between exposure concentrations were not statistically significant due to the high variance of results. Apparently, the low number of individuals (n = 5) could explain this result. A positive correlation was observed between the concentration of MeHg and the levels of MDA, r = 0.56, \(p<0.05\) after 12 h of exposure (Spearman). Studies with a higher n should be performed to verify this possible correlation due to the low number of samples used. In this study, we found that an in vitro increase in MeHg concentration caused a corresponding increase in MDA concentration (LPO), as well as decreases in the activities of SOD and GST, result which is in agreement with previous studies [76; 77; 78; 84]. Nonetheless, it is important to mention that there are other sensitive enzymes that respond to MeHg that should be studied as biomarkers of toxicity to mercury compounds. Peroxiredoxin and thioredoxins are found in all tissues and are involved in multiple functions that are associated with cellular maintenance and survival, including protein repair, regulation of the cell cycle, and cell signaling [85]. These enzymes could function in host defense against oxidative stress, modulate immune response, and control intracellular peroxide levels [86; 87], as well as participate in the antioxidant defense system [88; 89]. Furthermore, it was reported for Chinese soft-shelled turtle (_Pelodiscus sinensis_) that peroxidiredoxins was implicated defense against microbial pathogens and oxidants and would provide information of functional mechanism of immunity [90]. Studies that were carried out with these enzymes in human cell lines have reported that mercury and MeHg bind to their thiol groups (-SH), decreasing activity and causing inactivation in most cases [91; 92; 93]. Peroxiredoxins and thioredoxins are present in blood and, thus, are very important for living beings. They are very sensitive to oxidation [94; 95], but, thanks to their plasticity, they have the ability of making post-translational modifications that prevent them from oxidizing, maintaining the homeostatic balance of cells [96]. Thioredoxins and peroxiredoxins function structurally and they have been barely studied in turtles [90]. Some of these studies have focused on evaluating how prevotoxins affect thioredoxin, GST, and SOD levels [97]. Moreover, turtles' regulation of intracellular levels of GST and peroxiredoxins has been studied under oxidative stress caused by anoxia, freezing, and exposure to lipopolysaccharidesaccharides [98; 99; 100]. Overall, studies to date conclude that peroxiredoxins and thioredoxins are important targets for the compounds of mercury and their inhibition is likely to explain the results beyond Hg-induced toxicity [101]. For this reason, the evaluation in future studies of TrxR and Trx in blood and plasma could provide a useful tool for the prediction of adverse effects that are induced by mercury compounds. Additionally, the results indicate that mercury can inhibit the activities of many proteins/enzymes that are involved in combating oxidative stress in cells, including thioredoxin (Trx) (MeHg and HgCl\({}_{2}\)), glutaredoxin (Grx) (MeHg) [91; 102], glutathione peroxidase (Gpx) (MeHg) [103], glutathione reductase (GR) (MeHg and HgCl\({}_{2}\)), thioredoxin reductase (TrxR) (MeHg and HgCl\({}_{2}\)) [91; 104], superoxide dismutase (SOD) (MeHg and HgCl\({}_{2}\)) [105], neuronal nitric oxide synthase (nNOS) (MeHg) [105], and Keap1 [106]. In addition, mercury can also alter the ratio of GSH/glutathione disulfide (GSSH) either by directly binding to GSH, depleting GSH, or decreasing the levels of Gpx [107, 108, 102].