## 4 Discussion  
### Effect of Methomy on Antioxidant Defenses in Tilapia Testis  
GR, GPx, GST, GSSG and GSH are components of the glutathione-related antioxidant system and play important roles in intercellular defense against oxidative damage [20]. Various relationships exist between GST, GR, GPx, GSSG and GSH. GPx can catalyze GSH to GSSG, accompanied by the detoxification of hydroperoxides and ROS, and GSSG is then reduced to GSH during GR catalysis with the consumption of NADPH that is recycled into the pentose phosphate pathway [7]. The conjugation of GSH to xenobiotics can be catalyzed by GST, which facilitates chemical excretion by the addition of more polar groups [21]. Therefore, all these antioxidants work together to mitigate and prevent oxidative damage. GSH is a major low-molecular-weight sulfhydryl compound in the cytosol, and acts as a cellular reducing, protective reagent against numerous pollutants through the sulfhydryl (\(-\)SH) group. As an antioxidant enzyme substrate and a direct scavenger of oxyradical, GSH plays an important role in detoxification and the elimination of oxidative damage through the \(-\)SH group [7,22]. Our experiment showed that GSH levels in testis of tilapia exposed to methomyl decreased significantly, especially at the high concentrations (Figure 1f), which might be explained as follows. First, because of the conjugation of GSH to methomyl catalyzed by GST, GSH could conjugate with electrophilic intermediates catalyzed by GST, which will decrease GSH levels [21]. A similar decreasing trend in GSH content accompanied by a rise in the GST level was reported by other researchers [7,23]. In our experiment, a significant increase in GST was accompanied by a decrease of GSH in testis of fish exposed to 2 and 20 ug L\({}^{-1}\) methomyl (Figure 1c). Second, GSH can scavenge ROS through the oxidation of GSH to GSSG catalyzed by GPx, which will decrease the level of GSH [24]. Previous researchers have found decreasing levels of GSH accompanied by increases in GSSG and GPx in Nile tilapia exposed to methomyl [11], in rainbow trout  
Figure 1: SOD (**a**), CAT (**b**), GST (**c**), GR (**d**), GPx (**e**) activities and GSH (**f**), GSSG (**g**) contents in testis of tilapia exposed to methomyl for 30 days, and after an 18-day recovery period in methomyl-free water (R18). Note: All the data were expressed as Average \(\pm\) SD (\(n=6\)). _“\(\ast\)”_ means significant difference from the control. Different lower-case letters indicate significant difference among concentrations at the same exposure period, and different upper-case letters indicate difference between exposure and recovery periods at the same methomyl concentration, and with \(p<0.05\) being considered significant. (_Oncorhynchus mykiss_) exposed to carbamazepine [22], and in Nile tilapia exposed to domoic acid [25]. In the present study, the steady rise in GSSG and GPx was accompanied by a decrease in GSH in testis of the tilapia exposed to methomyl concentrations 2, 20, and 200 \(\upmu\)g L\({}^{-1}\) (Figure 1g). Overall, these results suggest that the transformation of GSH to GSSG happens under the oxidative stress caused by toxic materials [22]. Additionally, a loss of adaptive mechanisms and damage to GSH synthesis during the stress condition caused by methomyl exposure might be other reasons for declines in GSH content [22]. For instance, similar changes in GSH levels were observed in goldfish (_Carassius auratus_) exposed to 2-chlorophenol [21] and in rainbow trout exposed to the fungicide propiconazole [23]. GR can reduce GSSG to GSH with the consumption of NADPH, thereby effectively regulating the formation of ROS during exposure to exogenous pollutants. Under oxidative stress conditions, an increase of GR activity always reestablished the level of GSH, which is oxidized and effectively regulates ROS formation [26]. Likewise, in our own experiment, increases in GR were found at 2 \(\upmu\)g L\({}^{-1}\) and 20 \(\upmu\)g L\({}^{-1}\) methomyl.