**5. Conclusions**

Our investigations identify GSTO2 isoforms as functionally active surface-borne enzymes on mouse and boar spermatozoa. We propose that GSTO2 may facilitate the regulation of the oxidative environment of the plasmalemma indirectly, through the replenishment of antioxidant molecules such as ascorbic acid and glutathione. Overall, this study highlights the importance of oxidative-reductive regulation and demonstrates the wide-reaching negative implications its dysregulation can have on overall sperm fitness.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3921/8/12/601/s1, Figure S1: Mouse and boar GSTO sequence comparison, Figure S2: GSTO inhibitor binding specificity in whole mouse and boar spermatozoa, Figure S3: The Assessment of sperm viability between treatment groups, Figure S4: The level of protein tyrosine phosphorylation during in vitro capacitation in boar spermatozoa, Figure S5: The difference in cellular reactive oxygen species (ROS) levels of GSTO Inhibited and Control capacitated spermatozoa.

**Author Contributions:** Conceptualization, L.E.H., P.S., C.O. and R.O.; Data curation, L.E.H., M.Z., J.M. and W.X.; Formal analysis, L.E.H.; Funding acquisition, R.O.; Investigation, L.E.H., M.Z., J.M., W.X. and C.O.; Methodology, L.E.H., M.Z., W.X., C.O. and R.O.; Supervision, P.S. and R.O.; Writing—original draft, L.E.H. and R.O.; Writing—review and editing, M.Z., J.M., P.S., C.O. and R.O.

**Funding:** This research was funded by the Canadian Institute of Health Research (84440) and the Natural Science and Engineering Research Council of Canada (RGPIN/05305) to RO and the Agriculture and Food Research Initiative Competitive Grant no. 2015-67015-23231 from the USDA National Institute of Food and Agriculture and seed funding from the Food for the 21st Century Program of the University of Missouri to PS.

**Acknowledgments:** We would like to express our thanks to the National Swine Research and Resource Center sta ff for their help with semen collection and process.

**Conflicts of Interest:** The authors declare no conflict of interest.
