Oxidative stress plays a critical role in regulating sperm function, yet species-specific antioxidant mechanisms remain poorly understood. This study compared hydrogen peroxide (H₂O₂) tolerance in horse and human sperm and investigated the roles of catalase and glutathione peroxidase (GPx) in horses. A H₂O₂ dose–response assay (0–2000 µM) showed that horse sperm were significantly more resistant to oxidative damage, with an IC₅₀ for progressive motility over 14-fold higher than that of human sperm (391.6 µM vs. 27.3 µM). Horse sperm also accumulated more intracellular H₂O₂ without loss of motility or viability. DNA damage assays (Halo and SCSA) revealed H₂O₂-induced fragmentation in human but not horse sperm. Enzyme inhibition experiments in horse sperm using 3-amino-1,2,4-triazole (catalase inhibitor) and (1S,3R)-RSL3 (GPx inhibitor) at 250 µM H₂O₂ showed that catalase inhibition severely impaired motility and increased intracellular H₂O₂ > 100-fold, while GPx inhibition had a milder effect (~5-fold increase). Immunocytochemistry localized catalase to the sperm head, particularly the post-acrosomal region, challenging the notion that sperm lack peroxisomes. The dependence of horse sperm on oxidative phosphorylation may drive the need for enhanced antioxidant defenses. These findings reveal species-specific oxidative stress adaptations and highlight catalase as a key antioxidant in equine sperm.

(1) Background: The RoXsta^TM system has been developed as a rapid, effective means of profiling different types of antioxidant activity. The purpose of this study was to examine its performance utilizing a diverse array of biological fluids including semen, blood plasma, serum, urine, saliva, follicular fluid and plant extracts. (2) Methods: The RoXsta^TM system was used to assess the ability of different fluids to suppress free radical formation as well as scavenge a variety of toxic oxygen metabolites including free radicals and both hydrogen and organic peroxides. (3) Results: Human semen was shown to have significantly (p < 0.001) more peroxide scavenging power than any other fluid tested (10–14 mM vitamin C equivalent compared with 1–2 mM for blood serum or plasma), while urine was particularly effective in scavenging free radicals and preventing free radical formation (p < 0.001). The powerful antioxidant properties of human semen were shown to reside within the seminal plasma (SP) fraction, rather than the spermatozoa, and to be resistant to snap freezing in liquid nitrogen. Moreover, comparative studies demonstrated that human SP exhibited significantly (p < 0.001) higher levels of antioxidant potential than any other species examined (stallion, bull, dog) and that this intense activity reflected the relative vulnerability of human spermatozoa to peroxide attack. (4) Conclusions: The RoXsta^TM system provides valuable information on the antioxidant profile of complex biological fluids, supporting its diagnostic role in conditions associated with oxidative stress. Based on the results secured in this study, human semen is identified as a particularly rich source of antioxidants capable of scavenging both hydrogen and organic peroxides, in keeping with the high susceptibility of human spermatozoa to peroxide-mediated damage.

(1) Background: The RoXstaTM system has been developed as a potential diagnostic tool to profile the antioxidant activity of complex biological fluids. (2) Methods: This methodology was used to assess the ability of different biofluids (semen, blood plasma, serum, urine, saliva, follicular fluid and plant extracts) to suppress free radical formation, as well as scavenge a variety of toxic oxygen metabolites including free radicals and peroxides. (3) Results: Human semen was shown to have significantly (p < 0.001) more peroxide scavenging power than any other fluid tested, while urine was particularly effective in scavenging free radicals and preventing free radical formation. The powerful antioxidant properties of human semen were shown to reside within the seminal plasma (SP) fraction, rather than the spermatozoa, and to be resistant to snap freezing in liquid nitrogen. Moreover, comparative studies demonstrated that human SP exhibited significantly (p < 0.001) higher levels of antioxidant potential than any other species examined (stallion, bull, dog) and that this intense activity reflected the relative vulnerability of human spermatozoa to peroxide attack. (4) Conclusions: the RoXstaTM system provides valuable information on the antioxidant profile of complex biological fluids, supporting its diagnostic role in conditions associated with oxidative stress. Based on the results secured in this study, human semen is identified as a particularly rich source of antioxidants capable of scavenging both hydrogen-, and organic- peroxides, in keeping with the high susceptibility of human spermatozoa to peroxide-mediated damage.