Reactive oxygen species (ROS), such as the superoxide anion (O2−), hydrogen peroxide (H2O2) and nitric oxide (NO), when generated at low and controlled levels, act as second messengers. ROS regulate sperm capacitation, which is the complex series of changes allowing spermatozoa to bind to the zona pellucida surrounding the oocyte, induce the acrosome reaction (exocytotic event by which proteolytic enzymes are released) and fertilize the oocyte. Capacitating spermatozoa produce controlled amounts of ROS that regulate downstream events: first, the increase in cAMP, protein kinase A (PKA) activation and phosphorylation of PKA substrates (arginine-X-X-serine/threonine motif; 15–30 min); second, the phosphorylation of MEK (extracellular signal regulated kinase [ERK] kinase)-like proteins (30–60 min) and then that of the threonine–glutamate–tyrosine motif (> 1 h); finally, the late tyrosine phosphorylation of fibrous sheath proteins (> 2 h). Although all these events are ROS-dependent, the regulation by various kinases, protein kinase C, PKA, protein tyrosine kinases, the ERK pathway, etc. is different. ROS also regulate the acquisition of hyperactivated motility and the acrosome reaction by spermatozoa. ROS action is probably mediated via the sulfhydryl/disulfide pair on sperm proteins. Redundancy, cross talk, and multiple systems acting in parallel point to an array of safeguards assuring the timely function of spermatozoa.
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