Tyrosine nitration in human spermatozoa: a physiological function of peroxynitrite, the reaction product of nitric oxide and superoxide

Tyrosine nitration is a widely used marker of peroxynitrite (ONOO-) produce d from the reaction of nitric oxide (NO.) with superoxide (O-2(.-)). Since human spermatozoa are able to produce both NO. and O-2(.-) during capacitat ion in vitro, we investigated whether spontaneous tyrosine nitration of pro teins occurs in human spermatozoa and evaluated the physiological effects o f peroxynitrite on sperm function. We report here that human spermatozoa, i ncubated for 8 h under conditions conducive to capacitation, display a repr oducible pattern of protein tyrosine nitration. Several proteins with mot. wt of 105-14 kDa become increasingly tyrosine-nitrated after 15 min incubat ion and then minimal changes are observed. Treatment of capacitated spermat ozoa with human follicular fluid or calcium ionophore causes an increase of the nitrotyrosine content of proteins at the mot. wt of 85 kDa. Moreover, exposure of spermatozoa to ONOO- (2.5-50 mu mol/l) increases motility and p rimes spermatozoa to respond earlier to human follicular fluid. ONOO- also increases protein tyrosine nitration and phosphorylation in a concentration -dependent manner. Taken together, these results demonstrate that tyrosine nitration of sperm proteins occurs in capacitated human spermatozoa, and th at low concentrations of ONOO- modulate sperm functions, emphasizing the co ncept that capacitation is part of an oxidative process.