Inhibition of the pig sperm acrosome reaction by a decapacitation factor from pig seminal plasma

Decapacitation factors that inhibit the fertilising ability of spermatozoa have been identified in the seminal plasma of several species. The area study of these factors has broadened, as it seems that there is more than one decapacitation factor per species. The aim of the present study was to identify decapacitation factors in pig seminal plasma capable of inhibiting the acrosome reaction (AR) induced by zona pellucida (ZP). Spermatozoa were washed by centrifugation to remove the seminal plasma and incubated in TALP capacitation medium for 4 h at 37 degrees C. The seminal plasma was centrifuged, filtered, fractionated and kept frozen until use. Five seminal plasma fractions were obtained: <10, 10-30, 30-50, 50-100 and >100 kDa. These fractions were added to capacitated sperm, and after 40 min incubation, solubilised ZP was added to induce the AR. Sperm were stained with Hoechst 33258 and flourescein-conjugated lectin to assess the percentage of acrosome reacted sperm, further used as an index of sperm capacitation. Only the >100 kDa seminal plasma fraction inhibited the introduction of AR by ZP, and the inhibition was dose-dependent (r = 0.93). When the >100 kDa fraction was treated with trypsin, its inhibitory effect on the acrosome reaction was blocked. Tn conclusion, the pig seminal plasma contains decapacitation factors, protein, in nature, with molecular weight greater than 100 kDa, which inhibit the ZP-induced sperm acrosome reaction.