RELATIONSHIP BETWEEN MOLECULAR CONVERSIONS OF ACROSIN AND THE PROGRESSION OF EXOCYTOSIS IN THE CALCIUM IONOPHORE-INDUCED ACROSOME REACTION

In this study we used a previously characterized monoclonal antibody t o analyze the molecular conversions of acrosin during the acrosomal ex ocytosis induced by ionophore A23187. Before sperm exposure to the ion ophore, most of the sperm acrosin was in the form of proacrosin (55-kD a and 53-kDa forms). Upon exposure to the ionophore, the concentration of proacrosin in sperm samples decreased rapidly and was negatively c orrelated with the progression of exocytosis. After 1 h of ionophore t reatment, proacrosin was quantitatively converted into the two active acrosin forms, alpha-acrosin (49 kDa) and beta-acrosin (36 kDa). Howev er, products of further acrosin conversions were not found after this treatment. As compared with the speed of acrosin activation during spe rm contact with the ionophore, the ionophore-induced release of acrosi n from the sperm cells into the soluble fraction was apparently delaye d, and only the active acrosin forms (49 kDa and 36 kDa) were found in sperm incubation media. External Ca2+ influenced the speed of proacro sin conversion in a concentration-dependent manner. The ionophore-indu ced activation of proacrosin and acrosome reaction were partially inhi bited by trypsin inhibitors. The results suggest that proacrosin activ ation is an essential step in the mechanism of the acrosomal exocytosi s.