p-Aminobenzamidine, an acrosin inhibitor, inhibits mouse sperm penetration of the zona pellucida but not the acrosome reaction.

The effect of p-aminobenzamidine (pAB), an inhibitor of mouse sperm acrosin, on mouse sperm capacitation, motility, acrosome loss and fertility in vitro was examined using zona-intact and zona-free eggs. With intact eggs, concentrations of pAB ranging from 0.1 to 1.0 mM in the sperm preincubation medium effectively inhibited fertilization (13-0%, respectively), but these same suspensions (10(6) cells/ml) showed high rates of fertilization with zona-free eggs (100-95.3%); with the lower concentration of 10(5) cells/ml, fertilization rates of zona-free eggs decreased with increasing concentrations of pAB (100-55%). Washing of treated samples gave fertilization rates similar to control samples (87.1 and 84.6%, respectively), indicating that inhibition was reversible and that there had been no interference with the capacitation process. Whiplash motility was also observed in all samples, suggesting that the apparent inability to penetrate the zona might be due to an acrosomal defect. This was confirmed by electron microscopic examination of treated sperm samples. In high concentrations of pAB, many cells had undergone the acrosome reaction, i.e. membrane vesiculation, but acrosomal matrix dispersal was inhibited. These results are consistent, therefore, with a role for the acrosomal enzyme acrosin in matrix dispersal, but not the acrosome reaction itself, and in penetration of the zona pellucida.