Inhibition of Acrosin Activity with a Trypsin Inhibitor Blocks Human Sperm Penetration of the Zona Pellucida

To evaluate the role of acrosin in human sperm penetration of the zona pellucida (ZP), sperm-oocyte interaction was studied after acrosin activity was blocked with soybean trypsin inhibitor (SBTI). Oocytes that had failed to fertilize because of sperm pathology in a clinical in vitro fertilization program were used to assess sperm binding to and penetration into the ZP. The acrosome reaction of sperm bound to the ZP was determined using fluorescein-labeled Pisum sativum agglutinin after sperm were removed from the ZP. Acrosin activity, determined by a gelatin substrate film method, was severely inhibited by 2 mg/ml SBTI. Sperm motility and movement characteristics, assessed by a Hamilton-Thorn motility analyzer, were unchanged after 6-h incubation with SBTI. Inhibition of acrosin activity did not affect the number of sperm bound to the ZP but completely blocked sperm penetration of the ZP after a 5-h incubation. SBTI did not influence the spontaneous acrosome loss of sperm in culture medium after 6-h and 20-h incubations, but the percentage of acrosome-reacted sperm bound to the ZP was significantly reduced. It was concluded that acrosin activity plays a key role in sperm-zona interaction in humans. Motile sperm are unable to penetrate the ZP when acrosin activity is inhibited. This might result from interference with a phase of the sperm-ZP binding reaction or with a lytic action of acrosin. Also, acrosin may be involved in the acrosome reaction induced by sperm binding to the ZP.