Mammalian fertilization includes highly regulated biochemical interactions between complementary molecules located on the surfaces of both gametes as well as those present in the gamete natural environment in male and female reproductive tracts. The key proteinase of the fertilization process is acrosin, localized in the acrosome of the spermatozoon. Acrosin is involved in the acrosome reaction of spermatozoa, in the secondary binding of the sperm to the zona pellucida (ZP) and penetration of the ZP by sperm. The biological role of acrosin inhibitors in sexual fluids is to inactivate the prematurely released active form of acrosin from occasionally damaged spermatozoa. Seminal plasma proteins (spermadhesins) attached to the sperm head are acceptor molecules for the sperm-associated acrosin inhibitor. The attachement of the inhibitor to the surface molecules of the sperm can stabilize the binding site for ZP and protect the sperm against proteolytic degradation
