Characterization of a decapacitation factor associated with epididymal mouse spermatozoa

Earlier studies demonstrated that epididymal mouse spermatozoa have a surface-associated factor which inhibits fertilizing ability in a reversible manner. The factor can be removed from uncapacitated spermatozoa by gentle centrifugation, resulting in immediately highly fertile gametes, and it can be added back to capacitated spermatozoa, resulting in poorly fertile cells in which the acrosome reaction has been blocked. Using such inhibition of in-vitro fertilizing ability as an assay, we have carried out experiments to characterize the factor. It appears to be an anionic polypeptide with Mr of approximately 40,000 (according to its behaviour on gel filtration). It is stable to heating at 100 degrees C for 15 min and is not destroyed by proteases at pH 8.0, yet inhibitory activity decreases during sperm incubation in capacitating conditions and is also destroyed in partially purified preparations by endogenous enzyme action during incubation at pH 5.0. Activity is not adsorbed to either concanavalin A-agarose or wheat-germ agglutinin-agarose, suggesting that terminal mannose and N-acetylglucosamine residues are not abundant. The factor causes rapid changes in the patterns of chlortetracycline fluorescence seen on sperm heads, a parameter used to assess the capacitated state. Removal of the factor from uncapacitated cells results in a shift to a predominance of capacitated patterns, while the addition of crude or partially purified factor to capacitated cells inhibits the acrosome reaction and causes a shift to the uncapacitated pattern in acrosome-intact spermatozoa. The factor therefore behaves as a decapacitation factor. However, it appears to differ from other characterized decapacitation factors in terms both of molecular size and of abundance of mannose and N-acetylglucosamine residues.