Boar acrosin. II. Classification, inhibition, and specificity studies of a proteinase from sperm acrosomes.

Acrosin, a proteolytic enzyme located in the acrosome of sperm, exhibits amidase, esterase, and proteinase activity on synthetic and natural substrates containing arginyl and lysyl residues. Highly purified acrosin preparations from boar acrosomes have endopeptidase activity cleaving only the carboxyl bonds of arginine and lysine with a strong preference for arginine bonds. The Michaelis constant for hydrolysis of benzoyl arginine ethyl ester at pH 8.0 is 5 X 10e5 M. The enzyme is inhibited by diisopropyl fluorophosphate and tosyl lysine chloromethyl ketone, indicating that serine and histidine residues may be present in the active site. Acrosin is inhibited by several natural proteinase inhibitors and appears to be unique in that its amidase and esterase activities are competitively inhibited by free arginine with an apparent K; of 3 mu. The unusual specificity of acrosin may have implications in its biological role of sperm penetration of the zona pellucida of the ovum.