Effects of Acrosin Inhibitors on the Soluble and Membrane-Bound Forms of Ram Acrosin, and a Reappraisal of the Role of the Enzyme in Fertilization

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Abstract
When denuded ram spermatozoa were suspended in weakly buffered 0.25 M sucrose, the acrosin (EC 3.4.2.1.10) remained bound to the acrosomal membranes of the sperm heads. Media containing CaCl2 caused complete solubilization of the enzyme. Effects of acrosin inhibitors on soluble and bound enzyme were studied in Tris/HCl (pH 8.2) containing sucrose. Denuded spermatozoa were used as a preparation of bound acrosin. Trasylol (Kuntz basic pancreatic trypsin inhibitor) acted more strongly on bound acrosin than on soluble acrosin, but soybean trypsin inhibitor acted more strongly on soluble acrosin. At concentrations 0.5-2.0 .mu.M, the inhibitors isolated from ram acrosomes and from ram seminal plasma inhibited soluble acrosin but had negligible effects on bound acrosin. Bound acrosin was sensitive to high concentrations of the acrosomal inhibitor. The 2 forms of acrosin were inhibited to about the same degree by p-aminobenzamidine and also by L-1-chloro-3-tosylamido-7-amino-2-heptanone. Membrane-bound acrosin may be the form that functions in penetration of the zona pellucida; a role for acrosin inhibitors is possibly suppression of an antifertility effect of soluble acrosin on mammalian eggs. This hypothesis is supported by the impaired fertilizing capacity of rabbit spermatozoa treated with acrosin inhibitors, the anti-fertility effects on hamster eggs of solutions of acrosin and of bovine trypsin, and these results.