Acrosin does not appear to be bound to the inner acrosomal membrane of bull spermatozoa.

Ferritin-conjugated soybean trypsin inhibitor was used for the ultrastructural localization of acrosin in bull spermatozoa following acrosomal disruption. The ferritin label was observed in the anterior segment of the acrosome in disrupted cells only. Emptied acrosomes were labeled, mostly on the external surface of their outer membrane. Labeling was also found on the material bound to detached acrosomal caps. However, at no time could the ferritin label be found on the inner acrosomal membrane. It is concluded that acrosin activity is not present on the inner acrosomal membrane but is lost from the acrosomal matrix as the acrosomal reaction proceeds.