Proacrosin activation and acrosin release during the guinea pig acrosome reaction

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Abstract
The kinetics of proacrosin activation and release from guine pig spermatozoa during the nonsynchronous acrosome reaction were studied. Epididymal spermatozoa were incubated at 37°C in a defined medium (pH 7.8) containing 1.7 mM Ca2+. After 195 min, 78% of the motile spermatozoa had undergone the acrosome reaction as determined by light microscopy. Acrosin and proacrosin levels in the spermatozoa and medium were measured at the beginning of the incubation period. Most of the total acrosin activity (78%) was associated with the spermatozoa, of which greater than 90% was in the form of proacrosin. Proacrosin represented a small, stable fraction (23%) of the total acrosin in the medium; it did not activate to acrosin while in the medium. After 195 min, a decrease in sperm-associated total acrosin (42%; p < 0.05) was accompanied by an increase in the total acrosin level in the medium (115%; P < 0.05). No change in the relative proacrosin content (percent of total acrosin) was evident in either medium or spermatozoa. Additional experiments quantified acrosin and proacrosin during the progression of the acrosome reaction. Both the loss of sperm-associated total acrosin and the increase in total acrosin levels in the medium were highly correlated with the fraction of acrosome-reacted spermatozoa (r = 0.954 and 0.922, respectively; P < 0.001). However, the rate of acrosin appearance in the medium was only 60% (P < 0.001) of the rate of acrosin loss from the spermatozoa. The fractional proacrosin content of spermatozoa (94%) and medium (31%) remained unchanged during the acrosome reaction (r = 0.15 and 0.30, respectively; P > 0.1). From these data, it was possible to estimate the total acrosin content of acrosome-intact spermatozoa (434 mlU/107 spermatozoa), and the fractions of the total sperm acrosin levels that are (1) released as active acrosin (37%), (2) released as proacrosin (17%), (3) inactivated (36%), and (4) associated with the spermatozoa (10%) during the acrosome reaction. The data suggest that proacrosin activation occurs within the spermatozoa, prior to distruption of the acrosome; this finding is consistent with the contention that acrosin is an active component of the mechanism underlying the acrosome reaction.