Biochemical Studies of Proacrosin and Acrosin from Hamster Cauda Epididymal Spermatozoa1

Partially purified proacrosin, the zymogen form of acrosin was obtained from unwashed hamster cauda epididymal spermatozoa by acid extraction and Sephadex column chromatography. Hamster proacrosin (92 percent of the total acrosin activity) and active acrosin (8 percent of the total activity) were eluted together with an apparent molecular weight of 70,000 ± 3000. However, the acrosin produced by complete “autoactivation” of the proacrosin preparation had an apparent molecular weight of 41,000 ± 3000. Hamster proacrosin “autoactivation” was a second order autocatalytic process typical of zymogen conversion to active enzyme. “Autoactivation” was most rapid at pH 8.0, accelerated by Ca2+ and inhibited by Zn2+. Our earlier studies have shown that rabbit sperm proacrosin had similar properties which suggests that the molecular nature of the two proacrosins is similar. The following properties of hamster acrosin were similar to those of rabbit acrosin: pH optima; Km values with respect to TAME and BAEE; Ki’s for p-aminobenzamidine and soybean trypsin inhibitor; and stimulation by Ca2+. Hamster acrosin was also inhibited by the following inhibitors known to inhibit rabbit acrosin: lima bean and ovomucoid trypsin inhibitors; the synthetic trypsin inhibitors TLCK, NPGB and benzamidine; and Zn2+. TPCK and L-arginine did not inhibit hamster or rabbit acrosins. Two striking differences between the two acrosins were: 1) hamster acrosin did not hydrolyze the rabbit acrosin substrate BANA: 2) the Vmax BAEE/Vmax TAME ratios of hamster and rabbit acrosins were 0.2 and 2.0 respectively. The hydrolysis of the hamster egg zona pellucida by hamster sperm acrosin was demonstrated for the first time.