Activation of phospholipase A2 of human spermatozoa by proteases

The effect of various proteases (kallikrein, plasmin, and trypsin) on sperm phospholipase A2 activity (PA2: EC 3.1.1.4) has been studied. The addition of trypsin to spermatozoa, isolated and washed in the presence of the protease inhibitor benzamidine, increased PA2 activity optimally with trypsin concentrations of 1.0-1.5 units/assay. In kinetic studies, all of the above proteases stimulated the deacylation of phosphatidylcholine (PC); in fresh spermatozoa, trypsin showed a higher activation potential than kallikrein or plasmin. In the presence of benzamidine, the activity remained at basal levels. Endogenous protease activity due to acrosin (control) resulted in an increase in PC deacylation compared to the basal level. The maximum activation time of PA2 activity by proteases was 30 min. Natural protease inhibitors (soybean trypsin inhibitor and aprotinin) kept the PA2 activity at basal levels and a by-product of kallikrein, bradykinin, did not significantly affect the control level. Protein extracts of fresh spermatozoa exhibited the same pattern of PA2 activation upon the addition of proteases, thus indicating that the increase in PA2 activity was not merely due to the release of the enzyme from the acrosome. All of these findings suggest the presence of a precursor form of phospholipase A2 that can be activated by endogenous proteases (acrosin) as well by exogenous proteases present in seminal plasma and in follicular fluid (plasmin, kallikrein). Thus, this interrelationship of proteases and prophospholipase A2 could activate a dormant fusogenic system: the resulting effect would lead to membrane fusion by lysolipids, key components in the acrosome reaction.