Stimulation of phospholipase A2 activity in human platelets by trypsin and collagen

Type I collagen enhanced human platelet phospholipase A2 activity whether added to platelet-rich plasma or washed platelets. The stimulatory effect of type I collagen on platelet membrane phospholipase A2 activity was also observed in a cell-free system utilizing platelet membranes. The release of arachidonic acid was enhanced by types I and III but not by type V collagen. The activation of platelet phospholipase A2 by type I collagen was inhibited by soybean trypsin inhibitor and mimicked by trypsin. However, type I collagen addition was not associated with any detectable changes in platelet membrane proteins while trypsin altered many proteins. These results point to acid soluble phospholipase A2 activity of platelets as an enzyme activated by type I collagen.