Effect of berbamine on cytosolic phospholipase A2 activation in rabbit platelets

The effect of berbamine, a biscoclaurine alkaloid, on cytosolic phospholipase A₂ activation in rabbit platelets was investigated. Berbamine inhibited árachidonic acid liberation induced by thrombin but not that by ionomycin. The alkaloid did not affect thrombin-stimulated Ca²⁺ mobilization, Ca²⁺-dependent translocation of cytosolic phospholipase A₂ to membranes, or the activity of partially purified cytosolic phospholipase A₂. Furthermore, berbamine had no effect on the thrombin-elicited increase in cytosolic phospholipase A₂ activity. However, berbamine suppressed arachidonic acid liberation in platelets stimulated with GTP-binding protein activators. Although incubation of platelet membranes with a GTP analogue decreased the islet-activating protein-catalyzed ADP-ribosylation of an approximately 40 kDa protein in the membranes, pretreatment of the membranes with berbamine did not influence the decrease in ADP-ribosylation. These results suggest that berbamine may impair GTP-binding protein-mediated activation of cytosolic phospholipase A₂, probably without influencing the enzyme translocation to membranes or the increase in the enzyme activity, and thus may cause the suppression of thrombin-induced arachidonic acid liberation.