Effect of alkaline phosphatase on thromboxane mimetic induced platelet activation

Recently it has been reported that alkaline phosphatase selectively inhibited thromboxane mimetic induced platelet aggregation and secretion suggesting that the phosphorylation state on the platelet surface may be important for thromboxane induced platelet activation. We report here studies attempting to elucidate the mechanism of action of alkaline phosphatase. Washed human platelet aggregation induced by the thromboxane mimetic IBOP was completely abolished when incubated with alkaline phosphatase (1 unit/ml) for 5 min. The effect was inhibited by co-incubation with 5mM phosphate. Binding studies using [125I]BOP showed that neither the affinity of IBOP for the receptor (control: 9.2 +/- 2.1 nM, alkaline phosphatase: 7.9 +/- 1.8 nM) nor the Bmax (control: 1780 +/- 320 sites/plt. alkaline phosphatase: 1920 +/- 290 sites/plt) were effected by alkaline phosphatase treatment. GTPase activity was measured in platelet membranes treated with and without alkaline phosphatase as measured by IBOP induced hydrolysis of [gamma-32P]GTP. The EC50 values for IBOP induced GTPase were similar whereas the maximum amount of released Pi in the control membranes was more than two fold greater than in alkaline phosphatase treated membranes. These studies suggest that thromboxane induced platelet activation may be dependent upon the phosphorylation state of the thromboxane receptor and/or closely associated protein.