EFFECT OF ALKALINE-PHOSPHATASE ON THROMBOXANE MIMETIC INDUCED PLATELET ACTIVATION

Recently it has been reported that alkaline phosphatase selectively in hibited 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 h ere studies attempting to elucidate the mechanism of action of alkalin e phosphatase. Washed human platelet aggregation induced by the thromb oxane mimetic IBOP was completely abolished when incubated with alkali ne phosphatase (1 unit/ml) for 5 min. The effect was inhibited by co-i ncubation with 5mM phosphate. Binding studies using [I-125]BOP showed that miether the affinity of IBOP for the receptor (control: 9.2 +/- 2 .1 nM, alkaline phosphatase: 7.9 +/- 1.8 nM) nor the B-max (control: 1 780 +/- 320 sites/plt, alkaline phosphatase: 2920 +/- 290 sites/plt) w ere effected by alkaline phosphatase treatment. GTPase activity was me asured in platelet membranes treated with and without alkaline phospha tase as measured by IBOP induced hydrolysis of [gamma-P-32]GTP. The EC (50) values for IBOP induced GTPase were similiar whereas the maximum amount of released P-1 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 clo sely associated protein.