Stimulation of phosphoinositide kinases by thrombin is restricted to the plasma membrane in platelets.

This study was based on the hypothesis that lipid kinases in the different subcellular fractions would be differently affected by thrombin-treatment of platelets prior to subcellular fractionation. When using our previously reported method for subcellular fractionation on Percoll self-generated gradients, marker enzymes were detected as previously described. Stimulation of intact platelets with thrombin induced increased activities of PtdIns 4-kinase, PtdIns(4) P 5-kinase and PtdIns(4,5) P 4-kinase was also detected in internal membranes but was not modified by thrombin. We conclude that the production of phosphoinositides phosphorylated on the D 3 and D 5 positions of the inositol ring is restricted to the plasma membrane and that only the enzymes that are present in, or relocated to, the plasma membrane when platelets are activated are stimulated by thrombin. 3-kinase in the plasma membrane fraction. PtdIns 2