SPECIFIC BINDING-SITES FOR INOSITOL 1,3,4,5-TETRAKISPHOSPHATE ARE LOCATED PREDOMINANTLY IN THE PLASMA-MEMBRANES OF HUMAN PLATELETS

In the present study we describe the characterization and localization of Ins(1,3,4,5)P4-binding sites in human platelet membranes. Specific binding sites for Ins(1,3,4,5)P4 have been identified on mixed, plasm a and intracellular membranes from neuraminidase-treated platelets usi ng highly purified carrier-free [P-32]Ins(1,3,4,5)P4. The displacement of Ins(1,3,4,5)P4 from these sites by Ins(1,4,5)P3 and InsP6 occurs a t greater than two orders of magnitude higher concentrations and with Ins(1,3,4,5,6)P5 at about 40-fold higher concentrations than with Ins( 1,3,4,5)P4. The membranes were further separated by free-flow electrop horesis into plasma and intracellular membranes. The Ins(1,3,4,5)P4-bi nding sites separated with plasma membranes, and showed similar affini ties and specificities as mixed membranes, whereas Ins(1,4,5)P3-bindin g sites were predominantly in the intracellular membranes. These resul ts suggest a predominantly plasma membrane location for putative Ins(1 ,3,4,5)P4 receptors in human platelets.