Binding of Adenosine Diphosphate by Isolated Membranes from Human Platelets

Binding of adenosine diphosphate by isolated membranes from human platelets has been demonstrated. The process was reversible, temperature-dependent, supported by calcium or magnesium ions, and influenced by the integrity of membrane protein sulfhydryl groups. Prior exposure of the isolated membranes to trypsin, chymotrypsin, or pronase resulted in significant inhibition of [14C]ADP uptake. Extraction of the bound radioactivity and subsequent thin layer chromatographic analysis revealed that the ADP was unchanged. GDP at relatively high concentrations interfered with ADP binding, whereas cyclic AMP, α,β-methylene ADP, adenosine, and PGE1 had no effect. AMP, ATP, and 2-chloroadenosine, known inhibitors of platelet aggregation, significantly blocked [14C]ADP binding. Mersalyl (10-4 m), an inhibitor of actomyosin-type ATPase activity, interfered with [14C]ADP binding by the membrane particles. Scatchard plot analysis suggested that there are approximately 100,000 binding sites per platelet.