Binding of 14C-ADP by thrombasthenic platelet membranes

We have measured the binding of 14C-ADP to isolated human platelet membranes by a technique using 0.8-mum Millipore filters to separate unbound tracer from membrane-bound nucleotide. The binding was dependent on the time of incubation and on the nucleotide concentration in the medium. The affinity constant was found to be comprised between 0.35 x 10(6) and 0.55 x 10 (6) M-1. Platelet membranes prepared from different thrombasthenic patients bound 14C-ADP with the same kinetic parameters as those from normal subjects. The affinity constant as determined for two of these thrombasthenic platelet membrane preparations was in the normal range.