Cytochalasin B binding by human platelets

Intact human platelets bind cytochalasin B (CB) with a capacity of 100-120 p mols CB/mg protein or approximately 7 x 10(4) molecules/cell and dissociation constants (KD) ranging from 2 x 10(-8) to 10(-6) M. Up to 85% of this saturable binding is displaced by 10(-5) M cytochalasin E (CE). This CE-sensitive binding also appears heterogeneous with KD similar to those of the overall binding. The CE-insensitive binding, however, appears as a single component with KD approximately equal to 4 x 10(-7) M. The sedimentable constituents from frozen, thawed, and washed cells also bind CB with KD ranging from 2.4 x 10(-8) to 1.5 x 10(-6) M and a total capacity of approximately 39 p mols/mg protein which accounts for only 4% of the ligand binding to the intact cell. The major portion (60-80%) of this CB binding is displaceable by 500 mM D-glucose and has a KD of 1.5 x 10(-6) M, while only 10-15% is CE-sensitive with a KD of 2.4 x 10(-8) M. It is concluded that 95% of the saturable CB binding in platelets is associated with the cytosol of which 80-85% is sensitive to CE and that only 3% of the cellular binding is glucose sensitive, membrane-associated binding. If the CE-sensitive binding associated with the cytosol is entirely to actin, the stoichiometry of this binding is approximately one CB to 30 actin monomers, which is greater by an order of magnitude than that for CB binding to muscle actin.