Abstract
The properties of the Ca 2+-pump system of platelet microsomes isolated without Ca 2+-precipitating anions are studied. Passive Ca 2+ binding to the microsomes takes place in a noncooperative manner with K d = 0.7 μM. Half-maximal stimulation of ATP-dependent transport occurs at 0.4 μM Ca 2+. The velocity of Ca 2+ uptake, Ca 2+ capacity and the level of phosphoprotein in platelet microsomes are significantly lower than in cardiac microsomes. Energization of platelet and muscle microsomes and activation of intact platelets result in opposite charge redistribution in hydrophobic regions of the membranes. It is concluded that these charge movements are caused by Ca 2+ binding to and dissociation from nonpolar binding sites in the membranes.
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