The properties of the Ca2+‐pump system of platelet microsomes isolated without Ca2+‐precipitating anions are studied. Passive Ca2+ 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 Ca2+. The velocity of Ca2+ uptake, Ca2+ 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 Ca2+ binding to and dissociation from nonpolar binding sites in the membranes.
Changes in the charge of sarcoplasmic reticulum (SR) vesicles are studied using lipophilic ions, which are adsorbed by the membrane phase. Upon addition of MgATP, phenyldicarbaundecaborane (PCB-) and tetraphenylboron (TPB-) are taken up by the SR vesicles, while tetraphenylphosphonium (TPP+) is released into the water phase. The PCB- uptake occurs as well under conditions when SR membrane is shunted by high Cl- concentration. MgATP induces minor additional binding of PCB- in the presence of oxalate and it is followed by release of the lipophilic anion from the vesicles. EGTA partly reverses the ATP effect, and calcium ionophore A23187 plus EGTA reverses it completely. Vesicles that were preliminarily loaded by Ca2+ demonstrated higher passive and lower ATP-dependent PCB- binding. Activation of isolated Ca2+-ATPase in the presence of 0.1 mM EGTA results in PCB- release into the medium and additional TPP+ binding to the enzyme. We suggest that the redistribution of the lipophilic ions between the water phase and SR membrane reflects charge changes in Ca2+-binding sites inside both SR vesicles and Ca2+-ATPase molecules in the course of Ca2+ translocation.
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