title: Calcium binding by human erythrocyte membranes.
abstract: 1. The characteristics of Ca(2+) binding to haemoglobin-free human erythrocyte membranes were investigated by using (45)Ca and centrifugation partition of ;ghosts' from their external incubation medium. Equilibrium of ;ghosts' with external Ca(2+) required less than 15min. 2. The binding did not vary with temperature in the range 0-37 degrees C. 3. At pH7.4 ;ghosts' bound a maximum of 283mumol of Ca(2+)/g of ;ghost' protein, equivalent to 6.85x10(7) Ca(2+) ions per cell. 4. Increasing the ionic strength from 0.01 to 0.46 diminished Ca(2+) binding, as did ATP in concentrations ranging from 0 to 15mm in the incubation medium. 5. An increase of the pH from 3.0 to 9.3 caused a marked increase in the amount of Ca(2+) bound. 6. Extraction of (45)Ca-labelled ;ghosts' with chloroform-methanol showed that the distribution of Ca(2+) was: 79% protein-bound, 16% lipid-bound, 5% in the aqueous phase, presumably non-bound. Most of the lipid-bound Ca(2+) (about 80%) was associated with a phospholipid fraction containing phosphatidylserine, phosphoinositides and phosphatidylethanolamine, giving a molar Ca(2+): phosphorus ratio of about 1:2.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002127:Calcium Isotopes; D002462:Cell Membrane / Q000378:metabolism*; D002725:Chloroform; D002855:Chromatography, Thin Layer; D004912:Erythrocytes / Q000378:metabolism*; D006461:Hemolysis; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008055:Lipids / Q000032:analysis; D000432:Methanol; D010714:Phosphatidylethanolamines / Q000032:analysis; D010716:Phosphatidylinositols / Q000032:analysis; D011485:Protein Binding; D013696:Temperature
chemical_list: D002127:Calcium Isotopes; D008055:Lipids; D010714:Phosphatidylethanolamines; D010716:Phosphatidylinositols; D002725:Chloroform; D000255:Adenosine Triphosphate; D002118:Calcium; D000432:Methanol