title: Calcium-binding properties and ATPase activities of rat liver plasma membranes.
abstract: Plasma membranes from rat liver purified according to the procedure of Neville bind calcium ions by a concentration-dependent, saturable process with at least two classes of binding sites. The higher affinity sites bind 45 nmol calcium/mg membrane protein with a K(D) of 3 microM. Adrenalectomy increases the number of the higher affinity sites and the corresponding K(D). Plasma membranes exhibit a (Na(+)-K(+))-independent-Mg(2+)-ATPase activity which is not activated by calcium between 0.1 microM and 10 mM CaCl(2). Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na(+)-K(+))-independent ATPase. Both Mg(2+)- and Ca(2+)-ATPase activities are identical with respect to pH dependence, nucleotide specificity and sensitivity to inhibitors. But when calcium is substituted for magnesium, there is no detectable membrane phosphorylation from [gamma-(32)P] ATP as it is found in the presence of magnesium. The existence of high affinity binding sites for calcium in liver plasma membranes is compatible with a regulatory role of this ion in membrane enzymic mechanisms or in hormone actions. Plasma membranes obtained by the procedure of Neville are devoid of any Ca(2+)-activated-Mg(2+)-ATPase activity indicating the absence of the classical energy-dependent calcium ion transport. These results would suggest that the overall calcium-extruding activity of the liver cell is mediated by a mechanism involving no direct ATP hydrolysis at the membrane level.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000311:Adrenal Glands / Q000502:physiology; D000315:Adrenalectomy; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism*; D005260:Female; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000201:enzymology / Q000378:metabolism*; D008274:Magnesium; D008862:Microsomes, Liver / Q000378:metabolism; D011188:Potassium; D012964:Sodium
chemical_list: D012964:Sodium; D000251:Adenosine Triphosphatases; D008274:Magnesium; D011188:Potassium; D002118:Calcium