Calcium-Binding Properties and ATPase Activities of Rat Liver Plasma Membranes

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 µM. Adrenalectomy increases the number of the higher affinity sites and the corresponding K_D. Plasma membranes exhibit a (Na⁺-K⁺)-independent-Mg²⁺-ATPase activity which is not activated by calcium between 0.1 µM and 10 mM CaCl₂. Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na⁺-K⁺)-independent ATPase. Both Mg²⁺- and Ca²⁺-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 [γ-³²P] 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 Ca²⁺-activated-Mg²⁺-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.