A high affinity Ca2+ -ATPase in C57 black mouse liver plasma membranes.

Plasma membranes of a cell contain 2 kinds of Ca2÷-extruding mechanisms to maintain [Ca 2÷] of cytoplasm at submicromolar levels, ATP-dependent and extracellular NaLdependent mechanisms. In plasma membranes of several tissues, the ATP-dependent Ca2+-pumps associate with (Ca 2+ + Mg2÷)-ATPase activity [ 1-3 ]. In the case of erythrocyte plasma membrane, a reconstitution study has clearly demonstrated the identity of the (Ca 2÷ + Mg2+)-ATPase and the Ca2+-extruding pump [4]. A variant type of (Ca 2÷ + Mg2÷)-ATPase has been shown in adipocyte and corpus luteum plasma membranes [5,6]. This ATPase has a high affinity for, Ca 2÷ with Ko.s of 0.14-0.3/aM and some other characteristics consistent with those of erythrocyte (Ca 2÷ + Mg2+)ATPase. However, this ATPase shows different sensitivity to Mg 2÷, calmodulin and calmodulin-antagonists from that of erythrocyte enzyme. In rat liver plasma membrane, no (Ca 2÷ + Mg2÷)ATPase had been detected although a Ca2÷-extruding activity in rat liver cells was shown to be ATP-dependent and extracellular Na+-independent [7,8]. A (Ca 2+ + Mg~+)-ATPase distinct from (Ca 2÷ + Mg2+)ATPases of erythrocyte, adipocyte and corpus luteum has been reported in rat liver plasma membranes [9]. This enzyme has very high affinities for Ca 2÷ (Ko.s = 13 nM) and Mg 2÷ (Ko.s = <12/aM), and is activated