The possibility that Ca 2+-ATPase from the plasma membrane-rich fraction of bovine parotid gland is ecto-Ca 2+-dependent nucleoside triphosphatase

Abstract

1. 1. Parotid plasma membrane nonpump low-affinity Ca 2+-ATPase, which possesses high-affinity (Ca 2+ + Mg 2+ )-ATPase activity, was characterized. 2. 2. Purified Ca 2+-ATPase hydrolyzed the nucleoside triphosphates, GTP, ITP, CTP, UTP, TTP (67–93% of ATP) and nucleoside diphosphates, ADP. GDP, IDP, CDP, TDP (12–40% of ATP) but not AMP and p-NPP. 3. 3. The maximum activities of Ca 2+- and (Ca 2+ +Mg 2+ )-ATPases were obtained in the presence of 1 mM and 0.13 μ M Ca 2+, respectively. 4. 4. The K m values for Ca 2+ in Ca 2+- and (Ca 2++ Mg 2+ )-ATPases were 0.2 mM and 22 nM. respectively. 5. 5. The activities of both Ca 2+- and (Ca 2+ + Mg 2+ )-ATPases were found in the right-side-out-vesicles obtained from the plasma membrane-rich fraction. 6. 6. These features suggest that Ca 2+-ATPase is an ecto-Ca 2+-dependent nucleoside triphosphatase.