A CA2+-ATPASE FROM RAT PAROTID-GLAND PLASMA-MEMBRANES HAS THE CHARACTERISTICS OF AN ECTO-ATPASE

A Ca2+-ATPase with an apparent K(m) for free Ca2+ = 0.23-mu-M and V(max) = 44 nmol P(i)/mg/min was detected in a rat parotid plasma membrane-enriched fraction. This Ca2+-ATPase could be stimulated without added Mg2+. However, the enzyme may require submicromolar concentrations of Mg2+ for its activation in the presence of Ca2+. On the other hand, Mg2+ could substitute for Ca2+. The lack of a requirement for added Mg2+ distinguished this Ca2+-ATPase from the Ca2+-transporter ATPase in the plasma membranes and the mitochondrial Ca2+-ATPase. The enzyme was not inhibited by several ATPase inhibitors and was not stimulated by calmodulin. An antibody which was raised against the rat liver plasma membrane ecto-ATPase, was able to deplete this Ca2+-ATPase activity from detergent solubilized rat parotid plasma membranes, in an antibody concentration-dependent manner. Immunoblotting analysis of the pellet with the ecto-ATPase antibody revealed the presence of a 100,000 molecular weight protein band, in agreement with the reported ecto-ATPase relative molecular mass. These data demonstrate the presence of a Ca2+-ATPase, with high affinity for Ca2+, in the rat parotid gland plasma membranes. It is distinct from the Ca2+-transporter, and immunologically indistinguishable from the plasma membrane ecto-ATPase.