A Ca(2+)-ATPase from rat parotid gland plasma membranes has the characteristics of an ecto-ATPase.
A Ca(2+)-ATPase with an apparent Km for free Ca2+ = 0.23 microM and Vmax = 44 nmol Pi/mg/min was detected in a rat parotid plasma membrane-enriched fraction. This Ca(2+)-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 Ca(2+)-ATPase from the Ca(2+)-transporter ATPase in the plasma membranes and the mitochondrial Ca(2+)-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 Ca(2+)-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 Ca(2+)-ATPase, with high affinity for Ca2+, in the rat parotid gland plasma membranes. It is distinct from the Ca(2+)-transporter, and immunologically indistinguishable from the plasma membrane ecto-ATPase.
Cheung
P H
PH
Department of Pharmacology, University of Nebraska Medical Center, Omaha 68198-6260.
Dowd
F J
FJ
Porter
J E
JE
Li
L S
LS
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000502:physiology; D002462:Cell Membrane / Q000201:enzymology*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D010306:Parotid Gland / Q000201:enzymology*; D051381:Rats; D011919:Rats, Inbred Strains
D002147:Calmodulin; D000251:Adenosine Triphosphatases; C027201:ectoATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium