title: The high affinity (Ca2+-Mg2+)-ATPase in liver plasma membranes is a Ca2+ pump. Reconstitution of the purified enzyme into phospholipid vesicles.
abstract: The purified (Ca2+-Mg2+)-ATPase from rat liver plasma membranes (Lotersztajn, S., Hanoune, J., and Pecker, F. (1981) J. Biol. Chem. 256, 11209-11215) was incorporated into soybean phospholipid vesicles, together with its activator. In the presence of millimolar concentrations of Mg2+, the reconstituted proteoliposomes displayed a rapid, saturable, ATP-dependent Ca2+ uptake. Half-maximal Ca2+ uptake activity was observed at 13 +/- 3 nM free Ca2+, and the apparent Km for ATP was 16 +/- 6 microM. Ca2+ accumulated into proteoliposomes (2.8 +/- 0.2 nmol of Ca2+/mg of protein/90 s) was totally released upon addition of the Ca2+ ionophore A-23187. Ca2+ uptake into vesicles reconstituted with enzyme alone was stimulated 2-2.5-fold by the (Ca2+-Mg2+)-ATPase activator, added exogenously. The (Ca2+-Mg2+)-ATPase activity of the reconstituted vesicles, measured using the same assay conditions as for ATP-dependent Ca2+ uptake activity (e.g. in the presence of millimolar concentrations of Mg2+), was maximally activated by 20 nM free Ca2+, half-maximal activation occurring at 13 nM free Ca2+. The stoichiometry of Ca2+ transport versus ATP hydrolysis approximated 0.3. These results provide a direct demonstration that the high affinity (Ca2+-Mg2+)-ATPase identified in liver plasma membranes is responsible for Ca2+ transport.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D007700:Kinetics; D008081:Liposomes; D008099:Liver / Q000166:cytology* / Q000201:enzymology; D010743:Phospholipids; D051381:Rats
chemical_list: D008081:Liposomes; D010743:Phospholipids; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium