The rat liver plasma membrane high affinity (Ca2+-Mg2+)-ATPase is not a calcium pump. Comparison with ATP-dependent calcium transporter.

The high affinity (Ca2+-Mg2+)-ATPase purified from rat liver plasma membrane (Lin, S.-H., and Fain, J. N. (1984) J. Biol. Chem. 259, 3016-3020) has been further characterized. This enzyme also possesses Mg2+-stimulated ATPase activity with K0.5 of 0.16 microM free Mg2+. However, the Vm of the Mg2+-stimulated activity is only half that of the Ca2+-stimulated ATPase activity. The effects of Ca2+ and Mg2+ on this enzyme are not additive. Both the Ca2+-stimulated ATPase and Mg2+-stimulated ATPase activities have similar affinities for ATP (0.21 mM and 0.13 mM, respectively) and similar substrate specificities (they are able to utilize ATP, GTP, UTP, CTP, ADP, and GDP as substrates); both activities are not inhibited by vanadate, p-chloromercuribenzoate, ouabain, dicyclohexylcarbodiimide, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, oligomycin, F-, N-ethylmaleimide, La3+, and oxidized glutathione. These properties of the Mg2+- and Ca2+-ATPases indicate that both activities reside on the same protein. A comparison of the properties of this high affinity (Ca2+-Mg2+)-ATPase with those of the liver plasma membrane ATP-dependent Ca2+ transport activity reconstituted into artificial liposomes (Lin, S.-H. (1985) J. Biol. Chem. 260, 7850-7856) suggests that this high affinity (Ca2+-Mg2+)-ATPase is not the biochemical expression of the liver plasma membrane Ca2+ pump. The function of this high affinity (Ca2+-Mg2+)-ATPase remains unknown.