Comparison of high affinity Ca2+-ATPase and low affinity Ca2+-ATPase in rat liver plasma membranes.
Rat liver plasma membranes contained two types of calcium-dependent adenosine triphosphatase (Ca2+-ATPase, EC 3.6.1.3) activities. One of them had a high affinity for free calcium (Ca2+) with an apparent half maximal saturation constant (K0.5) of 0.2 microM (high affinity Ca2+-ATPase), and the other exhibited a low affinity with a K0.5 of 50 microM for Ca2+ (low affinity Ca2+-ATPase). The high affinity Ca2+-ATPase showed: independence from free magnesium (Mg2+), a wide range of optimum pH (7.2-7.5), inhibition by a large amount of calmodulin, and substrate preference for ATP, GTP and ITP. On the other hand, the low affinity Ca2+-ATPase showed: stimulation by Mg2+ as well as Ca2+, an optimum pH of 8, mild stimulation by calmodulin, reversible inhibition by calmodulin-antagonists, inhibition by dicyclohexylcarbodiimide, and substrate preference for UTP and GTP. Both Ca2+-ATPases were insensitive to Na+, K+, ouabain, NaN3 and KCN. Orthovanadate, a potent inhibitor for many ATPases, had no effect on both ATPases over a wide range of concentrations (7 nM-1.7 mM). The Ca2+-ATPases could be separated by gel filtration on a Sepharose 4B column after solubilization with Triton X-100. The high affinity Ca2+-ATPase showed a Stokes radius of about 49 A and a sedimentation coefficient of about 7.0 S with a molecular weight of 1.4 X 10(5). The frictional ratio was 1.4. The results suggest that the high affinity Ca2+-ATPase may be a possible candidate for an ATPase with Ca2+ pumping activity, and that the high affinity enzyme is distinct from the low affinity Ca2+-ATPase in the rat liver plasma membranes.
D000818:Animals; D000252:Calcium-Transporting ATPases / Q000032:analysis* / Q000302:isolation & purification; D002147:Calmodulin / Q000494:pharmacology; D002462:Cell Membrane / Q000201:enzymology; D004492:Edetic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000201:enzymology*; D008274:Magnesium / Q000494:pharmacology; D008970:Molecular Weight; D051381:Rats; D011919:Rats, Inbred Strains; D012995:Solubility
D002147:Calmodulin; C005390:CDTA; D004533:Egtazic Acid; D004492:Edetic Acid; D000252:Calcium-Transporting ATPases; D008274:Magnesium