Ca2+ transport by rat liver plasma membranes: the transporter and the previously reported Ca2+-ATPase are different enzymes.
A rat liver plasma membrane fraction showed an ATP-dependent uptake of Ca2+ which was released by the ionophore A23187. This activity represents a plasma membrane component and is not due to microsomal contamination. The Ca2+ transport displayed several properties which were different from those of the high-affinity Ca2+-ATPase previously observed in these membranes (Lotersztajn et al. (1981) J. Biol. Chem. 256, 11209-11215; Birch-Machin, M.A. and Dawson, A.P. (1986) Biochim. Biophys. Acta 855, 277-285). These observations have shown that Ca2+-ATPase does not require added Mg2+ whereas we have demonstrated that, in the same membrane preparation, Ca2+ uptake required millimolar concentrations of added Mg2+. The Ca2+-ATPase has a broad specificity for the nucleotides ATP, GTP, UTP and ITP while Ca2+ uptake remains specific for ATP. Ca2+ uptake also displayed different affinities for free Ca2+ and MgATP compared to Ca2+-ATPase activity, with apparent Km values of 0.25 microM Ca2+, 0.15 mM MgATP and 1.0 microM Ca2+, 4 microM MgATP respectively. The apparent maximum rate of Ca2+ uptake was about 150-fold less than Ca2+-ATPase activity. These features suggest that the high-affinity Ca2+-ATPase is not the enzymic expression of the ATP-dependent Ca2+ transport mechanism.
Birch-Machin
M A
MA
School of Biological Sciences, University of East Anglia, Norwich, U.K.
Dawson
A P
AP
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000493:pharmacokinetics*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000166:cytology* / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology; D009711:Nucleotides / Q000378:metabolism; D051381:Rats; D014638:Vanadates / Q000494:pharmacology
D009711:Nucleotides; D000001:Calcimycin; D014638:Vanadates; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium