Ca 2+ transport by rat liver plasma membranes: the transporter and the previously reported Ca 2+-ATPase are different enzymes

Abstract

A rat liver plasma membrane fraction showed an ATP-dependent uptake of Ca 2+ which was released by the ionophore A23187. This activity represents a plasma membrane component and is not due to microsomal contamination. The Ca 2+ transport displayed several properties which were different from those of the high-affinity Ca 2+-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 Ca 2+-ATPase does not require added Mg 2+ whereas we have demonstrated that, in the same membrane preparation, Ca 2+ uptake required millimolar concentrations of added Mg 2+. The Ca 2+-ATPase activity, with apparent K m values of 0.25 μM Ca 2+, 0.15 mM MgATP and 1.0 μM Ca 2+, 4 μM MgATP respectively. The apparent maximum rate of Ca 2+ uptake was about 150-fold less than Ca 2+-ATPase activity. These features suggest that the high-affinity Ca 2+-ATPase is not the enzymic expression of the ATP-dependent Ca 2+ transport mechanism.