Ca2+ uptake by corpus-luteum plasma membranes. Evidence for the presence of both a Ca2+-pumping ATPase and a Ca2+-dependent nucleoside triphosphatase.

Plasma-membrane vesicles from rat corpus luteum showed an ATP-dependent uptake of Ca2+. Ca2+ was accumulated with a K1/2 (concn. giving half-maximal activity) of 0.2 microM and was released by the bivalent-cation ionophore A23187. A Ca2+-dependent phosphorylated intermediate (Mr 100,000) was detected which showed a low decomposition rate, consistent with it being the phosphorylated intermediate of the transport ATPase responsible for Ca2+ uptake. The Ca2+ uptake and the phosphorylated intermediate (E approximately P) displayed several properties that were different from those of the high-affinity Ca2+-ATPase previously observed in these membranes. Both Ca2+ uptake and E approximately P discriminated against ribonucleoside triphosphates other than ATP, whereas the ATPase split all the ribonucleoside triphosphates equally. Both Ca2+ uptake and E approximately P were sensitive to three different Hg-containing inhibitors, whereas the ATPase was inhibited much less. Ca2+ uptake required added Mg2+ (Km = 2.2 mM), whereas the ATPase required no added Mg2+. The maximum rate of Ca2+ uptake was about 400-fold less than that of ATP splitting; under different conditions, the decomposition rate of E approximately P was 1,000 times too slow to account for the ATPase activity observed. All of these features suggested that Ca2+ uptake was due to an enzyme of low activity, whose ATPase activity was not detected in the presence of the higher-specific-activity Ca2+-dependent ATPase.