Evidence for Distinct Ca2+‐ATPase Activities in Gastric Microsomes

Abstract

Recently, a variant type of (Ca2+ + Mg2+)-ATPase stimulated by submicromolar Ca2+ has been reported in plasma membranes of erythrocyte,) Iiver,2 adipocyte,l corpus luteum,4 isolated bone cells,s and ascites hepatoma cells. The Ca2+ -stimulation of many of these A TPases2-5 was abolished by the addition of Mg2+ -a fact posing a major difficulty in demonstrating the high-affinity Ca2+ -activated ATPase. We previously reported? that purified dog gastric microsomes highly enriched in H+,K +ATPase activity also contain a high-affinity and a low-affinity Ca2+ -ATPase system. Our data suggested? the Ca2+ -ATPase activities to be a part of the Mg2+ -ATPase, which is always associated with the gastric microsomes. In this paper, we report complete separation of the H + ,K+ -ATPase from the Mg2 + -ATPase and the associated Ca2+ -ATPase activities, thus demonstrating that the latter activities are unrelated to the former.