The plasma membrane system in cancer cells is a site of many well-documented biological and biochemical changes. These include cell-cell interaction, response to growth factors, and enhanced transport of ions and nutrients, which may be driven by ATP. Relatively high Mg?2+?-ATPase activity that was insensitive to ouabain was observed in the purified plasma membranes of several human tumor xenografts in the athymic mice. One of our research objectives is to determine if the ATPase is implicated in the transport processes across the cell membrane. We have since discovered that the Mg?2+?-ATPase activity, i.e., Mg?2+?-dependent ATP hydrolysis, is contributed by several enzymes. Four ATP hydrolyzing activities can be demonstrated in the plasma membranes of human oat cell carcinoma. These are a low affinity Ca?2+?-ATPase; a Mg?2+?-ATPase; a monovalent ion-stimulated, Mg?2+?-dependent ATPase; and an ATP diphosphohydrolase. The low affinity Ca?2+?-ATPase, a 30 kilodalton protein, has been partially purified from oat cell carcinoma, which is especially abundant in this enzyme. The functions of these ATPases are not yet known, although it is certain that physiological functions are in part dictated by the sidedness of the ATPases. Both ecto-Mg?2+?-ATPase and ectoCa?2+?-ATPase are detected in cultured human tumor cells. In one hepatoma cell line, which appears to be an autocrine system, we found EGF, cholera toxin, and hydrocortisone induced the ecto-ATPase 5-to-10-fold. Our future research plan includes purification of these ATPases, their characterization and preparation of antibodies, and determination of their membrane orientation. Experimental evidence for the implication of these ATPases in transport will be provided by reconstitution techniques. (A)
