Comparison between Ca2+ and Mg2+ on Surface‐located ATPase of Intact Normal and Neoplastic Human Cells in Culture

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Abstract
The present results support an earlier finding that the activity of a Mg2+-stimulated ATPase at the outer surface of glia cells was high compared with that of glioma cells, although the effects of Ca2+ or Mg2+ plus Ca2+ were not tested. The possibility that the Mg2+-ATPase at the surface of the tumor cells changed its characteristics and, e.g., becoming exclusively Ca2+-dependent was not supported by the present results. Support for this hypothesis may be adduced from the results obtained by Stefanovic et al. who reported a Ca2+-activated ATPase at the external surface of neuroblastoma cells. Even slight additions of Mg2+ ions decreased the activity with Ca2+ alone in their experiments. Although the divalent cation concentration in the work by Stefanovic et al. and in the present investigation was about the same, the amount of ATP differed. In the former investigations the ratio between ATP and Ca2+ was around 4 for maximal activity, whereas in the present experiments the ratio between ATP and divalent cations was about 0.01. This difference in ATP concentration amounting to a factor of 400 might explain the divergent results. The conformation and properties of the proteins at the outer surface of the cells are probably different at a high and low concentration of ATP in the external medium. The effects of ATP in the external medium are manifold, e.g., as a necessary substrate for protein kinase at the cell surface; as a regulator of cell volume, permeability and various expressions of cytokinesis involving the plasma membrane.