Abstract: | The ionic influence and ouabain sensitivity of lymphocyte Mg2+-ATPase and Mg2+-(Na+ + K+)-activated ATPase were studied in intact cells, microsomal fraction and isolated plasma membranes. The active site of 5′-nucleotidase and Mg2+-ATPase seemed to be localized on the external side of the plasma membrane whereas the ATP binding site of (Na+ + K+)-ATPase was located inside the membrane.Concanavalin A induced an early stimulation of Mg2+-ATPase and (Na+ + K+)-ATPase both on intact cells and purified plasma membranes. In contrast, 5′-nucleotidase activity was not affected by the mitogen. Although the thymocyte Mg2+-ATPase activity was 3–5 times lower than in spleen lymphocytes, it was much more stimulated in the former cells (about 40 versus 20 %). (Na+ + K+)-ATPase activity was undetectable in thymocytes. However, in spleen lymphocytes (Na+ + K+)-ATPase activity can be detected and was 30 % increased by concanavalin A. Several aspects of this enzymic stimulation had also characteristic features of blast transformation induced by concanavalin A, suggesting a possible role of these enzymes, especially Mg2+-ATPase, in lymphocyte stimulation. |