Lysis of human red cells in vitro by an enzyme obtained from rabbit red cell hemolysates and the inhibition of this lytic activity by human stroma have been shown to require Mg++ and ATP, and ATP utilization has been demonstrated in both reactions. We find that sodium or potassium ions are also required for the lytic phenomenon and that they enhance the inhibition. The rate of hemolysis is not affected by the internal concentrations of these ions but depends only on the external concentration. The rate of influx and efflux of Na22Cl and K42Cl in surviving red cells is greatly enhanced both during and after treatment with rabbit hemolytic factor whereas the entry of C24-sucrose, a small foreign molecule, is mediated only in the presence of hemolytic factor. Glycolysis neither protects against lysis nor enhances the activity of this system, and cardiac glycosides which are known inhibitors of active transport of ions also have no effect. It appears that lysis in this system is not brought about by increased active transport of ions into the cell but that the rabbit factor degrades or combines with some membrane component, altering permeability and resulting in increased diffusion, first of sodium and potassium ions and other small molecules, and finally of large molecules (hemoglobin) out of the cell.

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