Effect of calcium ions on staphylococcal alpha-toxin-induced hemolysis of rabbit erythrocytes.

Calcium in millimolar concentrations protected rabbit erythrocytes from hemolysis caused by staphylococcal alpha-toxin. This effect was maximal at 30 mM CaCl2 and required the continued presence of calcium. The protection was not absolute and could be overcome by increased concentrations of alpha-toxin. Calcium did not block the binding of alpha-toxin to erythrocytes but inhibited the alpha-toxin-induced release of small ions from the cell as measured by 86Rb release. The transient removal of calcium was sufficient to abrogate its protective effect, suggesting that its action involves a reversible alteration in the state of the membrane. The three steps of the alpha-toxin-induced hemolytic sequence are: (i) binding to specific receptors, (ii) formation of transmembrane pores, and (iii) cell lysis. We concluded that calcium acted at step ii by impeding the lateral movement of alpha-toxin necessary to form the transmembrane hexamer pores.