Interaction of streptolysin-O with biomembranes: kinetic and morphological studies on erythrocyte membranes.

The kinetics of lytic events caused by the bacterial cytolytic toxin streptolysin-O (SLO) in red blood cells was examined using erythrocytes of several species of defined age and at different temperatures, by measurement of hemoglobin and ATP release. Lysis required much lower doses of SLO than hitherto described in the literature. Resistance to SLO varied within the different species, with the reaction temperature and increased with storage time (in vitro age). When erythrocytes treated with SH-activated SLO were examined in the electron microscope after negative staining or freeze-etching, ring- and arc-shaped structures were observed on the outer surface of their membranes. Identical, ring- and arc-shaped structures were also observed in SH-activated SLO solution alone. The findings indicate that SLO-SH complexes are formed upon activation and are not an SLO-cholesterol complex, as cholesterol was not detectable. These results led to a morphological model which proposes that the ring- and arc-shaped SLO complexes hitherto described are polymerized forms of single SLO molecules. A functional model which suggests a mode of action of SLO-SH complexes is also discussed. Analysis of freeze-fracture micrographs of SLO-treated erythrocytes revealed no indication of formation of membrane pores through which cell lysis could occur. Aggregation of inner membrane particles, however, indicated that the membrane integrity had been severely altered. Thus, hemoglobin and ATP most probably permeate the membrane at fragile areas.