Characterization of acidic and neutral streptolysin O

We previously confirmed that crude streptolysin O (SLO) consists of heterogeneous components with different isoelectric points (pIs) and hemolytic efficiency, as determined by the slope of the titration curve. The two preparations of SLO having different pI distributions were further characterized in this research. The pIs of the major components of the two SLO preparations are around pH 6 (acidic) and 7.5 (neutral). Although the neutral SLO was shown to be more hydrophobic than the acidic SLO based on the behavior on hydrophobic interaction chromatography, the capacities of the two types to bind to erythrocytes did not differ. On the other hand, the neutral SLO destroyed liposomes to some extent, whereas the acidic SLO did not, even at concentrations of 100 HD50 (50% hemolytic dose), suggesting that the mode of membrane damage by the two types of SLO are different. Furthermore, electron microscopic observation indicated that the acidic SLO preferably formed an irregular-shaped complex and semicircular pores on erythrocyte membranes, in contrast to the complete pore formation by the neutral SLO. Thus, we clarified that the two hemolytic forms of SLO have different pore formation efficiencies based on liposome membrane damage and electron micrographs of ghosts.