Streptolysin O (SLO) is a membrane-damaging extracellular toxin produced by hemolytic streptococci. The membrane-damaging activity is measured by hemolysis of red-blood cell. SLO is oxygen-sensitive and is easily inactivated in its presence but can be reactivated by thiol compounds , so it is also called thiol-activated cytolysin (2). SLO is produced not only by Group A hemolytic streptococci but also by Group C and Group G strains. The amino acid sequences are highly conserved among them and their homology is over 98%.
The product was highly purified from E.coli over-expressing SLO of Group C hemolytic streptococci. The specific activity is as high as 1,900,000 hemolytic units (HU) /mg and forms a big hole on the cell membrane, which enables the introduction of protein inside the cells in vivo (Ref.1).