Interaction of streptolysin-O with natural and artificial membranes.

1. Kinetic studies on the binding of 125I-streptolysin-O exhibited immediate fixation of activation toxin to natural and artificial membranes. Once fixed to the membrane no release of Streptolysin-O or Streptolysin-O-lipid-complexes has been observed. 2. In contrast to activated toxin (free SH-groups!), oxidized Streptolysin-O was shown to become also fixed to membranes, however, with different binding kinetics. The binding of oxidized amterial was clearly dependent on temperature and time. When the toxin was oxidized twice the amount of labelled material was bound as compared with the hemolytically active Streptolysin-O. This suggests that oxidized Streptolysin-O, too, possesses a "binding site" within the molecule, though free SH-groups were expected to be essential for toxin fixation at the membrane. It has been shown that oxidized (inactive) and reduced (active) Streptolysin-O forms stable "complexes" with liposomes in aqueous which could be separated by chromatography on Sepharose gels. 3. The binding of 125I-toxin to membranes and liposomes was specific since specific antisera against Streptolysin-O inhibited fixation of toxin completely. 4. Hydrolysis of phospholipids and release of lysophosphatides by Streptolysin O esterase (EC 2.1.2.2) has not been observed, thus providing no evidence for an enzymatic concept for membrane damage.