Role of oxygen and metal ions in the instability of streptolysin O

Thiol‐containing preparations of streptolysin O (SLO) and pure cysteine generate superoxide radicals in alkaline buffer on autoxidation of the thiol groups. Autoxidation is stimulated by cupric ions. Reconstituted SLO preparations accumulate hydrogen peroxide with a concomitant loss of activity on storage at room temperature. Short‐term protection of hemolytic activity was achieved by inclusion of catalase in the preparation; no apparent protection was observed by superoxide dismutase, whereas 1,10‐O‐phenanthroline offered long‐term protection of the hemolysin.