Role of the essential thiol group in the thiol‐activated cytolysin from Clostridium perfringens

A hemolysin, θ‐toxin, produced by Clostridium perfringens has one cysteinyl residue in the free thiol form which is essential for its hemolytic activity. The cysteinyl residue was shown to be located at a position about 5 kDa from the C terminus of the molecule by the method of cysteine‐specific chemical cleavage. Modification of the residue with a thiol‐blocking agent, 5,5′‐dithiobis(2‐nitrobenzoic acid), reduced the binding affinity of the toxin to sheep erythrocytes to 1/100 that of intact toxin, resulting in a failure of binding at low cell concentrations (0.5%). Thus the failure of hemolysis at low cell concentrations is primarily ascribed to a decreased affinity of the toxin for erythrocytes. Effects of the modification on the lytic processes were examined using high cell concentrations where considerable amounts of modified toxin bound to the cells. The modified toxin hemolyzes erythrocytes once it binds to them; however, the efficiency of hemolysis is reduced by the modification. These, and additional results indicating that modification alters the sensitivity of toxin molecules to protease digestion, show that thiol‐modification inactivates the toxin by affecting both binding and the subsequent lytic processes, probably through a conformational change introduced in the toxin molecules.