Effect of a zwitterionic surfactant (HPS) on the conformation and hemolytic activity of St I and St II,two isotoxins purified from Stichodactyla helianthus

N-hexadecyl-N-N-dimethyl-3-ammonio-1-propane-sulfonate (HPS) is a zwitterionic surfactant that readily binds to sticholysins I and II, two sea toxins isolated from Stichodactyla helianthus. The binding constants, evaluated from changes in fluorescence intensities elicited by the surfactant, are 0.5–0.7 M^–1. The binding of the surfactant changes the conformation of the tertiary protein, without significant changes in its secondary structure, as reported from far-ultraviolet circular dichroism spectra. The changes elicited by HPS lead to loss of the native conformation (as reported from near-ultraviolet circular dichroism spectra) and to a shift of the intrinsic protein fluorescence toward longer wavelengths, an increase in fluorescence intensities and lifetimes, and a faster quenching by acrylamide. All these changes are indicative of a more expanded tertiary conformation. Despite this, the toxins fully retain their hemolytic activities, indicating that spectroscopic changes can be poor predictors of toxin activity.