Comparative kinetic stabilities of staphylococcal enterotoxin types A, B, and C1.

Staphylococcal enterotoxin types A and C, were observed by viscosimetry and near-ultraviolet difference spectroscopy to unfold at concentrations of aqueous guanidine hydrochloride greater than 1 M. Apparent rate constants of unfolding calculated from spectral curves differed markedly for the two enterotoxins. Rate constants for the unfolding of enterotoxin A in 2 or 3 M guanidine hydrochloride solution were 30to 40-fold larger in value than those observed for enterotoxin C,. In contrast, rate constants for the unfolding of enterotoxin C, in 4 or 5 M guanidine hydrochloride solution were only 4to 5-fold larger than those previously reported for enterotoxin type B (Warren, J. R., Spero, L., and Metzger, J. F. (1974) Biochemistq 13, 1678-1683). In addition, the types B and C, enterotoxins unfolded at nearly identical rates in 6 M guanidine hydrochloride and 8 M urea solution. Enterotoxin A unfolded about 50-fold faster in 8 M urea than enterotoxin B and C,. Therefore, unfolding of enterotoxin A by guanidine hydrochloride or urea appears to have a considerably lower activation energy than unfolding of enterotoxin B or C, by the denaturants. It is suggested that the observed differences in kinetic stability reflect a significant dissimilarity of the native structure of enterotoxin A to the native structures of enterotoxin B and C,. Enterotoxin A is known to demonstrate greater biological activity than enterotoxin B. Consequently, the dissimilarity of enterotoxin A structure indicated by the isothermal denaturation data is probably of functional importance.