Thermal stability of different forms of diphtheria toxin

Diphtheria toxin and its enzymatically active A fragment have been examined by differential scanning calorimetry. The thermal stability was measured for different forms of these molecules, including tryptically nicked and intact, nucleotide-bound and free, cysteine alkylated, and cysteine oxidized and reduced. Three ranges of denaturation temperature have been observed among the different forms of diphtheria toxin studied, and there is a correlation of the thermal stability of these different forms with their biological activity. At the concentrations used for measurement, all forms of the 62,000-Da diphtheria toxin are irreversibly denatured by heating to 100 degrees C, while free A chain is reversibly denatured. In vivo and in vitro activities of the samples were measured before and after heating, and all were found to retain significant degrees of activity after heating.