Structure-Activity Relationships in Diphtheria Toxin

Abstract The intact diphtheria toxin molecule, a single polypeptide chain of about 62,000 daltons, has no enzymic activity. However, the transfer in vitro of ADP-ribose from NAD to aminoacyltransferase II can be catalyzed by any of several fragments of toxin. The smallest active fragment (A, 24,000 daltons) is normally connected to the remainder of the molecule (B, 38,000 daltons) by a peptide bond and a disulfide bond. Both of these bonds must be broken to activate Fragment A. Thus, for example, toxin may be activated by mild digestion with trypsin in the presence of a thiol-reducing agent. The subsequent removal of Fragment B has no effect on the enzymic activity of Fragment A in vitro, but both fragments are required for intoxication of whole cells or of animals. Any enzymic activity observed in preparations of diphtheria toxin in the presence of a thiol derives from degradation products formed from the intact oxin after its synthesis by the diphtheria bacilli.