Abstract
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.
Highlights
IntroductionThe 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 disull?de bond
II can be catalyzed by any of several fragments of toxin
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
Summary
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 disull?de bond. 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. At slightly alkaline pH values the equilibrium is far to the right, and the forward reaction may be followed by measuring the accumulation of isotopically labeled acid-insoluble material when labeled NAD is supplied. This provides a simple method for estimating the activity of diphtheria toxin in vitro. In the “Discussion” we consider whether toxin may be activated in a similar manner when it acts upon whole cells
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
