Observations on the Structure of Diphtheria Toxin

Diphtheria toxin is synthesized as a single polypeptide chain with a molecular weight of about 62,000. After being secreted it may be partially digested by proteolytic enzymes released from the bacterial cells into the culture medium and purified preparations of toxin usually contain some of the proteolytic breakdown products. In particular most toxin preparations contain a species (nicked toxin) in which two fragments of molecular weights 24,000 (A) and 38,000 (B) are linked by a single disulfide bond. Fragment B contains an internal disulfide bond. Fragment A is stable to denaturing agents, acid, alkali, and heat, whereas Fragment B forms aggregates in neutral buffers. Fragments similar to A and B are the first tryptic digestion products of reduced toxin but in the absence of thiol-reducing agents trypsin also rapidly hydrolyses a second peptide bond.