Investigations into the relationship between structure and function of diphtheria toxin

Studies on the structure-function relationship of diphtheria toxin are reported. New methods are described for the preparation of pure intact ("unnicked") toxin and for the preparation of the individual A and B chains. A biological assay method for the B chain is also presented, as well as a method for the labelling of "nicked" (one peptide bond broken) diphtheria toxin with 131I such that the label is confined to only one of the two polypeptide chains. Alterations of diphtheria toxin with specific reagents reveal that modifications of the tryptophan, methionine, and arginine residues did not result in a significant loss in toxicity, whereas treatment of the toxin with omicron-phthalaldehyde or by photooxidation with rose bengal results in a complete loss of the toxic activity. Modification of tyrosine by iodination results in active toxin, whereas modification by tetranitromethane causes a loss in activity. Preliminary results also indicate that the isolated A chain is about an order of magnitude more active in incorporating adenosine diphosphoribose into translocase (elongation factor 2) than whole or nicked toxin is under identical conditions. The observed structural properties are discussed in view of the functional activity of diphtheria toxin in cell-free systems as well as in cell cultures. Evidence is presented indicating that the B chain binds to membranes: it inhibits the action of nicked toxin on HeLa cells.