Investigations into the relationship between structure and function of diphtheria toxin.

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Abstract
Studies on the structure-function relationship of diphtheria toxin [DT] 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 and a method for the labeling of nicked (1 peptide bond broken) DT with 131I, such that the label is confined to only 1 of the 2 polypeptide chains, are presented. Alterations of DT with specific reagents reveal that modifications of the tryptophan, methionine and arginine residues did not result in a significant loss in [animal] toxicity; treatment of the toxin with o-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; modification by tetranitromethane causes a loss in activity. 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 DT in cell-free systems and cell cultures. The B chain binds to membranes; it inhibits the action of nicked toxin on [human cervical carcinoma] HeLa cells.