INTERACTION OF DIPHTHERIA-TOXIN AND ITS ACTIVE SUBUNIT, FRAGMENT-A, WITH TOXIN-SENSITIVE AND TOXIN-RESISTANT CELLS

Diphtheria toxin and purified fragment A, the active subunit of the toxin, were tested on toxin-sensitive and permeability class toxin-resistant cultured mammalian [human oral carcinoma KB-S, KB-R2A and KB-R2J cells, mouse fibroblast L929 and 3T3-FL cells, and HeLa cells] cells. Protein synthesis was inhibited to the same degree in sensitive or resistant cells by active concentrations of purified fragment A. Resistant cells required a concentration of whole toxin 5-6 logs greater than that required by sensitive cells to achieve the same degree of inhibition. On a molar basis, the toxicity of fragment A was equivalent to that of whole toxin on resistant cells. These results are evidence for the existence of 2 independent mechanisms for the entry of toxin or its active moiety into cells. One mechanism is a highly efficient toxin-specific entry mechanism, involving surface receptor and fragment B-mediated association, and is active in sensitive cells. The other is a less efficient nonspecific mechanism, probably related to endocytosis, which is operative in sensitive and resistant cells but is inapparent in sensitive cells when they are exposed to whole toxin because of the action of the specific mechanism.