Degradation of choleragen bound to cultured human fibroblasts and mouse neuroblastoma cells.

125I-choleragen bound to human fibroblasts was degraded slowly with a t1/2 of 2-3 days; the radiolabel in bound 125I-choleragen was present in both the A and B subunits. During degradation, radiolabel was lost more rapidly from the 125I-A1 (t1/2 approximately 2 days) than from the 125I-B peptides (t1/2 greater than 5 days). 125I-Choleragen bound to neuroblastoma cells showed a considerably shorter t1/2 for both the 125I-A1 and 125I-B peptides; as with the fibroblasts, radiolabel was lost more rapidly from the 125I-A1 than from the 125I-B peptides. The continued presence of choleragen in the fibroblasts and neuroblastoma cells was associated with a prolonged activation of adenylate cyclase. In addition, fibroblasts, previously exposed to toxin and then washed free of unbound choleragen, only slowly recovered their ability to bind 125I-choleragen with a t1/2 of 7 days. Fibroblasts exposed to choleragen also showed evidence of persistent toxin on the surface based on the ability of the cells to bind antitoxin, antisubunit A or antisubunit B antibodies followed by 3H-protein A. It appears that choleragen remains persistently bound to fibroblasts, is degraded at a slow rate, and may prevent the binding of new toxin molecules to the fibroblast. The relatively slow degradation of toxin by fibroblasts may explain the prolonged activation of adenylate cyclase by toxin. The loss of 125I-toxin binding capacity following incubation with toxin may result from continued presence of toxin subunits on the cell surface.