Reduction of protein disulfide bonds in an oxidizing environment

Abstract
Following retrograde transport to the endoplasmic reticulum (ER) the A‐subunit of cholera toxin (CTX‐A) is partially cleaved into CTX‐A1 and CTX‐A2 by reduction of a disulfide bridge [Majoul et al. (1996) J. Cell Biol. 133, 777–789], although the redox state in the ER favors disulfide formation. We show here that the disulfide bridge of CTX‐A is cleaved in vitro already at GSH/GSSG ratios between 1 and 3. Protein disulfide isomerase (PDI) exerts only a minor accelerating effect. Various mixed disulfide intermediates (CTX‐A1‐S‐S‐CTX‐A1; PDI‐S‐S‐A2; PDI‐S‐S‐A1) appear during CTX‐A reduction. These results indicate that in the ER protein disulfide formation and protein disulfide reduction can take place simultaneously.