Protein disulfide-isomerase (PDI) is the best known representative of a growing family of enzymes with thioredoxin domains. Two such proteins with thioredoxin (Trx) domains, CaBP1 and CaBP2 (ERp72), have previously been isolated from rat liver microsomes. Here we report that they, like PDI are substrates for thioredoxin reductase and will catalyze NADPH-dependent insulin disulfide reduction. The activity of CaBP1 and CaBP2 in this assay was higher than that of PDI but lower than that of E. coli Trx. Furthermore, as isolated the thioredoxin domains of CaBP1 and CaBP2 were in disulfide form as judged by stoichiometric oxidation of 2 and 3 mol of NADPH in CaBP1 and CaBP2, respectively. The redox potential of the active site disulfide/dithiol was estimated from the equilibrium with a mutant E. coli Trx, P34H Trx, with a known redox potential (−235 mV). This showed that CaBP1 and CaBP2, like PDI, have a much higher redox potential than wild type thioredoxin (−270 mV) in agreement with a role in formation of protein disulfide bonds. In conclusion, in vitro CaBP1 and CaBP2 share catalytic properties in thiol disulfide-interchange reactions with PDI. Thus, the well known activity of PDI is not unique in the endoplasmic reticulum and CaBP1 and CaBP2 may be regarded as functional equivalents.
