Required to facilitate the formation of correct disulfide bonds in some periplasmic proteins and for the assembly of the periplasmic c-type cytochromes. Acts by transferring electrons from cytoplasmic thioredoxin to the periplasm, thereby maintaining the active site of DsbC, DsbE and DsbG in a reduced state. This transfer involves a cascade of disulfide bond formation and reduction steps. [protein]-dithiol + NAD(+) = [protein]-disulfide + H(+) + NADH [protein]-dithiol + NADP(+) = [protein]-disulfide + H(+) + NADPH The consequences of replacement of the cysteines with alanines were found to depend on the conditions tested and on the reporter system used for the analysis, and then differ depending on the references. Belongs to the thioredoxin family. DsbD subfamily.