OBSOLETE. A small disulfide-containing redox protein that serves as a general protein disulfide oxidoreductase. Interacts with a broad range of proteins by a redox mechanism, based on the reversible oxidation of 2 cysteine thiol groups to a disulfide, accompanied by the transfer of 2 electrons and 2 protons. The net result is the covalent interconversion of a disulfide and a dithiol.