Oxidoreductase involved in disulfide bond formation in the endoplasmic reticulum. Efficiently reoxidizes P4HB/PDI, the enzyme catalyzing protein disulfide formation, in order to allow P4HB to sustain additional rounds of disulfide formation. Following P4HB reoxidation, passes its electrons to molecular oxygen via FAD, leading to the production of reactive oxygen species (ROS) in the cell. Required for the folding of immunoglobulins (By similarity). Enzyme activity is tightly regulated to prevent the accumulation of reactive oxygen species in the endoplasmic reticulum. Reversibly down-regulated by the formation of disulfide bonds between the active site Cys-90 and Cys-129, and between Cys-95 and Cys-100. Glutathione may be required to regulate its activity in the endoplasmic reticulum (By similarity). Predominantly monomer. May function both as a monomer and a homodimer (By similarity). The Cys-90/Cys-95 and Cys-392/Cys-395 disulfide bonds constitute the redox-active center. The Cys-90/Cys-95 disulfide bond may accept electron from protein disulfide isomerase (PDI) and funnel them to the active site disulfide Cys-392/Cys-395 (By similarity). Belongs to the EROs family.