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. Other protein disulfide isomerase family members can also be reoxidized, but at lower rates compared to P4HB, including PDIA2, PDIA3, PDIA4, PDIA6 and NXNDC12. Following P4HB reoxidation, passes its electrons to molecular oxygen via FAD, leading to the production of reactive oxygen species (ROS) in the cell (By similarity). Involved in oxidative proinsulin folding in pancreatic cells, hence required for glucose homeostasis in vivo. Glutathione may be required to regulate its activity in the endoplasmic reticulum. Homodimer; disulfide-linked (By similarity). Heterodimer with ERO1A; disulfide-linked (By similarity). Also detected as monomer. Homodimers may be somewhat less active than monomers. The abundance of monomers and homodimers may be tissue-specific. Interacts with P4HB (By similarity). Interacts with ERP44 (By similarity). The association with ERP44 may be essential for its retention in the endoplasmic reticulum. Predominantly expressed in stomach and pancreas (mostly in the islets of Langerhans) (at protein level). N-glycosylated. The Cys-90/Cys-95 and Cys-393/Cys-396 disulfide bonds constitute the redox-active center. The Cys-90/Cys-95 disulfide bond may accept electron from P4HB and funnel them to the active site disulfide Cys-393/Cys-396 (By similarity). Belongs to the EROs family.