Essential oxidoreductase that oxidizes proteins in the endoplasmic reticulum to produce disulfide bonds. Acts by oxidizing directly PDI1 isomerase through a direct disulfide exchange. Does not act as a direct oxidant of folding substrate, but relies on PDI1 to transfer oxidizing equivalent. Does not oxidize all pdi related proteins, suggesting that it can discriminate between PDI1 and related proteins. Its reoxidation probably involves electron transfer to molecular oxygen via FAD. Acts independently of glutathione. May be responsible for a significant proportion of reactive oxygen species (ROS) in the cell, thereby being a source of oxidative stress (By similarity). May function both as a monomer and a homodimer. Belongs to the EROs family.