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 proper folding of immunoglobulins (By similarity). Plays an important role in ER stress-induced, CHOP-dependent apoptosis by activating the inositol 1,4,5-trisphosphate receptor IP3R1. 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-94 and Cys-130, and between Cys-99 and Cys-104. 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. Interacts with PDILT. Interacts with ERP44; the interaction results in retention of ERO1A in the endoplasmic reticulum. The association with ERP44 is essential for its retention in the endoplasmic reticulum (By similarity). In neurons, it localizes to dendrites (By similarity). Widely expressed (at protein level) (PubMed:20308425). In the mammary gland, expressed at higher levels in lactating mice than in virgin mice (at protein level) (PubMed:29858230). Stimulated by hypoxia; suggesting that it is regulated via the HIF-pathway. By ER stress in a DDIT3/CHOP-dependent manner. N-glycosylated. The Cys-94/Cys-99 and Cys-390/Cys-393 disulfide bonds constitute the redox-active center. The Cys-94/Cys-99 disulfide bond may accept electron from P4HB and funnel them to the active site disulfide Cys-390/Cys-393. The regulatory Cys-99/Cys-104 disulfide bond stabilizes the other regulatory bond Cys-94/Cys-130 (By similarity). Phosphorylated on Ser-144 by FAM20C in the Golgi which increases its enzymatic activity (By similarity). Phosphorylation is induced by lactation (PubMed:29858230). It is also induced by hypoxia and reductive stress (By similarity). Belongs to the EROs family.