NADH-cytochrome b5 reductase involved in endoplasmic reticulum stress response pathway. Plays a critical role in protecting pancreatic beta-cells against oxidant stress, possibly by protecting the cell from excess buildup of reactive oxygen species (ROS). Reduces a variety of substrates in vitro, such as cytochrome c, feericyanide and methemoglobin. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) E(0) is -108 mV. Soluble protein. Widely expressed. Variants Arg-187 and Arg-223 do not influence the pathogenesis of non-autoimmune diabetes. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.