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). 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Soluble protein. Isoform 2 is expressed in testis, brain, skeletal muscle and in the male germline. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.