Transfers two electrons from NADH to two molecules of cytochrome b5 through an enzyme-bound FAD. Electrons from the reduced cytochrome b5 are then transferred to various electron acceptors, thereby participating in methemoglobin reduction in red blood cells and in the desaturation and elongation of fatty acids, cholesterol biosynthesis and cytochrome P-450-mediated drug metabolism other tissues. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Component of a complex composed of cytochrome b5, NADH-cytochrome b5 reductase (CYB5R3) and MTARC2 (By similarity). Interacts with MTLN; the interaction is required to maintain cellular lipid composition and leads to stimulation of mitochondrial respiratory complex I activity (By similarity). Produces the soluble form found in erythrocytes. Isoform 1 and isoform 3 are ubiquitously expressed. Isoform 2 is expressed only in erythroid tissues, reticulocytes and liver. Only the isoform 1 is myristoyled. Produced by alternative promoter usage. Produced by alternative promoter usage. Produced by alternative initiation. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.