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 (PubMed:16973608). Interacts with MTLN; the interaction is required to maintain cellular lipid composition and leads to stimulation of mitochondrial respiratory complex I activity (By similarity). The enzyme exists in two forms, a membrane-bound form on the cytoplasmic side of the endoplasmic reticulum and also on the mitochondrial outer membrane and in soluble form in erythrocytes. NADH-cytochrome b5 reductase 3 membrane-bound form: Endoplasmic reticulum membrane; Lipid-anchor; Cytoplasmic side. Mitochondrion outer membrane; Lipid-anchor; Cytoplasmic side. NADH-cytochrome b5 reductase 3 soluble form: Cytoplasm. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.