The outer membrane form may mediate the reduction of outer membrane cytochrome b5, and the soluble inter-membrane space form may transfer electrons from external NADH to cytochrome c, thereby mediating an antimycin-insensitive, energy-coupled oxidation of external NADH by yeast mitochondria. Involved in the reduction of D-erythroascorbyl free radicals (By similarity). 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) By osmotic stress. There are two isoforms of NADH-cytochrome b5 reductase, a 34 kDa form (p34) and a 32 kDa form (p32). The p34 form becomes firmly anchored to the outer mitochondrial membrane after an incomplete translocation arrest. The p32 form is formed after translocation of the p34 precursor to the inner mitochondrial membrane, where it is processed by mitochondrial inner membrane peptidase (IMP) complex and released to the intermembrane space (By similarity). Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.