The outer membrane form may mediate the reduction of outer membrane cytochrome b5 (PubMed:8001120). 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 (PubMed:8001120). Involved in the reduction of D-erythroascorbyl free radicals (PubMed:11420140). May play a minor role in complementing the function of CBR1 as a DPH3 reductase in the first step of diphthamide biosynthesis and tRNA Wobble base mcm5s 2U (5-methoxycarbonylmethyl-2-thiouridine) formation (PubMed:27694803). 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. Simultaneous disruption of CBR1 results in resistance to diphtheria toxin and K.lactis killer toxin. Present with 1920 molecules/cell in log phase SD medium. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.