NADH-dependent cytochrome b5 reductase that reduces coenzyme Q6 at the plasma membrane and mediates lifespan extension by calorie restriction by shifting fermentative to respiratory metabolism, probably through modulating the NAD(+)/NADH ratio. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Inhibited by diphenylene iodonium (DPI). By calorie restriction. Present with 29000 molecules/cell in log phase SD medium. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.