Conversion of NADPH, generated by peripheral catabolic pathways, to NADH, which can enter the respiratory chain for energy generation. NAD(+) + NADPH = NADH + NADP(+) Binds 1 FAD per subunit. Homooligomer; probably composed of four stacked rings of 7 or 8 monomers. Forms filamentous structures. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.