In Complex I assay with intact mitochondria, reduction of cytochrome c at 550 nm, is it important to use an inhibitor of complex IV like cyanide or azide in the assay to prevent the further oxidation of reduced cytochrome c. Or we can follow the 'King and Howard's 1967' method with only solubilized and purified preparation of NADH dehydrogenase. Further, do we need to express the results in terms of NADH oxidized or cytochrome C reduced? Please answer. Thank you.