Alternative NADH-ubiquinone oxidoreductase which catalyzes the oxidation of mitochondrial NADH does not translocate protons across the inner mitochondrial membrane. a quinone + H(+) + NADH = a quinol + NAD(+) a ubiquinone + H(+) + NADH = a ubiquinol + NAD(+) Binds 1 FAD per subunit. Expressed at low levels in seedlings, roots, stems, buds and flowers and, to a lower extent, in leaves and cotyledons. Belongs to the NADH dehydrogenase family.