Catalyzes the four-electron reduction of molecular oxygen to water. Active on beta-NADH, but not on alpha-NADH, beta-NADPH or alpha-NADPH. Under aerobic conditions, oxygen acts as the electron acceptor. Under anaerobic conditions, DCIP and MB can replace oxygen as the electron acceptor. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 FAD per subunit. Inhibited by hydrogen peroxide, sulfhydryl reagents and quinine, but not by EDTA. Optimum pH is 6.0-9.0. Below pH 6.0 the enzyme is reversibly inactivated, above pH 10.0 the enzyme is irreversibly inactivated. Homodimer. Belongs to the class-III pyridine nucleotide-disulfide oxidoreductase family.