Catalyzes the reduction of FAD with the concomitant oxidation of NADH. NAD is the physiological electron donor. Subsequently, the reduced flavins diffuse to the oxygenase component NphA2. a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH Optimum pH is 7.5. Homodimer. 4-nitrophenol 2-monooxygenase complex consists of an oxygenase component NphA1 and a flavin reductase component NphA2. By 4-NP in the presence of NphR. Belongs to the non-flavoprotein flavin reductase family.