Phosphorylates specifically NADH. Can phosphorylate NAD with a 100-fold decrease in efficiency compared to NADH. Prefers ATP as nucleoside triphosphate substrate. Can also utilize UTP, GTP and CTP. Key source of the cellular reductant NADPH which is an important antioxidant factor. ATP + NADH = ADP + H(+) + NADPH Two-fold decrease in activity in the presence of PPi, iodoacetate or para-chloromercuribenzoate. Measured at pH 7.9 and 25 degrees Celsius for all experiments. Optimum pH is 7.9 at 25 degrees Celsius. Homodimer. Ubiquitous. Belongs to the NAD kinase family.