Enzymes which use NAD(H) as an electron acceptor or as a cofactor. Nicotinamide adenine dinucleotide, an important redox coenzyme that participates in a variety of enzymatic reactions in which it serves as an electron carrier by being alternately oxidized (NAD+) and reduced (NADH). NAD also functions as an ADP-ribose donor in ADP-ribosylation reactions.