Nicotinamide adenine dinucleotide (NAD) is best known as an essential cofactor of biochemical reactions. In addition, it is involved in the regulation of virtually all major cellular events. These NAD-dependent regulatory functions are mediated by enzymes (e.g., sirtuins, poly–adenosine -diphosphate [ADP] ribose polymerases, ADP-ribosyl cyclases) that cleave the molecule to liberate nicotinamide ...