Nicotinamide adenine dinucleotide has several essential roles in metabolism. It acts as a coenzyme in redox reactions, as a donor of ADP-ribose moieties in ADP-ribosylation reactions, as a precursor of the second messenger molecule cyclic ADP-ribose, as well as acting as a substrate for bacterial DNA ligases and a group of enzymes called sirtuins that use NAD to remove acetyl groups from proteins. In addition to these metabolic functions, NAD emerges as an adenine nucleotide that can be released from cells spontaneously and by regulated mechanisms, and can therefore have important extracellular roles.