β-Nicotinamide adenine dinucleotide (NAD+) plays a major role in metabolism as a cofactor and mobile electron acceptor. NAD+ is a required oxidizing cosubstrate for many enzymes. It is reduced to NADH which carries electrons to the electron transport chain for subsequent oxidative phorphorylation and ATP production. NAD+ is capable of donating ADP-ribose moieties to a protein, producing nicotinamide in the process. Sirtuin enzymes use NAD+ as a substrate to deacetylate proteins and direct activity between the nucleus and mitochondria. NAD+ is regenerated by fermentation and by oxidative phosphorylation.