The pyridine dinucleotide, NAD+, is a substrate of sirtuins and coenzyme for hydride transfer enzymes and other NAD+-dependent ADP ribose transfer enzymes. It is a unique cellular molecule produced from the pyridine mononucleotides nicotinic acid mononucleotide (NaMN) or nicotinamide mononucleotide (NMN). It is exist in either the form NAD+ or NADH. When being cleaved, NAD+ and NADH can convert to nicotinamide (Nam) and acetylated ADP ribose. The acetylated ADP ribose in turn can be deacetylated by sirtuins.