The three-mammalian nicotinamide adenine dinucleotide (NAD+) biosynthesis pathways. De novo synthesis and the Preiss–Handler pathway start from nutritionally-derived tryptophan (essential amino acid) and nicotinic acid (NA), respectively. Both will eventually yield nicotinic acid mononucleotide (NAMN), which will be converted to nicotinic acid adenine dinucleotide (NAAD) via nicotinamide mononucleotide adenylyltransferases (NMNATs). NAAD conversion to NAD+ is catalyzed by nicotinamide adenine dinucleotide synthetase (NADS). Nicotinamide (NAM) constitute an important precursor for NAD+ inside the cell via the salvage (rescue) pathway. NAM is the product of several NAD+ dependent enzymes: sirtuins (SIRTs), poly (ADP-ribose) polymerases (PARPs), mono (ADP-ribose) transferases (MARTs), cluster of differentiation 38 (CD38) and CD157. NAM will be converted to nicotinamide mononucleotide (NMN) in the rate determining step of the salvage pathway via nicotinamide phosphoribosyltransferase (NAMPT) or visfatin. NAD+ may be regenerated from NMN via nicotinamide mononucleotide adenylyltransferase (NMNAT).