The rate-limiting step in mammalian NAD biosynthesis from nicotinamide is the transfer of a phosphoribosyl residue from 5-phosphoribosyl-1-pyrophosphate (PRPP) to nicotinamide catalysed by nicotinamide phosphoribosyltransferase (Nampt) to produce nicotinamide mononucleotide (NMN), which is then converted to NAD by nicotinamide mononucleotide adenylyltransferase (Nmnat).

Nampt functions as an intra- and extracellular NAD biosynthetic enzyme. Nampt catalyses the formation of nicotinamide mononucleotide (NMN) from nicotinamide (NAM). NMN is subsequently converted to NAD by three organelle-specific isoforms of nicotinamide mononucleotide adenylyltransferase (Nmnat1-3). Intracellularly, Nampt has been shown to be located in different cellular compartments. It affects the function of NAD degrading enzymes by raising cellular NAD levels and consequently influences the regulation of metabolism and stress resistance through sirtuins (Sirt1-7) and other NAD consuming enzymes, e.g. poly(ADP-ribose)polymerase-1 (PARP-1). The NAD metabolism of mitochondria and nucleus is possibly influenced by transport of NAD metabolites from and to the cytoplasm. Extracellularly, Nampt produces nicotinamide mononucleotide (NMN), which might act in an autocrine/paracrine fashion and/or be transported to other target tissues, where it acts on glucose stimulated insulin secretion in pancreatic β cells and might also elicit other biological responses. Possibly, Nampt also functions as a cytokine by binding to and activating an unidentified receptor.