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. Author manuscript; available in PMC: 2014 Sep 1.
Published in final edited form as: Diabetes Obes Metab. 2013 Sep;15(0 3):10.1111/dom.12171. doi: 10.1111/dom.12171

Figure 2. NAMPT-mediated NAD biosynthetic pathways in mammals.

Figure 2

In mammals, nicotinamide (NIC) is converted to nicotinamide mononucleotide (NMN) by the rate-limiting enzyme, nicotinamide phosphoribosyltranferase (NAMPT). There are two forms of NAMPT: intracellular and extracellular NAMPT (iNampt and eNampt, respectively). eNAMPT has significantly higher enzymatic activity than iNAMPT and likely synthesizes NMN in blood circulation. Extracellular NMN might be directly transferred into tissues/cells or is first converted to nicotinamide riboside (NR) and then transferred into tissues/cells. NR is re-converted to NMN by nioctinamide riboside kinase and utilized for NAD biosynthesis. NAD is generated from NMN by nicotinamide/nicotinic acid mononucleotide adenylyltransferase (NMNAT) and consumed by sirtuins, poly(ADP-ribose) polymerases (PARPs), and CD38.