# Nicotinamide Adenine Dinucleotide (NAD) Metabolism as a Relevant Target in Cancer
## 6 Nicotinamide Phosphoribosyltransferase (NAMPT)  
Nicotinamide phosphoribosyltransferase (NAMPT), in addition to being the rate-limiting enzyme of the NAD+ salvage pathway, also acts as a cytokine-modulating inflammatory, metabolic, and immune responses [47; 48]. In fact, NAMPT was discovered in lymphocytes in human peripheral blood and was initially called pre-B colony-stimulating factor (PBEF) since it is involved in the maturation of B lymphocytes in the presence of interleukin 7 (IL-7) and stem cell factor (SCF) [49; 50]. This extracellular form of NAMPT is commonly called visatin to differentiate it from the intracellular form. Visfatin is secreted mainly by adipocytes, macrophages, lymphocytes and inflamed endothelial cells and is involved in inflammatory processes and metabolic disorders such as obesity and cancer. For example, adipocytes release visatin into the extracellular medium, where it exerts an insulin-mimetic function and regulates glucose and visceral fat levels related to obesity. In fact, high levels of visfatin have been detected in obese people [47; 51]. Interestingly, the visatin protein sequence lacks a secretion signal, so the protein is thought to be the same as intracellular NAMPT released after cell lysis [47; 52]. The intracellular form of NAMPT is expressed in all tissues, mainly in those that require more energy, such as bone marrow, liver and muscle. Within the cell, NAMPT is located mainly in the cytoplasm and nucleus [43; 50]. The NAMPT gene is located on chromosome 7 at the 7q22.3 locus and contains 11 exons and 10 introns [43; 49; 53]. Approximately 19 messenger RNAs and 14 variants have been predicted to be produced through alternative splicing (Figure 3). Of all these variants, only four likely produce proteins: variant 1 with a size of 491 amino acids (aa), variant 2 with 368 aa, variant 3 with 88 aa and variant 4 with 60 aa. The predominant variant is variant 1, NAMPT1, which is the only variant that possesses the enzymatic activity necessary for the conversion of NAM to NMN [53; 54]. The protein contains two recognised domains: DUF5593, which comprises the N-terminal aa 10-116 and whose function is unknown, and NAPRTase, which comprises aa 188-466 and is responsible for the enzymatic activity of the protein [55; 56]. The amino acids that make up the catalytic site are found at positions D219 (asparatic acid), G384 (glycine) and R392 (arginine). Additionally, amino acids R196 (arginine), H247 (histidine) and R311 (arginine) form the binding site of cofactor PRPP [57] (Figure 3A). To be functional, NAMPT1 must form a homodimer with two catalytic sites, each of which is made up of two aa from one chain and one aa from the other chain [58; 59] (Figure 3B) In mammals, it appears that NAMPT can be activated by autophosphorylation. Thus, for NMN synthesis to occur, histidine 247 must first be phosphorylated by ATP hydrolysis and in the presence of Mg\({}^{2+}\). Phosphorylated NAMPT has been shown to be more active than its unphosphorylated form. Once the enzyme is active, its substrates bind, first PRPP and then NAM, each to its corresponding binding site [53; 60; 61]. Figure 3: Structure of the NAMPT gene and protein. **(A)** The NAMPT gene contains 11 exons and 10 introns that are translated into 4 possible variants by alternative splicing or splicing. NAMPT1 is the predominant variant and the only one that has enzymatic activity. The protein contains 2 domains, DUF5593 and NAPRTase, in which the amino acids that form the catalytic site of NAM (D219, G384 and R392) and PRPP (R196, H247 and R311) are located. H247 a must first be phosphorylated by ATP hydrolysis for NMN synthesis to take place. **(B)** Crystal structure of the NAMPT functional homodimer (3 DHF; [[https://www.rcsb.org/structure/3](https://www.rcsb.org/structure/3) DHF]([https://www.rcsb.org/structure/3](https://www.rcsb.org/structure/3) DHF), Accessed on 1 April 2022). Catalytic sites are marked with dashed circles.