<sod>Besides assembling NAD{{+}} ''de novo'' from simple amino acid precursors, cells also salvage preformed compounds containing nicotinamide. Although other precursors are known, the three natural compounds containing the nicotinamide ring and used in these salvage metabolic pathways are nicotinic acid (Na), nicotinamide (Nam) and [[nicotinamide riboside]] (NR).<ref name=Belenky/> These compounds can be taken up from the diet, where the mixture of nicotinic acid and nicotinamide is found in vitamin B{{sub|3}} or ''[[niacin]]''. However, these compounds are also produced within cells, when the nicotinamide moiety is released from NAD{{+}} in ADP-ribose transfer reactions. Indeed, the enzymes involved in these salvage pathways appear to be concentrated in the [[cell nucleus]], which may compensate for the high level of reactions that consume NAD{{+}} in this [[organelle]].<ref>{{cite journal | author = Anderson RM, Bitterman KJ, Wood JG, Medvedik O, Cohen H, Lin SS, Manchester JK, Gordon JI, Sinclair DA | title = Manipulation of a nuclear NAD{{+}} salvage pathway delays aging without altering steady-state NAD{{+}} levels | journal = J. Biol. Chem. | volume = 277 | issue = 21 | pages = 18881–90 | year = 2002 | pmid = 11884393 | doi = 10.1074/jbc.M111773200 }}</ref> Cells can also take up extracellular NAD{{+}} from their surroundings.<ref>{{cite journal | author = Billington RA, Travelli C, Ercolano E, Galli U, Roman CB, Grolla AA, Canonico PL, Condorelli F, Genazzani AA | title = Characterization of NAD Uptake in Mammalian Cells | journal = J. Biol. Chem. | volume = 283 | issue = 10 | pages = 6367–74 | year = 2008 | pmid = 18180302 | doi = 10.1074/jbc.M706204200 }}</ref><eod>
<sod>Besides assembling NAD{{+}} ''de novo'' from simple amino acid precursors, cells also salvage preformed compounds containing a pyridine base. The three vitamin precursors used in these salvage metabolic pathways are nicotinic acid (NA), nicotinamide (Nam) and [[nicotinamide riboside]] (NR).<ref name=Belenky/> These compounds can be taken up from the diet and are termed vitamin B{{sub|3}} or ''[[niacin]]''. However, these compounds are also produced within cells and by digestion of cellular NAD{{+}}. Some of the enzymes involved in these salvage pathways appear to be concentrated in the [[cell nucleus]], which may compensate for the high level of reactions that consume NAD{{+}} in this [[organelle]].<ref>{{cite journal | author = Anderson RM, Bitterman KJ, Wood JG, Medvedik O, Cohen H, Lin SS, Manchester JK, Gordon JI, Sinclair DA | title = Manipulation of a nuclear NAD{{+}} salvage pathway delays aging without altering steady-state NAD{{+}} levels | journal = J. Biol. Chem. | volume = 277 | issue = 21 | pages = 18881–90 | year = 2002 | pmid = 11884393 | doi = 10.1074/jbc.M111773200 }}</ref> There are some reports that mammalian cells can take up extracellular NAD{{+}} from their surroundings.<ref>{{cite journal | author = Billington RA, Travelli C, Ercolano E, Galli U, Roman CB, Grolla AA, Canonico PL, Condorelli F, Genazzani AA | title = Characterization of NAD Uptake in Mammalian Cells | journal = J. Biol. Chem. | volume = 283 | issue = 10 | pages = 6367–74 | year = 2008 | pmid = 18180302 | doi = 10.1074/jbc.M706204200 }}</ref><eod>