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). These compounds can be taken up from the diet and are termed vitamin B 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. There are some reports that mammalian cells can take up extracellular NAD from their surroundings, and both nicotinamide and nicotinamide riboside can be absorbed from the gut.