NAD+

Salvage pathways

Despite the presence of the de novo pathway, the salvage reactions are essential in humans; a lack of vitamin B3 in the diet causes the vitamin deficiency disease pellagra. This high requirement for NAD results from the constant consumption of the coenzyme in reactions such as posttranslational modifications, since the cycling of NAD between oxidized and reduced forms in redox reactions does not change the overall levels of the coenzyme. The major source of NAD in mammals is the salvage pathway which recycles the nicotinamide produced by enzymes utilizing NAD. The first step, and the rate-limiting enzyme in the salvage pathway is nicotinamide phosphoribosyltransferase (NAMPT), which produces nicotinamide mononucleotide (NMN). NMN is the immediate precursor to NAD+ in the salvage pathway.