Figure 1: Mammalian salvage pathway of NAD+ synthesis mediated by nicotinamide phosphoribosyltransferase. Nicotinamide, either derived from the degradation of NAD+ by NAD+ consuming enzymes or provided in the diet, is condensed with 5-phosphoribosyl-1-pyrophosphate to yield nicotinamide mononucleotide under the catalysis of nicotinamide phosphoribosyltransferase. Nicotinamide mononucleotide is then adenylylated to form NAD+, by Nicotinamide mononucleotide adenylyltransferase. NAD+ and NADH can be interconverted by a reduction and an oxidation reaction, respectively.