Fig. 1: Pathways of NAD⁺ synthesis.

The figure summarizes the different biochemical routes for NAD⁺ formation. NAD⁺ can be synthesized de novo from tryptophan (Trp), which merges with the Preiss–Handler pathway, which also uses nicotinic acid (NA) for the processing of deamidated precursors. The salvage pathway utilizes amidated precursors and allows recycling of the nicotinamide (NAM) byproduct of NAD⁺-consuming enzymes. Reduced NMN (NMNH) and NR (NRH) are proposed to proceed through NADH to generate NAD⁺.