FIG. 1.

Biosynthesis of NAD⁺. Five biosynthetic routes to NAD⁺ in mammalian cells are shown, with NAD⁺ itself in the gray box at lower right. (A) The Priess–Handler pathway is the main synthetic route to NAD from precursors such as NA. (B) NAD⁺ can also be salvaged from NAM, the product of many NAD⁺-consuming enzymes. (C) NMN can be synthesized from NR, which is orally bioavailable. (D) NAMN in the Priess–Handler pathway (A) can be synthesized de novo from the amino acid tryptophan. (E) NA or NAMN can be synthesized from NAR. For full discussion, see text (the NAD⁺ Synthesis and Compartmentation section). NA, nicotinic acid; NAD⁺, nicotinamide adenine dinucleotide; NAM, nicotinamide; NAMN, nicotinic acid mononucleotide; NAR, nicotinic acid riboside; NMN, nicotinamide mononucleotide; NR, nicotinamide riboside.