Fig. 1.

NAD⁺ biosynthetic pathways. Two of the three pathways through which NAD⁺ can be synthesised in cells include the Preiss-Handler pathway and more commonly through the salvage of other metabolites. Nicotinic acid is consumed in feed and is the initial metabolite for the Preiss-Handler pathway. NAD⁺ is consumed by Sirtuins and Poly-ADP-ribose-polymerases, the first reaction in the salvage pathway. The metabolites in white boxes represent metabolites in the Preiss-Handler pathway while the metabolites in the light grey boxes represent those from the salvage pathway. NAD⁺ in the black box represents the end point for both NAD⁺ biosynthetic pathways.