NRH salvage and conversion to NAD+ requires NRH kinase activity by adenosine kinase.

Abstract

Dihydronicotinamide riboside (NRH) was suggested to act as a precursor for the synthesis of NAD+ but the biochemical pathway converting it has been unknown. Here we show that NRH can be converted into NAD+ via a salvage pathway in which adenosine kinase (AK) acts as an NRH kinase. Using isotope labeling approaches, we demonstrate NRH is fully incorporated into NAD+, with NMNH acting as an intermediate. We further show that AK is enriched in fractions from cell lysates with NRH kinase activity and that AK can convert NRH into NAD+. In cultured cells and the mouse liver, pharmacological or genetic inhibition of AK blocks formation of nicotinamide mononucleotide (NMNH) and inhibits NRH-stimulated NAD+ biosynthesis. Finally, we confirm the presence of endogenous NRH in the liver with metabolomics. Our findings establish NRH as a natural precursor of NAD+ and reveal a new route for NAD+ biosynthesis via an NRH salvage pathway involving AK.