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

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.