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

Dihydronicotinamide riboside (NRH) has been 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 (ADK, also known as AK) acts as an NRH kinase. Using isotope-labelling approaches, we demonstrate that 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 mouse liver, pharmacological or genetic inhibition of AK blocks formation of reduced 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.