MNADK, a novel liver-enriched mitochondrion-localized NAD kinase

NADP(+) and its reducing equivalent NADPH are essential for counteracting oxidative damage. Mitochondria are the major source of oxidative stress, since the majority of superoxide is generated from the mitochondrial respiratory chain. Because NADP(+) cannot pass through the mitochondrial membrane, NADP(+) generation within mitochondria is critical. However, only a single human NAD kinase (NADK) has been identified, and it is localized to the cytosol. Therefore, sources of mitochondrial NADP(+) and mechanisms for maintaining its redox balance remain largely unknown. Here, we show that the uncharacterized human gene C5ORF33, named MNADK (mouse homologue 1110020G09Rik), encodes a novel mitochondrion-localized NAD kinase. In mice MNADK is mostly expressed in the liver, and also abundant in brown fat, heart, muscle and kidney, all being mitochondrion-rich. Indeed, MNADK is localized to mitochondria in Hep G2 cells, a human liver cell line, as demonstrated by fluorescence imaging. Having a conserved NAD kinase domain, a recombinant MNADK showed NAD kinase activity, confirmed by mass spectrometry analysis. Consistent with a role of NADP(+) as a coenzyme in anabolic reactions, such as lipid synthesis, MNADK is nutritionally regulated in mice. Fasting increased MNADK levels in liver and fat, and obesity dramatically reduced its level in fat. MNADK expression was suppressed in human liver tumors. Identification of MNADK immediately suggests a model in which NADK and MNADK are responsible for de novo synthesis of NADP(+) in cytosol and mitochondria, respectively, and therefore provides novel insights into understanding the sources and mechanisms of mitochondrial NADP(+) and NADH production in human cells.