NADPH‐dependent coenzyme Q reductase is the main enzyme responsible for the reduction of non‐mitochondrial CoQ in cells

Abstract We purified an NADPH‐dependent coenzyme Q reductase (NADPH‐CoQ reductase) in rat liver cytosol and compared its enzymatic properties with those of the other CoQ 10 reductases such as NADPH: quinone acceptor oxidoreductase 1 (NQO1), lipoamide dehydrogenase, thioredoxine reductase and glutathione reductase. NADPH‐CoQ reductase was the only enzyme that preferred NADPH to NADH as an electron donor and was also different from the other CoQ 10 reductases in the sensitivities to its inhibitors and stimulators. Especially, Zn 2+ was the most powerful inhibitor for NADPH‐CoQ reductase, but CoQ 10 reduction by the other CoQ 10 reductases could not be inhibited by Zn 2+ . Furthermore, the reduction of the CoQ 9 incorporated into HeLa cells was also inhibited by Zn 2+ in the presence of pyrithione, a zinc ionophore. Moreover, NQO1 gene silencing in HeLa cells by transfection of a small interfering RNA resulted in lowering of both the NQO1 protein level and the NQO1 activity by about 75%. However, this transfection did not affect the NADPH‐CoQ reductase activity and the reduction of CoQ 9 incorporated into the cells. These results suggest that the NADPH‐CoQ reductase located in cytosol may be the main enzyme responsible for the reduction of non‐mitochondrial CoQ in cells.