Iron (III) reduction: A novel activity of the human NAD(P)H:oxidoreductase

NAD(P)H:quinone oxidoreductase (NQO1; EC 1.6.99.2) catalyzes a two-electron transfer involved in the protection of cells from reactive oxygen species. These reactive oxygen species are often generated by the one-electron reduction of quinones or quinone analogs. We report here on the previously unreported Fe(III) reduction activity of human NQO1. Under steady state conditions with Fe(III) citrate, the apparent Michaelis–Menten constant (Kmapp) was ∼0.3 nM and the apparent maximum velocity (Vmaxapp) was 16 U mg−1. Substrate inhibition was observed above 5 nM. NADH was the electron donor, Kmapp=340μM and Vmaxapp=46Umg-1. FAD was also a cofactor with a Kmapp of 3.1 μM and Vmaxapp of 89 U mg−1. The turnover number for NADH oxidation was 25 s−1. Possible physiological roles of the Fe(III) reduction by this enzyme are discussed.