Description
A zinc enzyme, specific for NADPH. Catalyses the one-electron reduction of certain quinones, with the orthoquinones 1,2-naphthoquinone and 9,10-phenanthrenequinone being the best substrates. Dicoumarol [cf. EC 1.6.5.2 NAD(P)H dehydrogenase (quinone)] and nitrofurantoin are competitive inhibitors with respect to the quinone substrate. The semiquinonefree-radical product may be non-enzymically reduced to the hydroquinone or oxidized back to quinone in the presence of O2. In some mammals, the enzyme is abundant in the lens of the eye, where it is identified with the protein ζ-crystallin