Reaction: NADPH + H+ + 2 quinone = NADP+ + 2 semiquinone
Other name(s): NADPH2:quinone reductase
Systematic name: NADPH:quinone oxidoreductase
Comments: 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 [1]. Dicoumarol [cf. EC 1.6.5.2 NAD(P)H dehydrogenase (quinone)] and nitrofurantoin are competitive inhibitors with respect to the quinone substrate. The semiquinone free radical product may be non-enzymically reduced to the hydroquinone or oxidized back to quinone in the presence of O2 [1]. In some mammals the enzyme is abundant in the lens of the eye, where it is identified with the protein ζ-crystallin.
Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, PDB, CAS registry number: 9032-20-6
References:
1. Rao, P.V., Krishna, C.M. and Zigler, J.S., Jr. Identification and characterization of the enzymatic activity of ζ-crystallin from guinea pig lens. A novel NADPH:quinone oxidoreductase. J. Biol. Chem. 267 (1992) 96-102. [PMID: 1370456]
2. Duhaiman, A.S. Kinetic properties of camel lens ζ-crystallin. Int. J. Biochem. Cell Biol. 28 (1996) 1163-1168. [PMID: 8930141]
3. Bazzi, M.D. Interaction of camel lens ζ-crystallin with quinones: portrait of a substrate by fluorescence spectroscopy. Arch. Biochem. Biophys. 395 (2001) 185-190. [PMID: 11697855]
4. Tang, A. and Curthoys, N.P. Identification of ζ-crystallin/NADPH:quinone reductase as a renal glutaminase mRNA pH response element-binding protein. J. Biol. Chem. 276 (2001) 21375-21380. [PMID: 11294877]