UPDATE ON GENE COMPLETIONS AND ANNOTATIONS Update of the NAD(P)H:Quinone oxidoreductase (NQO) gene family Vasilis Vasiliou,1* David Ross1and Daniel W. Nebert2 1 Molecular Toxicology and Environmental Health Sciences Program, Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Health Sciences Center, Denver, CO 80262, USA 2 Department of Environmental Health and Center for Environmental Genetics (CEG), University of Cincinnati Medical Center, Cincinnati, OH 45267-0056, USA * Correspondence to: Tel: þ 1 303 315 6153; Fax: þ 1 303 315 6281; E-mail: vasilis.vasiliou@uchsc.edu Date received (in revised form): 6th February 2006 Abstract The NAD(P)H:quinone acceptor oxidoreductase (NQO) gene family belongs to the ﬂavoprotein clan and, in the human genome, consists of two genes (NQO1 and NQO2). These two genes encode cytosolic ﬂavoenzymes that catalyse the beneﬁcial two-electron reduction of quinones to hydroquinones. This reaction prevents the unwanted one-electron reduction of quinones by other quinone reductases; one-electron reduction results in the formation of reactive oxygen species, generated by redox cycling of semiquinones in the presence of molecular oxygen. Both the mammalian NQO1 and NQO2 genes are upregulated as a part of the oxidative stress response and are inex- plicably overexpressed in particular types of tumours. A non-synonymous mutation in the NQO1 gene, leading to absence of enzyme activity, has been associated with an increased risk of myeloid leukaemia and other types of blood dyscrasia in workers exposed to benzene. NQO2 has a melatonin-binding site, which may explain the anti-oxidant role of melatonin. An ancient NQO3 subfamily exists in eubacteria and the authors suggest that there should be additional divisions of the NQO family to include the NQO4 subfamily in fungi and NQO5 subfamily in archaebacteria. Interestingly, no NQO genes could be identiﬁed in the worm, ﬂy, sea squirt or plants; because these taxa carry quinone reductases capable of one- and two-electron reductions, there has been either convergent evolution or redundancy to account for the appearance of these enzyme functions whenever they have been needed during evolution. Keywords: NAD(P)H:quinone oxidoreductase gene family, diaphorase, cytochrome b5 reductase, quinone reductase, aminoazo dye reductase, ﬂavoprotein clan, ﬂavodoxins, nitric oxide synthases, ribonucleotide reductases, ﬂavin reductases, melatonin Introduction Quinones comprise a large class of aromatic compounds, found widely in plants, in the general (benzoyl) form of C6H4O2, but also found in all organisms as ﬂavonoids, electron-carrying coenzymes and metabolic end-products of oxidation. Commercially, quinones are used in making dyes, tanning hides and in photography. Quinones are potentially dangerous, in that they can be involved in a one-electron reduction to form semiquinones that will elicit oxidative stress in any cell; benzoquinones and naphthoquinones are among the most reactive in this category. Quinones were undoubtedly present in the environment and in unicellular organisms between 2 and more than 3 billion years ago (at the time that partial pressure of ambient oxygen on the planet was rising). Hence, during this time it is very likely there was a strong selection to develop an enzyme that would reduce quinones to hydroquinones by a two-electron step, thereby avoiding free radical formation, which is highly destructive to the living cell. Background and history The NAD(P)H:quinone acceptor oxidoreductase (NQO) gene family is therefore believed to be ancient — probably between 2 and more than 3 billion years old. Cytosolic NQO ﬂavoenzymes catalyse the beneﬁcial two-electron reduction of quinones to hydroquinones. The two-electron reaction prevents the reduction of quinones by one-electron reductases, which would result in the formation of reactive oxygen species (ROS) generated by redox cycling of semiquinones in the presence of molecular oxygen.1,2 In animals, in addition to the likely role in detoxication of many dietary quinones derived from plants, NQO enzymes have been shown to catalyse the reductive activation of chemotherapeutic quinones such as mitomycins and aziridinyl- benzoquinones.3,4 The hydroquinones derived from these agents appear to be chemically more reactive, either because of rapid autoxidation or rearrangement — which, in either case, will produce reactive species that can kill malignant cells.5,6 q HENRY STEWART PUBLICATIONS 1473-9542. HUMAN GENOMICS. VOL 2. NO 5. 329–335 MARCH 2006 329