quinones 
 are a subset of the quinoid family which also contains 
 the quinone imines and the quinone methides ( figure 1 ) . quinones , which contain the cyclohexadienedione 
 structure , are the most numerous of the quinoids and are commonly 
 found in several natural products , endogenous biochemicals , drugs , 
 and environmental chemicals and/or are generated through the metabolism 
 of aromatic compounds ( figure 2 ) . in general , the biological properties of quinone precursors 
 ( good , bad , or equivocal ) are often mediated by their oxidative metabolism 
 to quinones . however , the importance of quinone formation to the off - target 
 effects of drugs , natural products , environmental chemicals , and xenobiotic / endogenous 
 compounds in general is often underestimated . for example , two recent 
 reviews on biological reactive intermediates give varying estimates 
 on the importance of quinone formation . for carcinogenic processes , quinone formation was estimated at 7% of all mutagenic reactive intermediates , whereas for all toxic reactions ( 7% of total 
 metabolism ) , quinone formation represents closer to 41% . both numbers are likely low since quinones are 
 reactive intermediates and usually can not be observed directly . in 
 biological systems , as quickly as quinones are formed they react with 
 cellular nucleophiles ( gsh , protein sulfhydryls ) and/or they redox 
 cycle forming ros ( figure 3 ) . this perspective will focus on our limited knowledge of 
 the varied biological targets of quinones , resulting in numerous biological 
 effects , which could be beneficial , toxic , equivocal , and/or unknown . quinone methides and quinone imines have been reviewed recently and 
 are beyond the scope of this perspective . for 
 the purpose of this perspective , quinones that exist 
 in nature as the quinone form were defined as stable . in general , p - quinones are considerably more stable compared to o - quinones due to the strained 1,2-diketone functionality 
 of the latter . some p - quinones can be crystallized 
 ( t - butylquinone , menadione ) , and there are examples 
 of natural products with stable p - quinone structures 
 ( e.g. , juglone , ansamycin antibiotics ) as well as endogenous compounds 
 ( vitamin k , ubiquinone ) ( table 1 ) . ubiquinone is an antioxidant in the mitochondria , 
 and its analogues could be useful for the treatment of both alzheimer s 
 and parkinson s disease since mitochondrial failure , often 
 due to ros , is involved in the progression of both neurodegenerative 
 diseases . mitoquinone is a ubiquinone analogue in clinical 
 trials for the treatment of parkinson s disease . this novel quinone drug is designed to target the mitochondria through 
 its triphenylphosphine cationic moiety that facilitates drug accumulation 
 because of the negative mitochondrial membrane potential . idebenone 
 is another ubiquinone analogue which may have promise in the treatment 
 of alzheimer s disease ( table 1 ) . several potent chemotherapeutic 
 agents [ daunorubicin , diaziquone , doxorubicin ( adriamycin ) , emodin , 
 mitomycin c , mitoxantrone , and streptonigrin ; table 1 ] have highly substituted stable p - quinone functionalities , which likely contribute to their 
 ability to kill cancer cells through redox mechanisms ( discussed below ) 
 as well as cause reactive oxygen species ( ros)-induced toxicity to 
 normal cells . the naphthalene o - quinones miltirone isolated from 
 danshen and -lapachone from the pink trumpet tree are examples 
 of stable o - quinones ( table 1 ) . alphabetical 
 list ( 16 total , partial 
 list ) . hydroquinones and catechols 
 are very easily converted to quinones by oxidation catalyzed by virtually 
 any oxidative enzyme including p450s , cox-2 , peroxidase , tyrosinase , 
 xanthine oxidase and monoamine oxidase , and metal ions and in some 
 cases molecular oxygen ( table 2).p - quinones are formed from oxidation of t - butylhydroquinone , hydroquinone , naphthohydroquinone , 
 and thymohydroquinone . oxidation of catechols in natural 
 products [ caffeic acid , carnosic acid , catechin , ellagic acid , epigallocatechin 
 ( ecgc ) , hydroxychavicol , luteolin , procyanidin , quercetin , taxifolin , 
 and urushiols ] gives o - quinones with a variety of 
 biological effects ( table 2 ) . most of these o - quinones are short - lived , and their 
 contribution to the overall biological profiles of the parent compounds 
 is not clear . the endogenous catechol amines 
 ( 5,6-dihydroxytryptamine and dopamine ) are also oxidized to o - quinones which may play a role in the etiology of parkinson s 
 disease . dopamine oxidation to dopamine o - quinone as well as other quinones is believed to contribute to neurodegeneration 
 through induction of mitochondria and protein dysfunction and oxidative 
 stress ( table 2 ) . the selective estrogen receptor modulators ( serms ) 
 acolbifene and raloxifene form extended quinones in in vitro experiments , although both have very short lifetimes and may not 
 have any beneficial / toxic effects ( table 2 ) . extended quinone formation from the estrogen diethylstilbestrol 
 ( des ) may contribute to the teratogenic effects of des in women exposed 
 to this drug to prevent miscarriages and premature births in utero 
 up until its withdrawal in 1971 ( table 2 ) . [ o ] refers to any oxidative enzyme or metal ions and in some 
 cases molecular oxygen . aromatic rings represent 
 the most common functional group in drugs which suggests that the 
 general bioactivation scheme generating quinones should readily occur 
 as shown for benzene in figure 2 . for example , the anticonvulsant phenytoin causes skin toxicity in 
 510% of patients , and p450-catalyzed o - quinone 
 formation has been implicated as the culprit ( table 3 ) . the hepatotoxic effects of 
 the epilepsy drug , carbamazepine , has been shown to proceed through 
 a similar mechanism ( table 3 ) . the teratogen thalidomide is 
 metabolized to a phenol , catechol , and o - quinone 
 catalyzed by p450 which could contribute to the teratogenic mechanism 
 through o - quinone - induced oxidative stress and/or 
 protein binding ( table 3 ) . the environmental contaminants naphthalene , 
 bisphenol a , and some polychlorinated biphenyls ( pcbs ) readily form o- and p - quinones through this mechanism 
 ( table 3 ) . the serms ( droloxifene , lasofoxifene , ly2066948 , raloxifene , tamoxifen , 
 and toremifene ) all undergo one or two p-450 catalyzed aromatic hydroxylation 
 reactions followed by two - electron oxidation to o - quinones ( table 3 ) . generally , o - quinone formation from serms is a 
 minor bioactivation pathway , and the contribution of quinone metabolites 
 to the overall chemopreventive / toxic properties of the parent serms 
 are not known . the synthetic estrogen hexestrol , which is the saturated 
 derivative of des , is converted to the catechol by p450 followed by 
 two - electron oxidation to the o - quinone ( table 3 ) . two o - quinones are formed from the endogenous estrogens 
 ( estrone and estradiol , table 3 ) ; the 2-hydroxyestrogen o - quinone pathway 
 is likely a detoxification pathway since the o - quinone 
 formed is very unstable and readily isomerizes to a quinone methide . in contrast , the 4-hydroxyestrogen o - quinone is sufficiently long - lived to produce a variety of mutagenic / carcinogenic 
 effects including dna oxidation and adduct formation as described 
 below . estrogens in hormone therapy ( equilin , equilenin , 
 and 8,9-dehydroequilin ) , all form o - quinones through 
 initial p450-catalyzed aromatic hydroxylation followed by two - electron 
 oxidation ( table 3 ) . quinone formation from these estrogens could contribute 
 to the increased incidence of hormone - dependent cancer associated 
 with hormone therapy for menopausal symptoms . 17-ethinyl 
 estradiol , which is the most common estrogen in birth control pills , 
 is also converted to o - quinones which may contribute 
 to liver toxicity observed at high doses in animal models . however , the doses of ethinyl estradiol are 
 significantly lower than ht drugs , and quinone formation is unlikely 
 to contribute to clinical toxicity for birth control pills . finally , the natural products resveratrol and 
 genistein both form catechols and o - quinones by this 
 mechanism which may contribute to their chemopreventive properties in vivo . alphabetical 
 list ( 20 total , partial 
 list ) . [ o ] refers to any oxidative enzyme or metal ions and in some 
 cases molecular oxygen . another mechanism 
 of quinone formation involves p-450-catalyzed unmasking of phenols / catechols 
 through o - dealkylation reactions followed by two - electron oxidation . for example , the methoxy substituent in etoposide 
 is o - dealkylated generating a catechol which is readily oxidized to 
 an o - quinone ( table 4).o - quinone formation from etoposide likely 
 contributes to redox toxicity in cells through the generation of ros 
 contributing to side effects associated with this chemotherapeutic 
 drug . a similar mechanism is involved in the generation 
 of the extended quinone from arzoxifene ; initial o - dealkylation of 
 arzoxifene generates desmethylarzoxifene followed by two - electron 
 oxidation to the extended quinone ( table 4 ) . like the 
 raloxifene extended quinone described above , the arzoxifene quinone 
 has a very short half - life and may not make a significant contribution 
 to the overall biological effects of arzoxifene . eugenol ( cloves ) can also be metabolized by this pathway 
 initially generating hydroxychavicol followed by two - electron oxidation 
 to the o - quinone ( table 4).o - quinone formation from eugenol represents a minor 
 bioactivation pathway since direct two electron oxidation to the p - quinone methide is a more facile metabolic process . in an analogous mechanism , curcumin can be o - demethylated giving 
 the catechol , which is further oxidized to an o - quinone 
 which was implicated in curcumin induced oxidative dna damage ( table 4 ) . the synthetic phenolic antioxidant t - butylhydroxyanisole 
 ( bha ) also is o - demethylated by p450 to t - butylhydroquinone 
 and further oxidized to the t - butyl - p - benzoquinone ( table 4 ) . p450-catalyzed cleavage of the methylenedioxy 
 groups in isosafrole , 3,4-methylenedioxyamphetamine ( mda ) , 3,4-methylenedioxymethamphetamine 
 ( mdma , ecstasy ) , methysticin , myristicin , paroxetine , safrole , and 
 sitaxentan generate catechols which are readily oxidized to o - quinones by virtually any oxidative enzyme or metal ions 
 ( table 4 ) . these data suggest that quinone formation from compounds containing 
 a methylenedioxy substituent might be a general mechanism for o - quinone formation . [ o ] refers to any oxidative enzyme or metal ions and in some 
 cases molecular oxygen . polycyclic aromatic hydrocarbons ( pahs ) 
 are metabolized to o - quinones through a complex mechanism 
 involving initial p450-catalyzed arene oxide formation , ring opening 
 catalyzed by epoxide hydrolase , aldo - keto reductase ( akr ) mediated 
 formation of the catechol , and two - electron oxidation to o - quinone ( table 5 ) . many akrs 
 can also catalyze redox cycling ( see below ) by reducing the pah o - quinones to the catechol which autoxidize in air back 
 to the o - quinones consuming nadph . this pathway was found to be equally important with the 
 more commonly known diol - epoxide pathway to the activation of benzo[a]pyrene to a human lung carcinogen.o - quinone from pahs present in industrial 
 pollutants and tobacco smoke could make a major contribution to a 
 variety of human cancers . [ o ] refers to any oxidative enzyme or metal ions and in some 
 cases molecular oxygen . only one representative reference 
 is listed for each example . a more complex mechanism is also involved in quinone formation 
 from diabetic / anti - inflammatory drug troglitazone and the tocopherols 
 ( vitamin e ) ( table 5 ) . the troglitazone quinone likely contributes 
 to the idiosyncratic liver toxicity which led to the withdrawal of 
 troglitazone from the u.s . the highly substituted tocopherol p - quinone likely 
 contributes to the antioxidant effects and signaling mechanisms attributed 
 to the parent vitamin ( table 5 ) . the quinone formed from -tocopherol 
 has one less methyl group compared to that from -tocopherol 
 which could influence its biological effects in vivo . even though humans consume considerably more -tocopherol 
 in the diet compared to -tocopherol , -tocopherol is 
 selectively retained likely due to the enhanced reactivity / toxicity 
 of the less substituted -tocopherol quinone . quinones are 
 michael acceptors and readily react with soft nucleophiles like sulfhydryl 
 residues on cysteine in gsh and proteins ( figure 4 ) . p - quinones react via 
 1,4-reductive addition reactions regenerating the hydroquinone with 
 covalent attachment to the cysteine residue . unsubstituted o - quinones generally undergo 1,6-reductive addition reactions 
 with thiol nucleophiles due to extended conjugation , although 1,4-reductive 
 addition is often observed as a minor product ( figure 4 ) . there is some evidence 
 to suggest that the conjugation reaction with cysteine residues is 
 reversible , which could regenerate the quinone and free cysteine and 
 could play a role in quinone signaling mechanisms . the hydroquinone / catechol thioethers are more readily oxidized compared 
 to unsubstituted parent compounds , and quinone thioethers with multiple 
 cysteine substituents are often reported . reaction with nitrogen nucleophiles such as lysine , histidine , n - terminal 
 amino acids , and purine and pyrimidine bases on dna are much slower 
 compared to that of sulfur nucleophilic additions , although they do 
 occur . generally , reductive 1,4-michael additions 
 predominate over schiff base formation . very unstable o - quinones ( t1/2 < 1 s ) like those formed from genistein ( table 3 ) , luteolin , and quercetin o - quinones ( table 2 ) which have a c2c3 double bond 
 in the c ring are rapidly hydrated and unlikely to damage important 
 biological molecules . instead , quinone formation from these natural products might be 
 a chemopreventive mechanism through the keap1/nrf2 pathway as discussed 
 below . they are readily reduced 
 by one electron catalyzed by p450/nadph oxidoreductase generating 
 semiquinone radical anions ( figure 3 ) . these intermediates are very unstable and are easily 
 oxidized back up to quinones by molecular oxygen . the reduction of 
 molecular oxygen generates the first of the ros , the superoxide anion 
 radical . hydrogen peroxide can be reduced 
 by fenton chemistry through oxidation of iron and/or copper generating 
 the highly reactive hydroxyl radical . this futile redox cycling is 
 responsible for oxidative stress within cells and is a major contributor 
 to overall quinone toxicity . most quinones 
 can also be reduced by two electrons back to the hydroquinone or catechol 
 catalyzed by nadph / quinone oxidoreductase ( nqo1 ) , which represents 
 a major detoxification mechanism . in some cases 
 as with the pah o - quinones the 
 relative ability of quinones to redox cycle is highly dependent on 
 the ring substituents . electron rich substituents , extended ring systems , 
 and extensive substitution promote redox cycling since they stabilize 
 the semiquinone radical anion . this 
 often leads to a paradox where gsh conjugation does not result in 
 detoxification since the quinone - thioether quinone is more redox active 
 than the unsubstituted quinone due to the electron donating sulfur 
 substituent . in normal cells , a 
 strict balance is maintained between oxidation and reduction , and 
 anything that changes this delicate redox balance is thought to contribute 
 to a number of diseases . quinones cause 
 oxidative stress through the consumption of reducing equivalents including 
 gsh ( alkylation and oxidation ) and nad(p)h ( oxidation ) . production of ros can alter redox balance within 
 cells through the formation of oxidized cellular macromolecules including 
 lipids , proteins , and dna . formation of oxidatively damaged bases 
 such as 8-oxodeoxyguanosine ( 8-oxo - dg ) has been associated with aging 
 and carcinogenesis . furthermore , ros can activate 
 a number of signaling pathways including protein kinase c and ras . the redox potential between the quinone and 
 hydroquinone / catechol pair has a major effect on their prooxidant 
 and/or antioxidant effects and ultimately their cytotoxic versus cytoprotective 
 biological properties . for example , 
 it has recently been shown that the more easily p - hydroquinones are oxidized , the more potent their antiproliferative 
 effects are in mc38 cells . it is likely 
 that the equine catechol estrogens ( 4-hydroxyequilin , 4-hydroxyequilenin ) 
 and pah quinones , which only require oxygen to autoxidize to their 
 respective highly redox active o - quinones , cause 
 considerably more oxidative stress as compared to those formed from 
 the endogenous catechol estrogens from estradiol and estrone . these differences in redox activities 
 may explain variations in epidemiology studies on the link between 
 different types of hormone therapies and risk of breast cancer . the most important nonprotein 
 sulfhydryl 
 in cells is glutathione ( gsh ) whose major purpose is to protect cells 
 from reactive electrophiles and free radicals . as soft electrophiles , quinones are particularly susceptible 
 to gsh conjugation reactions , and usually , the reaction is so facile 
 that enzymatic catalysis by glutathione s - transferase 
 ( gsts ) is not required . model oxidation reactions with microsomes , 
 peroxidase / hydrogen peroxide , and tyrosinase , in the presence of gsh , 
 are all effective methods for generating and trapping quinones . under circumstances where quinone 
 formation is low and gsh levels are high ( i.e. , healthy liver cells , 
 510 mm gsh ) , nonenzymatic reaction with gsh generates reduced 
 gsh hydroquinone / catechol conjugates , which are readily excreted through 
 the mercapturic acid pathway ( figures 3 and 4 ) . along with two - electron 
 reduction catalyzed by nqo1 and or akr , gsh can also be 
 oxidized to gssg by hydroxyl radicals generated during redox cycling 
 of quinones . several drugs , environmental 
 chemicals , natural products , and endogenous compounds form quinone 
 gsh conjugates and/or cause gsh depletion . as gsh concentrations 
 are depleted , other sulfhydryl nucleophiles such as cysteine residues 
 on proteins become quinone biological targets . for highly reactive 
 quinones , the major proteins alkylated are those present in the highest 
 concentration containing the largest number of cysteine residues . for example , heat shock proteins [ hsp60 , hsp70 , hsp90 , and protein 
 disulfide isomerases ( pdis ) ] are produced in cells in response to 
 stressful conditions including the generation of electrophilic / redox 
 active quinones . these stress proteins are cysteine - rich 
 and thus ideal protein targets of quinones both for alkylation and 
 oxidation ( figure 5 ) . for example , using covert oxidatively activated tag ( coatag ) methodology 
 the extended quinone formed from two - electron oxidation of raloxifene 
 was shown to mainly modify the stress protein grp78/bip , three pdis , 
 and a microsomal gst in rat liver microsomes . pcb p - quinones also modify grp78/bip and 
 induce the unfolded protein response . the arylating quinone from -tocopherol but not the more substituted 
 -tocopherol the ansamycin quinones have potent antitumor 
 activity through mechanisms involving inactivation , destabilization , 
 and degrading hsp90 proteins . some dopamine o - quinones cause mitochondrial protein damage which has 
 been implicated in parkinson s disease . the dopamine quinones were found to inhibit the 20/26s 
 proteasome , which is a cytosol protein with numerous cysteines . there are a variety of protein targets of quinones , 
 and identifying reversible unstable protein adducts is challenging 
 for proteomic research , as well as determining the relative importance 
 of the identified biological targets to the overall cytotoxic / cytoprotective 
 effects . some specific quinone protein 
 targets are enzymes that catalyze their formation and detoxification 
 as well as proteins involved in cell signaling ( gst / mapk , keap1/nrf2 , 
 ikk / nf-b ) and nuclear receptor proteins [ arylhydrocarbon receptor 
 ( ahr ) , estrogen receptor ( er ) ] as outlined below . since p450 is the most likely 
 enzyme catalyzing quinone formation , it is to be expected that particularly 
 reactive quinone metabolites have been reported to be inhibitors of 
 p450s . for example , 4-hydroxytamoxifen 
 and raloxifene were found to be potent mechanism - based inactivators 
 of p450 as a result of quinone - mediated alkylation of the apoprotein . with raloxifene , analysis of the p450 3a4 peptide showed that cys239 
 was the site of the raloxifene extended quinone reaction . sitaxentan contains a methylenedioxy ring , 
 which is metabolized by p450 to the catechol and oxidized to the o - quinone , which could be responsible for the reported inhibition 
 of p450 3a4 ( table 4).o - quinone formation 
 by sitaxentan could be responsible for the reports of idiosyncratic 
 drug toxicity , which led to its worldwide withdrawal in december , 
 2010 . similarly , the methyenedioxy group in myristicin is oxidized 
 to an o - quinone leading to mechanism - based inhibition 
 of p4501a2 . gsh was shown to block the 
 inhibition which suggests that the o - quinone was 
 the inhibitor and not the carbene formed from the methylenedioxy moiety . the ssri paroxetine was shown to be an inhibitor 
 of p450 2d6 , and the o - quinone metabolite was implicated 
 as the reactive intermediate responsible for the inhibitory effect 
 which ultimately leads to clinical drug  drug interactions with 
 other p450 2d6 substrates . finally , the methoxyestrogens 
 are feedback inhibitors of p450 1a1 and p450 1b1 likely through the 
 formation of the catechol estrogen o - quinones . several 
 synthetic and natural 
 stable quinones have been reported to have anti - inflammatory activity 
 through a cox-2 inhibitory mechanism with a potency similar to indomethacin . for example , nanomolar ic50s have 
 been reported for jugalone and thymoquinone against human recombinant 
 cox-2 activity ( table 1 ) . the catechol metabolite of resveratrol , 
 piceatannol ( table 3 ) , was found to be a selective cox-2 inhibitor with potency similar 
 to that of celecoxib which may contribute to the chemopreventive properties 
 of both compounds . the diethylester prodrug , diacerein ( rhubarb ) , which is hydrolyzed 
 to the stable p - quinone rhein ( table 1 ) , has been shown to be effective for treatment 
 of osteoarthritis . like p450 , the peroxidase activity 
 of cox-2 catalyzes the oxidation of catechols and hydroquinones during 
 prostaglandin biosynthesis , and these quinones can inhibit the enzyme . for example , the peroxidase activity of cox-2 
 catalyzes the formation of the dopamine quinone , which is implicated 
 in the pathogenesis of parkinson s disease ( table 2 ) . it has been suggested that cox-2 inhibitors may be valuable therapies 
 aimed at slowing the progression of this neurodegenerative disease . similarly , cox-2 catalyzed formation of the 
 methamphetamine o - quinone as well as endogenous neurotransmitter o - quinones causing oxidative stress and mitochondrial protein 
 damage has been implicated in neurotoxicity and aging . finally , cox-2 has been shown to oxidize diethylstilbestrol and 
 catechol estrogens to the extended quinone and o - quinones , 
 respectively ( table 3 ) . in addition 
 to inhibiting enzymes 
 which catalyze their formation , quinones can also inhibit enzymes 
 which detoxify them or their hydroquinone / catechol precursors . for 
 example , catechol o - methyl transferase ( comt ) catalyzes 
 the conversion of catechols to methoxyethers which can not be further 
 oxidized to quinones and therefore represents a catechol detoxification 
 mechanism . the equine estrogen metabolites 4-hydroxyequilenin and 
 4-hydroxyequilin have been shown to be potent inhibitors of comt likely 
 because they can autoxidize to o - quinones ( table 3 ) . inhibition of comt - mediated catechol estrogen clearance may play 
 a role in the toxicity of endogenous estrogens and estrogens in ht . the catecholamine quinones are also potent 
 irreversible inhibitors of comt which could contribute to the neurotoxicity 
 mechanism ( table 3 ) . two - electron reduction of quinones 
 to catechols and hydroquinones is a major detoxification mechanism 
 for most quinones . however , some quinones 
 have been shown to be mechanism - based inhibitors of nqo1 . for example , 
 the p - quinone es936 ( table 1 ) is a nanomolar inhibitor of nqo1 in cell - based 
 assays . similarly , the antitumor agent 
 mitomycin c ( table 1 ) undergoes bioreductive activation catalyzed by nqo1 , but it is 
 also a mechanism - based inhibitor of the enzyme both in vitro and in vivo . since 
 nqo1 activity is much higher in cancer cells compared to that in normal 
 cells , quinone - mediated inhibition of nqo1 could be an effective anticancer 
 strategy . for example , the natural product 
 deoxynyboquinone is a potent nqo1 substrate and inhibitor through 
 an nqo1-catalyzed redox cycling mechanism and deoxynyboquinone could 
 have considerable potential as a chemotherapeutic agent ( table 2 ) . gsts are 
 often covalently modified 
 and/or oxidized by quinones likely in an attempt by gsts to catalyze 
 detoxification of quinones . this usually results in the inhibition of gsts , and the effect on 
 gst p11 in particular could play a role in regulating the 
 c - jun n - terminal kinase ( jnk ) cell signaling pathway . jnk is a mapk 
 which effects cell growth , differentiation , stress response , and cellular 
 transformation . gst p11 forms an inhibitory complex with jnk  c - jun 
 that inhibits the phosphorylation of c - jun by jnk . quinone mediated 
 oxidative or covalent modification of gst p11 leads to activation 
 of jnk phosphorylation of c - jun resulting in the induction of stress 
 response , differentiation , and proliferation . some specific examples 
 include metabolites of the equine estrogens 4-hydroxyequilenin o - quinone and 4-hydroxyequilin o - quinone , 
 which were found to be potent irreversible inhibitors of gst p11 
 ( table 3 ) . in contrast , the o - quinones of the endogenous catechol estrogen 4-hydroxyestrone o - quinone did not significantly inhibit gst p11 
 activity in human breast cancer cells perhaps due to differences in 
 the reactivity of these estrogen quinones . plant polyphenols including quercetin , ellagic acid , juglone , luteolin , 
 and caffeic acid have been reported as gst inhibitors likely through 
 quinone formation . para and o - quinones were among the first characterized 
 small molecule inducers of the keap1/nrf2/are pathway . this pathway controls gene expression of an 
 elaborate network of protective proteins including heme oxygenase , 
 nqo1 , akt , and gst that defend cells from electrophiles and free radicals . under normal conditions , the levels of these protective enzymes 
 is low due to the repressor function of keap1 , which sets up nrf2 
 for ubiquitination and proteosomal degradation ( figure 6 ) . modification of keap1 by either alkylation or oxidation 
 of crucial cysteine residues ( cys23 , 151 , 273 , and 288 ) leads to the 
 loss of the repressor function of keap1 , increased stability of nrf2 , 
 translocation of nrf2 to the nucleus , and activation of are - mediated 
 target genes . for example , bha and its metabolite t - butylhydroquinone 
 are classical activators of nrf2 genes resulting in potent induction 
 of heme oxygenase and nqo1 ( table 4 ) . it has been shown 
 that catechol estrogen o - quinones ( table 3 ) can covalently modify keap1 
 leading to nrf2 activation and induction of heme oxygenase . similarly , catechols from green tea including 
 egcg and its epimer gallocatechin gallate and flavonoids such as quercetin 
 form o - quinones can activate the keap1/nrf2/are pathway 
 leading to cytoprotection ( table 2 ) . in addition , the quercetin quinone methide formed by isomerization 
 of the quercetin o - quinone could also contribute 
 to keap1/nrf2/are activation . interestingly , 
 the level of induction could be enhanced by pretreatment with the 
 gsh synthesis inhibitor buthionine sulfoximine and diminished in the 
 presence of the electrophile scavenger n - acetylcysteine . other natural product catechols including carnosic acid and caffeic 
 acid had analogous keap1 modulatory effects through their corresponding o - quinone mediated mechanisms ( table 2 ) . activators of the keap1-nrf2 
 pathway are generally considered as 
 chemopreventive agents since they enhance the levels of detoxification 
 enzymes . however , it is important to realize that nrf2 is very much 
 involved in the initiation , promotion , and metastasis of cancer . in fact , cancer cells hijack nrf2 to support their malignant growth , 
 and activators of nrf2 could have unwanted effects including the development 
 of resistance to chemotherapeutic agents . for example , t - butylhydroquinone upregulated nrf2 and enhanced the resistance of 
 cancer cells to cisplatin , doxorubicin , and etoposide in neuroblastoma 
 cells . similarly , the electrophilic 
 and prooxidant signals produced from pah o - quinones 
 can lead to the induction of are - regulated genes which could influence 
 the initiation and promotion stages of pah carcinogenesis ( table 4 ) . quinone nrf2 activators could be an effective strategy for cancer 
 chemoprevention ; however , the effect of quinone exposure on long - term 
 nrf2 levels are not known , and they may not be safe for cancer patients . finally , based on the crucial role of nrf2 in redox homeostasis in 
 cells , it is likely that the dose response curve for quinone 
 nrf2 activators is u - shaped and that cytoprotection / chemoresistance 
 depends on the amount of quinone formed as well as its electrophilic / redox 
 activity ( figure 7 ) . inverse u - shaped modulation of toxicity as a function 
 of dose and 
 reactivity of quinone . nf-b proteins are a family 
 of transcription factors that play a major role in inflammation and 
 carcinogenesis . nf-b activity is regulated 
 by the ib proteins , which are controlled through phosphorylation 
 by upstream ib kinases ( ikk ) . these kinases are attractive 
 targets for electrophilic quinones , which can covalently modify ikk 
 leading to the inhibition of ikk and nf-b - mediated gene transcription 
 resulting in anti - inflammatory activities . for example , it has been 
 shown that 1,2-naphthoquinone and -lapachone ( table 1 ) inhibited phosphorylation 
 of ib by ikk and disrupted nf-b signaling . similarly , the catechol metabolite of resveratrol , piceatannol ( table 3 ) , is oxidized to 
 an o - quinone , which targets cys179 of ikk leading 
 to inhibition of phorbol ester induced nf-b activation . the antitumor anthroquinone denbinobin also inhibits nf-b 
 through the modulation of ikk in human leukemic cells , and the inhibitory 
 effects could be prevented by n - acetylcysteine . the ansamycin antibiotic herbimycin a ( table 1 ) preferentially inhibits 
 ikk likely through interaction with cys59 , resulting in the prevention 
 of expression of nf-b - dependent genes and anti - inflammatory 
 activity . in contrast , the carcinogenic 
 metabolite of estradiol , 4-hydroxyestradiol ( table 3 ) , is oxidized to an o - quinone 
 which generates ros causing the activation of ikk and nf-b 
 leading to the transformation of mcf-10a breast cells into a malignant 
 phenotype . it has 
 been shown that pah o - quinones and 1,2-naphthoquinone 
 can covalently modify ahr leading to the activation of the ahr / xre 
 pathway and induction of p450 1a1 . similar results 
 have been reported for 1,4-benzoquinone , t - butyl-1,4-benzoquinone , 
 and pcb quinones suggesting that some quinones could be bifunctional 
 inducers of both phase i and phase ii enzymes . interaction of these genotoxic quinones with ahr targets them into 
 the nucleus where they can redox cycle generating ros and cause oxidative 
 damage to dna in a manner similar to the trojan horse estrogen receptor 
 mechanism and 4-hydroxyequilenin ( see below ) . in support of this mechanism , the levels of dna strand 
 breaks and 8-oxo - dg levels were lower in ahr - deficient hepa and ahr 
 knockdown h358 cells treated with the pah o - quinone , 
 benzo[a]pyrene-7,8-dione . a similar trojan horse dna damage mechanism was observed with 
 the estrogen receptor ( er ) and the equine catechol estrogen , 4-ohen . the rate of 4-ohen - induced oxidative dna damage 
 was significantly enhanced in er positive cells . the mechanism 
 likely involves er acting as a trojan horse , which is covalently 
 modified by 4-ohen o - quinone , concentrating the er - quinone 
 complex in the nucleus , accelerating the rate of ros formation , and 
 enhancing dna oxidation . the trojan horse 
 mechanism of estrogen carcinogenesis could explain why there is a 
 positive association between 8-oxo - dg excretion levels and risk for 
 er positive breast cancer . several quinones are known to 
 cause a variety of dna lesions , including depurinating bases resulting 
 from n7-guanine or n3-adenine adduction , stable adducts at the exocyclic 
 amino groups of guanine and adenine , as well as oxidized bases ( figure 5 ) . depurinating n7-guanine and n3-adenine adducts have been 
 reported from dopamine o - quinones as well as from 
 4-hydroxyestrone / estradiol o - quinones . the estrogen depurinating dna 
 adducts have been detected in much higher levels in prostate and breast 
 cancer patients compared to that in samples from disease - free individuals . there is speculation and some in vitro studies 
 suggesting that similar depurinating adducts could be formed by a 
 number of quinones including those from diethylstilbestrol , bisphenol 
 a , pahs , and benzene . in contrast to these unstable depurinating adducts which would be 
 repaired by base excision repair enzymes , the equine estrogens 4-hydroxyequilin 
 and 4-hydroxyequilenin o - quinones also form stable 
 cyclic adducts similar to those reported for the pah o - quinones . highly mutagenic cyclic dna adducts have also been reported for p - benzoquinone which could contribute to the carcinogenic 
 effects of benzene . dna adducts have been reported from quercetin , 
 although these adducts likely result from covalent modification by 
 the quinone methide tautomer of the quercetin o - quinone . in addition , these quercetin dna adducts were 
 found to be transient and are unlikely to contribute to mutagenicity in vivo . although quercetin has tested positive in the ames 
 test in the presence of s9 activation , the transient nature of the dna adducts likely biomarkers for oxidative damage to dna include the formation 
 of 8-oxo - dg which is considered to be an important biomarker in carcinogenesis . pah o - quinones and pcb quinones have been reported 
 to cause oxidative dna damage . pah o - quinones generated oxidized pyrimidines and 
 8-oxo - dg through redox cycling resulting in g  t transversions 
 which may contribute to lung cancer risk . 8-oxo - dg formation has been 
 correlated with p53 mutations observed in human lung cancer which 
 strongly implicates the pah o - quinones as ultimate 
 carcinogens . the excessive production 
 of ros in breast cancer tissue has been 
 linked to metastasis of tumors in women with breast cancer . the source of ros has been suggested to be 
 the result of redox cycling between the estrogen o - quinones and their semiquinone radicals generating superoxide , hydrogen 
 peroxide , and ultimately reactive hydroxyl radicals ( figure 3 ) which cause oxidative cleavage 
 of the phosphate - sugar backbone as well as oxidation of the purine / pyrimidine 
 residues of dna . in support of this 
 mechanism , a variety of ros mediated damage has been reported in hamsters 
 treated with 17-estradiol including dna single - strand breaks , 
 8-oxo - dg formation , and chromosomal abnormalities . the equine catechol estrogens , 4-hydroxyequilin , and 4-hydroxyequilenin 
 are much more redox active since they autoxidize to o - quinones generating extensive oxidative dna damage . for example , 4-ohen induced 
 dna single - strand breaks as well as the formation of 8-oxo - da and 
 8-oxo - dg . these and other data are evidence for a 
 mechanism of estrogen - induced tumor initiation by redox cycling of 
 estrogen quinones generating ros which damage dna . finally , a variety 
 of redox active halogenated quinones have been shown to induce epigenetic 
 modulation of dna demethylation affecting the expression of thousands 
 of genes involved in a broad range of cellular processes . it is now becoming generally accepted 
 that quinones and compounds 
 that form them likely have inverse u - shaped dose this is because they have opposing 
 effects in biological systems which depending on the dose , cellular 
 targets , and time of exposure could result in toxicity or cytoprotection . 
 at low doses and for relatively stable / selective quinones , cytoprotection 
 comes from the electrophilic counterattack resulting in the induction 
 of detoxification enzymes through the keap1/nrf2 pathway . at higher 
 doses and for less selective / more reactive quinones , gsh depletion , 
 nad(p)h oxidation , and protein degradation led to the endoplasmic 
 reticulum stress response and altered signal transduction . dna adducts 
 especially from estrogen and pah quinones which are not repaired efficiently 
 over long - term exposure can initiate and promote the carcinogenic 
 process . very reactive quinones likely only target the enzymes that 
 sythesize them ( p450 , cox-2 , etc . ) or react with water and have little 
 toxic / cytoprotective effects . in general , the higher the potency 
 of the drug , the lower is the 
 dose and the decreased risk of toxicity . for example , paroxetine does form an o - quinone 
 which can decrease gsh and inhibit p450 2d6 ; however , the dose of 
 the drug is low , and o - quinone formation is unlikely 
 to contribute to significant clinical problems . similarly , o - quinones are the major metabolites 
 of endogenous estrogens and estrogens in estrogen replacement formulations . 
 as with paroxetine , the dose is very low for these drugs especially 
 for oral contraceptives , and o - quinone formation 
 may not be an issue . however , in addition to the generally accepted 
 hormonal carcinogenesis mechanism , long - term exposure to low doses 
 of estrogen o - quinones in estrogen replacement formulations 
 could contribute to enhanced risk of breast cancer especially in older 
 women . the 
 above are several examples of both structurally simple and 
 complex aromatic compounds for which data strongly implicate quinone 
 intermediates as mediators of toxicity and/or cytoprotection for a 
 variety of drugs , natural products , environmental contaminants , and 
 endogenous chemicals . these reactive metabolites could be considerably 
 more important to the metabolism and biological properties of synthetic 
 and naturally occurring aromatic compounds than is currently recognized . quinones are formed both enzymatically and nonenzymatically , but the 
 details of these processes and relationships to the structures of 
 aromatic compounds are just beginning to emerge . variations in the 
 contributions of electrophilicity / redox activity modulate quinone 
 reactivity over a wide range , suggesting substantial differences in 
 the biological targets and intracellular effects of quinones . it is 
 clear that covalent modification of both proteins and dna competes 
 with detoxification mechanisms such as reaction with gsh and that 
 quinones are capable of inducing cytotoxic and cytoprotective responses . future studies will seek to clarify relationships between reactivities 
 and biological actions of these electrophiles and to gain insight 
 into the mechanisms involved in cellular damage / cytoprotection . these 
 data obtained will assist in clarifying the complex biological properties 
 of a number of aromatic drugs and natural products and provide new 
 information on intracellular targets as a function of electrophile / redox 
 reactivity , which may be applicable to other types of electrophilic 
 intermediates . developing a better understanding of factors affecting 
 phase ii conjugation and rapid elimination of the parent aromatic 
 compounds versus formation of quinones , their reactivities , and biological 
 targets will allow advances in the drug discovery process to either 
 enhance or prevent this pathway in vivo . given the 
 high cost of drug withdrawal especially for idiosyncratic drug toxicity , 
 it is important to screen for potential quinone formation early in 
 the drug discovery process .
quinones 
 represent a class of toxicological intermediates , which 
 can create a variety of hazardous effects in vivo including , acute cytotoxicity , immunotoxicity , and carcinogenesis . 
 in contrast , quinones can induce cytoprotection through the induction 
 of detoxification enzymes , anti - inflammatory activities , and modification 
 of redox status . 
 the mechanisms by which quinones cause these effects 
 can be quite complex . 
 the various biological targets of quinones depend 
 on their rate and site of formation and their reactivity . 
 quinones 
 are formed through a variety of mechanisms from simple oxidation of 
 catechols / hydroquinones catalyzed by a variety of oxidative enzymes 
 and metal ions to more complex mechanisms involving initial p450-catalyzed 
 hydroxylation reactions followed by two - electron oxidation . 
 quinones 
 are michael acceptors , and modification of cellular processes could 
 occur through alkylation of crucial cellular proteins and/or dna . 
 alternatively , 
 quinones are highly redox active molecules which can 
 redox cycle with their semiquinone radical anions leading to the formation 
 of reactive oxygen species ( ros ) including superoxide , hydrogen peroxide , 
 and ultimately the hydroxyl radical . 
 production of ros can alter redox 
 balance within cells through the formation of oxidized cellular macromolecules 
 including lipids , proteins , and dna . 
 this perspective explores the 
 varied biological targets of quinones including gsh , nadph , protein 
 sulfhydryls [ heat shock proteins , p450s , cyclooxygenase-2 ( cox-2 ) , 
 glutathione s - transferase ( gst ) , nad(p)h : quinone 
 oxidoreductase 1 , ( nqo1 ) , kelch - like ech - associated protein 1 ( keap1 ) , 
 ib kinase ( ikk ) , and arylhydrocarbon receptor ( ahr ) ] , and dna . 
 the evidence strongly suggests that the numerous mechanisms of quinone 
 modulations ( i.e. , alkylation versus oxidative stress ) can be correlated 
 with the known pathology / cytoprotection of the parent compound(s ) 
 that is best described by an inverse u - shaped dose 
 response 
 curve .