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1
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Cytotoxicity of anticancer aziridinyl-substituted benzoquinones in primary mice splenocytes

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Miliukienė V., Nivinskas H., Čėnas N.
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2014
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vol. 61
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issue 4
833-836
EN
The anticancer activity of aziridinyl-quinones is mainly attributed to their NAD(P)H:quinone oxidoreductase 1 (NQO1)-catalyzed two-electron reduction into DNA-alkylating products. However, little is known about their cytotoxicity in primary cells, which may be important in understanding their side effects. We found that the cytotoxicity of aziridinyl-unsubstituted quinones (n = 12) in mice splenocytes with a low amount of NQO1, 4 nmol × mg-1 × min-1, was caused mainly by the oxidative stress. Aziridinyl-benzoquinones (n = 6) including a novel anticancer agent RH1 were more cytotoxic than aziridinyl-unsubstituted ones with the similar redox properties, and their cytotoxicity was not decreased by an inhibitor of NQO1, dicumarol. The possible reasons for their enhanced cytotoxicity are discussed.
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Correlation between mammalian cell cytotoxicity of flavonoids and the redox potential of phenoxyl radical/phenol couple

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Marozienė A., Nemeikaitė-Čėnienė A., Vidžiūnaitė R., Čėnas N.
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2012
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vol. 59
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issue 2
299-306
EN
Flavonoids exhibit prooxidant cytotoxicity in mammalian cells due to the formation of free radicals and oxidation products possessing quinone or quinomethide structure. However, it is unclear how the cytotoxicity of flavonoids depends on the ease of their single-electron oxidation in aqueous medium, i.e., the redox potential of the phenoxyl radical/phenol couple. We verified the previously calculated redox potentials for several flavonoids according to their rates of reduction of cytochrome c and ferricyanide, and proposed experimentally-based values of redox potentials for myricetin, fisetin, morin, kaempferol, galangin, and naringenin. We found that the cytotoxicity of flavonoids (n=10) in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) and murine hepatoma (line MH-22a) increases with a decrease in their redox potential of the phenoxyl radical/phenol couple and an increase in their lipophilicity. Their cytotoxicity was decreased by antioxidants and inhibitors of cytochromes P-450, α-naphthoflavone and isoniazide, and increased by an inhibitor of catechol-O-methyltransferase, 3,5-dinitrocatechol. It shows that although the prooxidant action of flavonoids may be the main factor in their cytotoxicity, the hydroxylation and oxidative demethylation by cytochromes P-450 and O-methylation by catechol-O-methyltransferase can significantly modulate the cytotoxicity of the parent compounds.
3
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Role of NAD(P)H:quinone oxidoreductase (NQO1) in apoptosis induction by aziridinylbenzoquinones RH1 and MeDZQ

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Nemeikaitė-Čėnienė A., Dringelienė A., Šarlauskas J., Čėnas N.
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2005
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vol. 52
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issue 4
937-942
EN
We aimed to characterize the role of NAD(P)H:quinone oxidoreductase (NQO1) in apoptosis induction by antitumour quinones RH1 (2,5-diaziridinyl-3-hydroxymethyl-6-methyl-1,4-benzoquinone) and MeDZQ (2,5-dimethyl-3,6-diaziridinyl-1,4-benzoquinone). Digitonin-permeabilized FLK cells catalyzed NADPH-dependent single- and two-electron reduction of RH1 and MeDZQ. At equitoxic concentrations, RH1 and MeDZQ induced apoptosis more efficiently than the nonalkylating duroquinone or H2O2. The antioxidant N,N'-diphenyl-p-phenylene diamine, desferrioxamine, and the inhibitor of NQO1 dicumarol, protected against apoptosis induction by all compounds investigated, but to a different extent. The results of multiparameter regression analysis indicate that RH1 and MeDZQ most likely induce apoptosis via NQO1-linked formation of alkylating species but not via NQO1-linked redox cycling.
4
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Quinone- and nitroreductase reactions of Thermotoga maritima thioredoxin reductase

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Valiauga B., Rouhier N., Jacquot J.-P., Čėnas N.
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EN
The Thermotoga maritima NADH:thioredoxin reductase (TmTR) contains FAD and a catalytic disulfide in the active center, and uses a relatively poorly studied physiological oxidant Grx-1-type glutaredoxin. In order to further assess the redox properties of TmTR, we used series of quinoidal and nitroaromatic oxidants with a wide range of single-electron reduction potentials (E17, -0.49-0.09 V). We found that TmTR catalyzed the mixed single- and two-electron reduction of quinones and nitroaromatic compounds, which was much faster than the reduction of Grx-1. The reactivity of both groups of oxidants increased with an increase in their E17, thus pointing to the absence of their structural specificity. The maximal rates of quinone reduction in the steady-state reactions were lower than the maximal rates of reduction of FAD by NADH, obtained in presteady-state experiments. The mixed-type reaction inhibition by NAD+ was consistent with its competition for a NADH binding site in the oxidized enzyme form, and also with the reoxidation of the reduced enzyme form. The inhibition data yielded a value of the standard potential for TmTR of -0.31±0.03 V at pH 7.0, which may correspond to the FAD/FADH2 redox couple. Overall, the mechanism of quinone- and nitroreductase reactions of T. maritima TR was similar to the previously described mechanism of Arabidopsis thaliana TR, and points to their prooxidant and possibly cytotoxic role.
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Reduction of nitroaromatic compounds by NAD(P)H:quinone oxidoreductase (NQO1): the role of electron-accepting potency and structural parameters in the substrate specificity

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Misevičienė L., Anusevičius Ž., Šarlauskas J., Čėnas N.
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2006
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vol. 53
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issue 3
569-576
EN
We aimed to elucidate the role of electronic and structural parameters of nitroaromatic compounds in their two-electron reduction by NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2). The multiparameter regression analysis shows that the reactivity of nitroaromatic compounds (n = 38) increases with an increase in their single-electron reduction potential and the torsion angle between nitrogroup(s) and the aromatic ring. The binding efficiency of nitroaromatics in the active center of NQO1 exerted a less evident role in their reactivity. The reduction of nitroaromatics is characterized by more positive entropies of activation than the reduction of quinones. This points to a less efficient electronic coupling of nitroaromatics with the reduced isoalloxazine ring of FAD, and may explain their lower reactivity as compared to quinones. Another important but poorly understood factor enhancing the reactivity of nitroaromatics is their ability to bind at the dicumarol/quinone binding site in the active center of NQO1.
6
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Two-electron reduction of quinones by Enterobacter cloacae PB2 pentaerythritol tetranitrate reductase: quantitative structure-activity relationships

76%
Misevičienė L., Anusevičius Ž., Šarlauskas J., Harris R. J., Scrutton N. S., Čėnas N.
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2007
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vol. 54
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issue 2
379-385
EN
In order to clarify the poorly understood mechanisms of two-electron reduction of quinones by flavoenzymes, we examined the quinone reductase reactions of a member of a structurally distinct old yellow enzyme family, Enterobacter cloacae PB2 pentaerythritol tetranitrate reductase (PETNR). PETNR catalyzes two-electron reduction of quinones according to a 'ping-pong' scheme. A multiparameter analysis shows that the reactivity of quinones increases with an increase in their single-electron reduction potential and pKa of their semiquinones (a three-step (e-,H+,e-) hydride transfer scheme), or with an increase in their hydride-transfer potential (E7(H-)) (a single-step (H-) hydride transfer scheme), and decreases with a decrease in their van der Waals volume. However, the pH-dependence of PETNR reactivity is more consistent with a single-step hydride transfer. A comparison of X-ray data of PETNR, mammalian NAD(P)H : quinone oxidoreductase (NQO1), and Enterobacter cloacae nitroreductase, which reduce quinones in a two-electron way, and their reactivity revealed that PETNR is much less reactive, and much less sensitive to the quinone substrate steric effects than NQO1. This may be attributed to the lack of π-π stacking between quinone and the displaced aromatic amino acid in the active center, e.g., with Phe-178' in NQO1.
7
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Single-electron reduction of quinone and nitroaromatic xenobiotics by recombinant rat neuronal nitric oxide synthase

76%
Anusevičius Ž., Nivinskas H., Šarlauskas J., Sari M.-A., Boucher J.-L., Čėnas N.
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2013
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vol. 60
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issue 2
217-222
EN
We examined the kinetics of single-electron reduction of a large number of structurally diverse quinones and nitroaromatic compounds, including a number of antitumour and antiparasitic drugs, and nitroaromatic explosives by recombinant rat neuronal nitric oxide synthase (nNOS, EC 1.14.13.39), aiming to characterize the role of nNOS in the oxidative stress-type cytotoxicity of the above compounds. The steady-state second-order rate constants (kcat/Km) of reduction of the quinones and nitroaromatics varied from 102 M-1s-1 to 106 M-1s-1, and increased with an increase in their single-electron reduction potentials (E17). The presence of Ca2+/calmodulin enhanced the reactivity of nNOS. These reactions were consistent with an 'outer sphere' electron-transfer mechanism, considering the FMNH./FMNH2 couple of nNOS as the most reactive reduced enzyme form. An analysis of the reactions of nNOS within the 'outer sphere' electron-transfer mechanism gave the approximate values of the distance of electron transfer, 0.39-0.47 nm, which are consistent with the crystal structure of the reductase domain of nNOS. On the other hand, at low oxygen concentrations ([O2] = 40-50 μM), nNOS performs a net two-electron reduction of quinones and nitroaromatics. This implies that NOS may in part be responsible for the bioreductive alkylation by two-electron reduced forms of antitumour aziridinyl-substituted quinones under a modest hypoxia.
8
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Enzymatic redox reactions of the explosive 4,6-dinitrobenzofuroxan (DNBF): implications for its toxic action.

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Nemeikaitė-Čėnienė A., Šarlauskas J., Misevièienė L., Anusevièius Ž., Marozienė A., Čėnas N.
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vol. 51
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issue 4
1081-1086
EN
With an aim to understand the toxicity mechanisms of the explosive 4,6-dinitro- benzofuroxan (DNBF), we studied its single-electron reduction by NADPH:cytochrome P450 reductase and ferredoxin:NADP+ reductase, and two- electron reduction by DT-diaphorase and Enterobacter cloacae nitroreductase. The enzymatic reactivities of DNBF and another explosive 2,4,6-trinitrotoluene (TNT) were similar, except for the much lower reactivity of DNBF towards nitroreductase. DNBF was less cytotoxic in FLK cells than TNT. However, their action shared the same mechanisms, oxidative stress and activation by DT-diaphorase. The lower cytotoxicity of DNBF may be explained by the negative electrostatic charge of its adduct with water which may impede cellular membrane penetration, and by the formation of its less reactive adducts with intracellular reduced glutathione.
9
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Redox properties and prooxidant cytotoxicity of a neuroleptic agent 6,7-dinitrodihydroquinoxaline-2,3-dione (DNQX)

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Šarlauskas J., Nemeikaitė-Čėnienė A., Misevičienė L., Krikštopaitis K., Anusevičius Ž., Čėnas N.
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2013
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vol. 60
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issue 2
227-231
EN
In order to characterize the possible mechanism(s) of cytotoxicity of a neuroleptic agent 6,7-dinitrodihydroquinoxaline-2,3-dione (DNQX) we examined the redox properties of DNQX, and its mononitro- (NQX) and denitro- (QX) derivatives. The irreversible electrochemical reduction of the nitro groups of DNQX was characterized by the reduction peak potentials (Ep,7) of -0.43 V and -0.72 V vs. Ag/AgCl at pH 7.0, whereas NQX was reduced at Ep,7 = -0.67 V. The reactivities of DNQX and NQX towards the single-electron transferring enzymes NADPH:cytochrome P-450 reductase and NADPH:adrenodoxin reductase/adrenodoxin complex were similar to those of model nitrobenzenes with the single-electron reduction potential (E17) values of -0.29 V - -0.42 V. DNQX and NQX also acted as substrates for two-electron transferring mammalian NAD(P)H:quinone oxidoreductase (DT-diaphorase). The cytotoxicity of DNQX in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) was prevented by antioxidants and an inhibitor of NQO1, dicoumarol, and was enhanced by the prooxidant alkylating agent 1,3-bis(2-chloromethyl)-1-nitrosourea. A comparison with model nitrobenzene compounds shows that the cytotoxicity of DNQX and NQX reasonably agrees with the ease of their electrochemical reduction, and/or their reactivities towards the used enzymatic single-electron reducing systems. Thus, our data imply that the cytotoxicity of DNQX in FLK cells is exerted mainly through oxidative stress.
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A preliminary assessment of singlet oxygen scavenging, cytotoxic and genotoxic properties of Geranium macrorrhizum extracts

64%
Venskutonis P. R., Dedonytė V., Lazutka J., Slapšytė G., Marozienė A., Nemeikaitė-Čėnienė A., Čėnas N., Miliauskas G.
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2010
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vol. 57
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issue 2
157-163
EN
Strong radical-scavenging activity of Geranium macrorrhizum extracts isolated by using various solvent systems has been reported previously. This study aimed at expanding the knowledge on the bioactivities of antioxidatively active G. macrorrhizum butanol fraction, which was isolated from ethanolic extract (EB), and water fraction, which was isolated from water extract (WW) by measuring their singlet oxygen scavenging properties, as well as preliminary assessment of cytotoxicity and genotoxicity toward mammalian cells. The cytotoxicity (necrosis induction) of the extracts in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) was partly prevented by antioxidants and stimulated by the prooxidant BCNU (N,N'-bis(2-chloroethyl)-N-nitrosourea). This indicates that the cytotoxicity of G. macrorrhizum extracts is at least partly attributed to their prooxidant action, presumably due to the formation of quinoidal products of their (auto)oxidation. The latter was evidenced by the nature of the peroxidase-catalyzed oxidation products, which supported DT-diaphorase-catalyzed oxidation of NADPH and participated in conjugation reactions with reduced glutathione. The genotoxic properties were studied using chromosome aberration (CA) and sister chromatid exchange (SCE) tests in human lymphocytes in vitro and Drosophila melanogaster somatic mutation and recombination test (SMART) in vivo. In the CA test, only the highest doses of both fractions significantly increased chromosome aberration frequency. In the SCE test, both fractions induced SCEs in a clear dose-dependent manner. G. macrorrhizum extracts were not genotoxic in the SMART test in vivo. Our data indicate that in spite of the possible beneficial (antioxidant) effects of Geranium extracts, the possibilities of their use as ingredients of functional foods and/or food supplements should be further examined due to their cyto- and genotoxic effects resulting mainly from the action of quercetin-derived components abundant in the extracts.
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