Epidermal growth factor receptor is a common mediator of quinone-induced signaling leading to phosphorylation of connexin-43: role of glutathione and tyrosine phosphatases

Rat liver epithelial cells were exposed to three quinones with different properties: menadione (2-methyl-1,4-naphthoquinone, vitamin K3), an alkylating as well as redox-cycling quinone, the strongly alkylating p-benzoquinone (BQ), and the non-arylating redox-cycler, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ). All three quinones induced the activation of extracellular signal-regulated kinase (ERK) 1 and ERK 2 via the activation of epidermal growth factor receptor (EGFR) and MAPK/ERK kinases (MEK) 1/2. ERK activation resulted in phosphorylation at Ser-279 and Ser-282 of the gap junctional protein, connexin-43, known to result in the loss of gap junctional intercellular communication. Another EGFR-dependent pathway was stimulated, leading to the activation of the antiapoptotic kinase Akt via phosphoinositide 3-kinase. The activation of EGFR-dependent signaling by these quinones was by different mechanisms: (i) menadione, but not BQ or DMNQ, inhibited a protein-tyrosine phosphatase regulating the EGFR, as concluded from an EGFR dephosphorylation assay; (ii) although menadione-induced activation of ERK was unimpaired by pretreatment of cells with N-acetyl cysteine, activation by BQ and DMNQ was prevented; (iii) cellular glutathione (GSH) levels were strongly depleted by BQ. The mere depletion of GSH by application of diethyl maleate EGFR-dependently activated ERK and Akt, thus mimicking BQ effects. GSH levels were only moderately decreased by menadione and not affected by DMNQ. In summary, EGFR-dependent signaling was mediated by protein-tyrosine phosphatase inactivation (menadione), GSH depletion (BQ), and redox-cycling (DMNQ), funneling into the same signaling pathway.     

Degradation of 1,4-naphthoquinones by Pseudomonas putida

Pseudomonas putida J1 and J2, enriched from soil with juglone, are capable of a total degradation of 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, and 2-chloro-1,4-naphthoquinone. Naphthazerin and plumbagin are only converted into the hydroxyderivatives 2-hydroxynaphthazerin and 3-hydroxyplumbagin, respectively, whereas 2-amino-1,4-naphthoquinone is not attacked at all. The degradation of 1,4-naphthoquinone begins with a hydroxylation of the quinoid ring, yielding 2-hydroxy-1,4-naphthoquinone (lawsone). Lawsone is reduced to 1,2,4-trihydroxynaphthalene with consumption of NADH. The fission product of the quinol could not be detected by direct means because of its instability. However, the presence of 2-chromonecarboxylic acid, a secondary product of lawsone degradation, leads to the conclusion, that the cleavage of the quinol takes place in the meta-position. The resulting ring fission product is converted into salicylic acid by removal of the side chain, presumably as pyruvate. Further degradation of salicyclic acid leads to the formation of catechol, which is then cleaved in the ortho-position and then metabolized via the 3-oxoadipate pathway. The initial steps in the degradation of 2-chloro-1,4-naphthoquinone, namely, the hydroxylation of the quinone to 2-chloro-3-hydroxy-1,4-naphthoquinone, followed by the elimination of the chlorine substituent lead to lawsone, which is further degraded through the pathway described. The degradation steps could be verified by the accumulation products of mutant strains blocked in different steps of lawsone metabolism. Generation of mutants was carried out by chemical and by transposon mutagenesis. The regulation of the first steps of the pathway catalysed by juglone hydroxylase and lawsone reductase, was investigated by induction experiments.     

5-Hydroxy-1,4-naphthoquinone (juglone) and 2-hydroxy-1,4-naphthoquinone (lawsone) influence on jack bean urease activity: Elucidation of the difference in inhibition activity

The aim of this study was elucidation of the difference in inhibition influence of 5-hydroxy-1,4-naphthoquinone (juglone) and 2-hydroxy-1,4-naphthoquinone (lawsone) on jack bean urease activity. It was found that juglone acted as a strong, time and concentration dependent inactivator of urease. On the contrary, lawsone showed an inconsiderable inhibition influence. The reactivation of juglone modified urease showed the participation of reversible and irreversible contribution in the inactivation. In the presence of an excess of DTT, urease inactivated by juglone regained 70% of its activity. The reversible inactivation was attributed to oxidation of the essential urease thiols by reactive oxygen species (ROS) realizing during reduction of juglone to seminaphthoquinone. Presence of hydrogen peroxide in the incubation system was proved by direct determination and by application of catalase. The irreversible contribution in the inhibition was assumed as an arylation of urease thiol groups by juglone. The insignificant urease inhibition by lawsone was concluded as an effect of a low hydrogen peroxide generation and lawsone resistance for reaction with protein thiols. It was found that lawsone well reacted with l-cysteine, poorly with glutathione and hardly with urease thiols. The observed sequence was arranged according the rule the more complex thiol the less susceptible for reaction with lawsone. On the other hand, juglone displayed an excellent reactivity towards both thiols and urease. Thus, this indicated a significance of a steric hindrance which appeared when the hydroxyl group changing position from 5 in juglone (5-hydroxy-1,4-naphthoquinone) to 2 in lawsone (2-hydroxy-1,4-naphthoquinone).     

Inhibition of apoptosis by menadione on exposure to UVA

Quinones are widely distributed in the environment, both as natural products and as pollutants. This paper reports that one of the simplest quinones, 2-methyl-1,4-naphthoquinone (menadione), effectively inhibited apoptosis in the presence of UVA. Menadione suppressed the apoptosis induced by serum depletion and cell detachment. This effect was significantly enhanced by UVA irradiation. An antioxidant, N-acetylcysteine, completely inhibited the antiapoptotic effects of both menadione itself and menadione plus UVA, and peroxidation of the cells after treatment was observed using a probe to detect the intracellular production of peroxides. By contrast, 2-hydroxy-1,4-naphtoquinone (lawsone) showed no antiapoptotic effect in the presence or absence of UVA. Lawsone is reported not to undergo the redox process that produces reactive oxygen species. These results indicated that intracellular peroxidation contributed to the antiapoptotic effects of both menadione itself and menadione plus UVA. Dysregulation of the apoptotic process is critical to carcinogenesis. The photosensitization of quinone compounds as it relates to the inhibition of apoptosis should be examined in the future.     

Activation of ErbB2 by 2-methyl-1,4-naphthoquinone (menadione) in human keratinocytes: role of EGFR and protein tyrosine phosphatases

Activation of ErbB receptor tyrosine kinases triggers multiple signaling pathways that regulate cellular proliferation and survival. We here demonstrate that ErbB2 is activated via the epidermal growth factor receptor (EGFR) upon exposure of cultured human keratinocytes to 2-methyl-1,4-naphthoquinone (menadione). Both ErbB2 and EGFR are shown to be regulated by protein tyrosine phosphatases that are inhibited by menadione, giving rise to the hypothesis that phosphatase inhibition by menadione may result in a net activation of EGFR and an enhanced ErbB2 phosphorylation. Isolated PTP-1B, a protein tyrosine phosphatase known to be associated with ErbB receptors, is demonstrated to be inhibited by menadione.     

Inhibitory effect of novel 5-O-acyl juglones on mammalian DNA polymerase activity, cancer cell growth and inflammatory response

We previously found that vitamin K(3) (menadione, 2-methyl-1,4-naphthoquinone) inhibits the activity of human mitochondrial DNA polymerase γ (pol γ). In this study, we focused on juglone (5-hydroxy-1,4-naphthoquinone), which is a 1,4-naphthoquinone derivative, and chemically synthesized novel juglones conjugated with C2:0 to C22:6 fatty acid (5-O-acyl juglones). The chemically modified juglones enhanced mammalian pol inhibition and their cytotoxic and anti-inflammatory activities. The juglone conjugated with oleic acid (C18:1-acyl juglone) showed the strongest inhibition of DNA replicative pol α activity and human colon carcinoma (HCT116) cell growth in 10 synthesized 5-O-acyl juglones. C12:0-Acyl juglone was the strongest inhibitor of DNA repair-related pol λ, as well as the strongest suppression of the production of tumor necrosis factor (TNF)-α production induced by lipopolysaccharide (LPS) in the compounds tested. Moreover, this compound caused the greatest reduction in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced acute inflammation in mouse ears. C12:0- and C18:1-Acyl juglones selectively inhibited the activities of mammalian pol species, but did not influence the activities of other pols and DNA metabolic enzymes tested. These data indicate that the novel 5-O-acyl juglones target anti-cancer and/or anti-inflammatory agents based on mammalian pol inhibition. Moreover, the results suggest that acylation of juglone is an effective chemical modification to improve the anti-cancer and anti-inflammation of vitamin K(3) derivatives, such as juglone.     

Synthesis and preliminary pharmacological evaluation of new (+/-) 1,4-naphthoquinones structurally related to lapachol

Seven new 1,4-naphthoquinones structurally related to lapachol were synthesized from lawsone and oxygenated arylmercurials. These compounds can also be seen as pterocarpan derivatives where the A-ring was substituted by the 1,4-naphthoquinone nucleus. Pharmacological screening provided evidence of significant biological activities, including effects against proliferation of the MCF-7 human breast cancer cell line, against Herpes Simplex Virus type 2 infection, and against snake poison-induced myotoxicity. One derivative displaced flunitrazepam binding and showed benzodiazepine-like activity, suggesting novel neuroactive structural motifs.     

The toxicity of menadione (2-methyl-1,4-naphthoquinone) and two thioether conjugates studied with isolated renal epithelial cells

Menadione (2-methyl-1,4-naphthoquinone) was used as a model compound to test the hypothesis that thioether conjugates of quinones can be toxic to tissues associated with their elimination through a mechanism involving oxidative stress. Unlike menadione, the glutathione (2-methyl-3-(glutathion-S-yl)-1,4-naphthoquinone; MGNQ) and N-acetyl-L-cysteine (2-methyl-3-(N-acetylcysteine-S-yl)-1,4-naphthoquinone; M(NAC)NQ) thioether conjugates were not able to arylate protein thiols but were still able to redox cycle with cytochrome c reductase/NADH and rat kidney microsomes and mitochondria. Interestingly, menadione and M(NAC)NQ were equally toxic to isolated rat renal epithelial cells (IREC) while MGNQ was nontoxic. The toxicity of both menadione and M(NAC)NQ was preceded by a rapid depletion of soluble thiols and was associated with a depletion of soluble thiols and was associated with a depletion of protein thiols. Treatment of IREC with the glutathione reductase inhibitor, 1,3-bis(2-chloroethyl)-1-nitrosourea, potentiated the thiol depletion and toxicity observed with menadione and M(NAC)NQ indicating the involvement of oxidative stress in this model of renal cell toxicity. The lack of MGNQ toxicity can be attributed to an intramolecular cyclization reaction which destroys the quinone nucleus and therefore eliminates its ability to redox cycle. These findings have important implications with regard to our understanding of the toxic potential of quinone thioether conjugates and of quinone toxicity in general.     

DNA single-strand breakage in mammalian cells induced by redox cycling quinones in the absence of oxidative stress

Quinone-induced cell death is often attributed to oxidative stress during which the formation of DNA strand breaks is thought to play an important role. In this study, extensive DNA damage was observed in human chronic myelogenous leukemic cells (K562) exposed for 15 minutes to low concentrations (15–100 μM) of the redox cycling quinones 2,3-dimethoxy-1,4-naphthoquinone (2,3-diOMe-1,4-NQ) and menadione. However, DNA strand breakage and cell death could not be attributed to oxidative stress as the intracellular level and redox status of the reducing equivalents NADP(H) and GSH were unaffected. The intracellular level of NAD⁺ was found to correlate well with the extent of DNA repair (r = 0.93, P < 0.02) and cell proliferation (r = 0.96, P < 0.01) in cells exposed to the quinones. In contrast, a significant decrease in the level of intracellular ATP was only observed in cells exposed to menadione (50–100 μM). These results suggest that redox cycling quinones are capable of inducing DNA damage in mammalian cells by a mechanism that does not involve oxidative stress. Following DNA damage, cell death is dependent on the availability of NAD⁺, which may be key to the rapid repair of strand breaks. © 1995 John Wiley & Sons, Inc.     

The metabolism of menadione (2-methyl-1,4-naphthoquinone) by isolated hepatocytes. A study of the implications of oxidative stress in intact cells

The cytotoxic effects of many quinones are thought to be mediated through their one-electron reduction to semiquinone radicals, which subsequently enter redox cycles with molecular oxygen to produce active oxygen species and oxidative stress. The two-electron reduction of quinones to diols, mediated by DT-diaphorase (NAD(P)H: (quinone-acceptor) oxidoreductase), may therefore represent a detoxifying pathway which protects the cell from the formation of these reactive intermediates. By using menadione (2-methyl-1,4-naphthoquinone) and isolated hepatocytes, the relative contribution of the two pathways to quinone metabolism has been studied and a protective role for DT-diaphorase demonstrated. Moreover, in the presence of cytotoxic concentrations of menadione rapid changes in intracellular thiol and Ca2+ homeostasis were observed. These were associated with alterations in the surface structure of the hepatocytes which may be an early indication of cytotoxicity.     

Pyridine Nucleotide Changes In Hepatocytes Exposed To Quinones

Quinones may be toxic by a number of mechanisms. including arylation and oxidative stress caused by redox cycling. Using isolated hepatocytes, we have studied the cytotoxicity of four quinones. with differing abilities to arylate cellular nucleophiles and redox cycle. in relation to their effects on cellular pyridine nucleotides. High concentrations of menadione (redox cycles and arylates). 2-hydroxy-1,4-naphthoquinone (neither arylates nor redox cycles via a one electron reduction) 2.3-dimethoxy-1.4-naphthoquinone (a pure redox cycler) and p-benzoquinone (a pure arylator) caused an initial decrease in NAD⁺ and loss of viability, which was not prevented by 3-aminobenzamide. an inhibitor of poly(ADP-ribose)polymerase. In contrast. 3-aminobenzamide inhibited the loss of NAD' and viability caused by dimethyl sulphate so implicating poly(ADP-ribose)polymerase in its toxicity but not that of the quinones. Non-toxic concentrations of menadione. 2.3-dimethoxy-1.4-naphthoquinone and 2-hydroxy-1.4-naphthoquinone all caused markedly similar changes in cellular pyridine nucleotides. An initial decrease in NAD⁺ was accompanied by a small. transient increase in NADP⁺ and followed by a larger. prolonged increase in NADPH and total NADP⁺ + NADPH. Nucleotide changes were not observed with non-toxic concentrations of p-benzoquinone. Our findings suggest that a primary event in the response of the cell to redox cycling quinones is to bring about an interconversion of pyridine nucleotides. in an attempt to combat the effects of oxidative stress     

Adaptation of the phytopathogenic fungus Fusarium decemcellulare to oxidative stress

The adaptive response of the phytopathogenic fungus Fusarium decemcellulare to the oxidative stress induced by hydrogen peroxide and juglone (5-hydroxy-1,4-naphthoquinone) was studied. At concentrations higher than 1 mM, H2O2 and juglone completely inhibited the growth of the fungus. The 60-min pretreatment of logarithmic-phase cells with nonlethal concentrations of H2O2 (0.25 mM) and juglone (0.1 mM) led to the development of a resistance to high concentrations of these oxidants. The stationary-phase cells were found to be more resistant to the oxidants than the logarithmic-phase cells. The adaptation of fungal cells to H2O2 and juglone was associated with an increase in the activity of cellular catalase and superoxide dismutase, the main oxidative stress defense of enzymes.     

Parallel synthesis of a library of 1,4-naphthoquinones and automated screening of potential inhibitors of trypanothione reductase from Trypanosoma cruzi

Solid- and solution-phase parallel syntheses of 1,4-naphthoquinones (1,4-NQ) are described. A library of 1360 amides was constructed from the combination of 12 newly synthesised 1,4-NQ carboxylic acid and 120 amines, and was screened for inhibition of trypanothione reductase (TR) from Trypanosoma cruzi. The most active hits from a primary screening were re-synthesised and confirmed. This approach proves that it is possible to design potent and highly specific TcTR inhibitors deriving from menadione, juglone and plumbagin.     

The anti-sickling drug lawsone (2-OH-1,4-naphthoquinone) protects sickled cells against membrane damage

The ability of an anti-sickling drug lawsone, 2-OH-1,4-naphthoquinone, and two related compounds to inhibit the haematoporphyrin-sensitised photohaemolysis of normal and sickle cell erythrocytes has been investigated. The compounds appear to protect the erythrocyte membranes by reaction with transient oxidative species. Differential effects between normal and sickle cells are shown and these are attributed to the different membrane composition of irreversibly sickled erythrocytes. This report describes a possible basis for the decreased formation of irreversibly sickled cells in the presence of lawsone.     

Pyridine Nucleotide Changes In Hepatocytes Exposed To Quinones

Quinones may be toxic by a number of mechanisms. including arylation and oxidative stress caused by redox cycling. Using isolated hepatocytes, we have studied the cytotoxicity of four quinones. with differing abilities to arylate cellular nucleophiles and redox cycle. in relation to their effects on cellular pyridine nucleotides. High concentrations of menadione (redox cycles and arylates). 2-hydroxy-1,4-naphthoquinone (neither arylates nor redox cycles via a one electron reduction) 2.3-dimethoxy-1.4-naphthoquinone (a pure redox cycler) and p-benzoquinone (a pure arylator) caused an initial decrease in NAD⁺ and loss of viability, which was not prevented by 3-aminobenzamide. an inhibitor of poly(ADP-ribose)polymerase. In contrast. 3-aminobenzamide inhibited the loss of NAD' and viability caused by dimethyl sulphate so implicating poly(ADP-ribose)polymerase in its toxicity but not that of the quinones. Non-toxic concentrations of menadione. 2.3-dimethoxy-1.4-naphthoquinone and 2-hydroxy-1.4-naphthoquinone all caused markedly similar changes in cellular pyridine nucleotides. An initial decrease in NAD⁺ was accompanied by a small. transient increase in NADP⁺ and followed by a larger. prolonged increase in NADPH and total NADP⁺ + NADPH. Nucleotide changes were not observed with non-toxic concentrations of p-benzoquinone. Our findings suggest that a primary event in the response of the cell to redox cycling quinones is to bring about an interconversion of pyridine nucleotides. in an attempt to combat the effects of oxidative stress     

Chemosterilant activity of naturally occurring quinones and their analogues in the red cotton bug, Dysdercus koenigii (Het., Pyrrhocoridae)

The chemosterilant activity of two naturally occurring napthaquinones, plumbagin from Plumbago sp. and juglone from Juglans regia has been evaluated using the red cotton bug Dysdercus koenigii . Their activity were compared with synthetic napthaquinones, menadione, two benzoquinones, 2,6-dimethyl and 2,3,6-trimethylbenzoquinone and a hydroquinone 2,6-dimethylhydroquinone. As far as the authors are aware, the present investigation is the first systematic attempt to investigate the effects of quinones on different aspects of reproduction, namely mating behaviour, fecundity and fertility. All the above types of quinones have revealed adverse effects on the reproduction of D. koenigii , which were topically treated as 1-day-old adults with different doses of the compounds depending on their 50% lethality (LD₅₀) values. Analysis of the data revealed a highly statistically significant reduction in the number of eggs, their hatching and further development up to the final instar stage. The sterility index increased with the increase in the dose of any of the above compounds and was highest when both sexes were treated. Among the natural products, juglone induced more sterilizing effects than plumbagin at 5–10  μ g/insect. Menadione caused sterility at a very low dose such as 0.5  μ g. Of the two benzoquinones, the dimethylbenzoquinone acted at a much lower dose.     

Naphthoquinone inhibitors of Periplaneta americana and Scolytus multistriatus feeding: ultraviolet difference spectra of reactions of juglone, menadione, and 1,4-naphthoquinone with amino acids and the indicated mechanism of feeding inhibition

The relative reactivity of 3 naphthoquinones, which are feeding inhibitors for Periplaneta americana and Scolytus multistriatus, with each of 7 amino acids was measured by ultraviolet difference spectroscopy. Juglone (5-hydroxy-1,4-naphthoquinone), menadione(2-methyl-1,4-naphthoquinone) or 1,4-naphthoquinone produced difference spectra immediately upon mixing with cysteine, but not with valine, serine, glutamic acid, arginine, tryptophan or proline in phosphate buffer (pH 7.0). The K_s values for the reactions indicated that the affinities of 1,4-naphthoquinone (K_s = 4.4 · 10⁻⁴M) and juglone (K_s = 8.3 · 10⁻⁴M) for cysteine were comparable, but were both approx. 10 times greater than that for menadione (K_s = 3.2 · 10⁻³M). The extinction coefficient of the complex formed by cysteine with juglone (A = 3.448 · 10⁻¹M) was approx. 2 times greater than that of 1,4-naphthoquinone (A = 1.290 · 10⁻¹M) or menadione (A = 1.176 · 10⁻¹ M). The importance of these results to explaining the mechanism of chemoreception in P. americana and S. multistriatus is discussed.     

Using of liquid chromatography coupled with diode array detector for determination of naphthoquinones in plants and for investigation of influence of pH of cultivation medium on content of plumbagin in Dionaea muscipula

The interest of many investigators in naphthoquinones is due to their broad-range of biological actions from phytotoxic to fungicidal. The main aim of this work was to investigate the influence of different pH values of cultivation medium on naphthoquinone content in Dionaea muscipula. For this purpose, we optimized the simultaneous analysis of the most commonly occurring naphthoquinones (1,4-naphthoquinone, lawsone, juglone and plumbagin) by high performance liquid chromatography coupled with diode array detector (HPLC-DAD). The most suitable chromatographic conditions were as follows: mobile phase: 0.1 mol l-1 acetic acid:methanol in ratio of 33:67 (%, v/v), flow rate: 0.75 ml min-1 and temperature: 42 degrees C. Moreover, we looked for the most suitable technique for preparation of plant samples (D. muscipula, Juglans regia, Paulownia tomentosa, Impatience glandulifera, Impatience parviflora, Drosera rotundifolia, Drosera spathulata and Drosera capensis) due to their consequent analysis by HPLC-DAD. It clearly follows from the results obtained that sonication were the most suitable technique for preparation of J. regia plants. We also checked the recoveries of the determined naphthoquinones, which were from 96 to 104%. Finally, we investigated the changes in content of plumbagin in D. muscipula plants according to different pH of cultivation medium. The content increased with increasing pH up to 5 and, then, changed gradually. The lower content of plumbagin at lower pH values was of interest to us. Therefore, we determined the content of this naphthoquinone in the cultivation medium, what has not been studied before. We discovered that the lower tissue content of plumbagin was due to secretion of this naphthoquinone into the cultivation medium.     

Quinone induced stimulation of hexose monophosphate shunt activity in the guinea pig lens: role of zeta-crystallin.

The response of the hexose monophosphate shunt (HMS) in organ-cultured guinea pig lens to 1,2-naphthoquinone and 5-hydroxy-1,4-naphthoquinone (juglone) has been investigated. Both these compounds, which are substrates of guinea pig lens zeta-crystallin (NADPH:quinone oxidoreductase), were found to cause increases in the rate of 14CO2 production from 1-14C-labelled glucose. Exposure of lenses to 15 microM 1,2-naphthoquinone or 20 microM juglone yielded 5.9- and 7-fold stimulation of HMS activity, respectively. Unlike hydrogen peroxide-induced stimulation of HMS activity, these effects were not abolished by preincubation with the glutathione reductase inhibitor, 1,3-bis(2-chloroethyl)-1 nitrosourea (BCNU). While hydrogen peroxide produced substantial decrements in lens glutathione (GSH) levels, incubation with quinones was not associated with a similar reduction in GSH concentration. Protein-bound NADPH content in quinone-exposed guinea pig lenses was decreased, with a concomitant increase in the amounts of free NADP+. This finding supported the involvement of zeta-crystallin bound NADPH in the in vivo enzymic reduction of quinones. Hydrogen peroxide, on the other hand, caused decreases in the level of free NADPH alone, serving to confirm our earlier inference that quinone stimulated increases in the guinea pig lens HMS could be mediated through zeta-crystallin NADPH:quinone oxidoreductase activity.     

Tolerance of the yeast Yarrowia lipolytica to oxidative stress

The adaptive response of the yeast Yarrowia lipolytica to the oxidative stress induced by the oxidants hydrogen peroxide, menadione, and juglone has been studied. H2O2, menadione, and juglone completely inhibited yeast growth at concentrations higher than 120, 0.5, and 0.03 mM, respectively. The stationary-phase yeast cells were found to be more resistant to the oxidants than the exponential-phase cells. The 60-min pre-treatment of logarithmic-phase cells with nonlethal concentrations of H2O2 (0.3 mM), menadione (0.05 mM), and juglone (0.005 mM) made the cells more resistant to high concentrations of these oxidants. The adaptation of yeast cells to H2O2, menadione, and juglone was associated with an increase in the activity of cellular catalase, superoxide dismutase, glucose-6-phosphate dehydrogenase, and glutathione reductase, the main enzymes involved in cell defense against oxidative stress.