Quinone toxicity in hepatocytes: studies on mitochondrial Ca2+ release induced by benzoquinone derivatives

Drug Discovery Insights from Medicinal Beetles in Traditional Chinese Medicine

Traditional Chinese medicine (TCM) was the primary source of medical treatment for the people inhabiting East Asia for thousands of years. These ancient practices have incorporated a wide variety of materia medica including plants, animals and minerals. As modern sciences, including natural products chemistry, emerged, there became increasing efforts to explore the chemistry of this materia medica to find molecules responsible for their traditional use. Insects, including beetles have played an important role in TCM. In our survey of texts and review articles on TCM materia medica, we found 48 species of beetles from 34 genera in 14 different families that are used in TCM. This review covers the chemistry known from the beetles used in TCM, or in cases where a species used in these practices has not been chemically studied, we discuss the chemistry of closely related beetles. We also found several documented uses of beetles in Traditional Korean Medicine (TKM), and included them where appropriate. There are 129 chemical constituents of beetles discussed.

Comparison of the potential mechanisms for hepatotoxicity of p-dialkoxy chlorobenzenes in rat primary hepatocytes for read-across

Read-across based on only structural similarity is considered to have a risk of error in chemical risk assessment. Under these circumstances, considering biological similarity based on adverse outcome pathways using in vitro omics technologies is expected to enhance the accuracy and robustness of conclusions in read-across. However, due to a lack of practical case studies, key considerations and use of these technologies for data gap filling are not well discussed. Here we extracted and compared the potential mechanisms for hepatotoxicity for structural analogs of p-dialkoxy chlorobenzenes including 1,4-dichloro-2,5-dimethoxybenzene (DDMB), 2,5-dichloro-1,4-diethoxybenzene (DDEB), 2-chloro-1,4-dimethoxybenzene (CDMB), and 1-chloro-2,5-diethoxybenzene (CDEB) using in vitro omics technologies for read-across. To reveal the potential mechanisms for hepatotoxicity, we conducted microarray analysis with rat primary hepatocytes. The results showed that three (DDMB, DDEB, CDEB) of the four chemicals affected similar biological pathways such as peroxisome proliferation, oxidative stress, and mitochondrial dysfunction. Furthermore, these biological pathways are consistent with in vivo hepatotoxicity in the source chemical, DDMB. In contrast, CDMB did not affect a specific toxicological pathway. Taken together, these data show the potential mechanisms for hepatotoxicity for three chemicals (DDMB, DDEB, CDEB) and provide novel insights into grouping chemicals using in vitro toxicogenomics for read-across.

The in vitro effect of duroquinone on functional competence, genomic integrity, and oxidative stress indices of sterlet (Acipenser ruthenus) spermatozoa

The sturgeon is a highly endangered fish species mostly due to over-fishing, habitat destruction, and water pollution. Duroquinone (derivative of 1,4-benzoquinone) is a xenobiotic compound widespread in the environment. The effect of duroquinone on motility, DNA integrity, and oxidative stress indices in sterlet, Acispenser ruthenus, spermatozoa was investigated. Sterlet sperm was exposed for 2h to duroquinone at concentrations of 25, 50, 100, and 150 μM. Spermatozoa motility, velocity, and ATP content were significantly decreased with exposure to duroquinone. The level of DNA damage significantly increased at concentrations of 50 μM and above. Oxidative stress indices (lipid peroxidation and content of carbonyl proteins) and superoxide dismutase (SOD) activity increased significantly with increasing concentrations of duroquinone. Oxidative stress in sterlet spermatozoa induced by duroquinone was shown to impair spermatozoa DNA integrity, motility parameters, and the antioxidant defense system. Spermatozoa motility, content of carbonyl proteins, and SOD activity were shown to be sensitive biomarkers, exhibiting strong responses to low concentrations of the xenobiotic. Results also suggested that fish spermatozoa in vitro assays may provide a simple and efficient means of monitoring residual pollutants in the aquatic environment.

Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide

The quinone/semiquinone/hydroquinone triad (Q/SQ(*-)/H(2)Q) represents a class of compounds that has great importance in a wide range of biological processes. The half-cell reduction potentials of these redox couples in aqueous solutions at neutral pH, E degrees ', provide a window to understanding the thermodynamic and kinetic characteristics of this triad and their associated chemistry and biochemistry in vivo. Substituents on the quinone ring can significantly influence the electron density "on the ring" and thus modify E degrees' dramatically. E degrees' of the quinone governs the reaction of semiquinone with dioxygen to form superoxide. At near-neutral pH the pK(a)'s of the hydroquinone are outstanding indicators of the electron density in the aromatic ring of the members of these triads (electrophilicity) and thus are excellent tools to predict half-cell reduction potentials for both the one-electron and two-electron couples, which in turn allow estimates of rate constants for the reactions of these triads. For example, the higher the pK(a)'s of H(2)Q, the lower the reduction potentials and the higher the rate constants for the reaction of SQ(*-) with dioxygen to form superoxide. However, hydroquinone autoxidation is controlled by the concentration of di-ionized hydroquinone; thus, the lower the pK(a)'s the less stable H(2)Q to autoxidation. Catalysts, e.g., metals and quinone, can accelerate oxidation processes; by removing superoxide and increasing the rate of formation of quinone, superoxide dismutase can accelerate oxidation of hydroquinones and thereby increase the flux of hydrogen peroxide. The principal reactions of quinones are with nucleophiles via Michael addition, for example, with thiols and amines. The rate constants for these addition reactions are also related to E degrees'. Thus, pK(a)'s of a hydroquinone and E degrees ' are central to the chemistry of these triads.

Antimicrobial management of sepsis and septic shock

Every patient who has sepsis and septic shock must be evaluated appropriately at presentation before the initiation of antibiotic therapy. However, in most situations, an abridged initial assessment focusing on critical diagnostic and management planning elements is sufficient. Intravenous antibiotics should be administered as early as possible, and always within the first hour of recognizing severe sepsis and septic shock. Broad-spectrum antibiotics must be selected with one or more agents active against likely bacterial or fungal pathogens and with good penetration into the presumed source. Antimicrobial therapy should be reevaluated daily to optimize efficacy, prevent resistance, avoid toxicity, and minimize costs. Consider combination therapy in septic shock Pseudomonas infections in neutropenic patients. Combination therapy should be continued for no more than 3 to 5 days and de-escalation should occur following availability of susceptibilities. The duration of antibiotic therapy typically is limited to 7 to 10 days. Longer duration is considered if response is slow, if there is inadequate surgical source control, or if immunologic deficiencies are evident. Antimicrobial therapy should be stopped if infection is not considered the etiologic factor for a shock state.

Redox-cycling compounds can cause the permeabilization of mitochondrial membranes by mechanisms other than ROS production

The participation of reactive oxygen species (ROS) in the regulation of mitochondrial permeability transition pore (mPTP) opening by the redox-cycling compounds menadione and lucigenin was explored. The level of ROS was modulated by antioxidants, anoxia, and switching the sites of the reduction of redox cyclers, the dehydrogenases of the inner and outer mitochondrial membranes. We found that the reduction of both lucigenin and menadione in the outer mitochondrial membrane caused a strong production of ROS. However, mPTP opening was accelerated only in the presence of the cationic acceptor lucigenin. The antioxidants and scavengers of ROS that considerably decreased the level of ROS in mitochondria did not prevent or delay the mPTP opening. If the transmembrane potential under anoxia was supported by exogenous ATP or ferricyanide, the permeabilization of mitochondrial membranes by menadione or lucigenin was the same as under normoxia or even more pronounced. Under anoxia, the lucigenin-dependent permeabilization of membranes was less sensitive to mPTP antagonists than under normoxia. We conclude that the opening of the mPTP by redox cyclers may be independent of ROS and is due to the direct oxidation of mitochondrial pyridine nucleotides by menadione and the modification of critical thiols of the mPTP by the cation radical of lucigenin.

Nutritional and biochemical factors influencing the biological effects of cyanide

Investigation of the dietary interaction between cyanide and selenium in the chick, whereby cyanide alleviates selenium toxicity, suggests that cyanide alters metabolic reductive potential. Cyanide enhances the elimination of selenium as dimethyl selenide, the formation of which requires both reducing equivalents and methyl groups. Even when the methionine supply is adequate, meeting the need for the methyl groups, the interaction can be lost if there is a deficiency of certain micronutrients or an excess of vitamin K. Cyanide reduces liver glycogen, implying greater emphasis on anaerobic metabolism through inhibition of cytochrome oxidase. This may increase reductive potential but may also result in increased free radical production, processes that can be modified by levels of micronutrients. There is no evidence that an excess of sulphur amino acids can markedly enhance cyanide detoxification, although, for reasons that are not yet clear, cystine may be beneficial. However, the balance of dietary amino acids may be more critical than had been realized, because an excess of alanine appears to exacerbate cyanide toxicity.

Human cytochrome P450 reductase can act as a source of endogenous oxidative DNA damage and genetic instability

Studies with repair-deficient mice and other experiments suggest that oxidative DNA modifications are generated in all types of cells even under physiological conditions and that this type of endogenous DNA damage contributes to spontaneous cancer incidence. However, the cellular sources of reactive oxygen species that are relevant for nuclear oxidative DNA damage are largely unknown. Here, we report that expression of human NADPH-cytochrome P450 reductase (hOR) in cultured V79 Chinese hamster cells gives rise to elevated basal levels of oxidative purine modifications after depletion of glutathione. Also, the basal levels of micronuclei are increased in the hOR-expressing cells, and again the effect is enhanced when the antioxidant defense system of the cells is diminished by depletion of glutathione. The oxidative DNA damage is increased when duroquinone, a substrate of hOR, is added, both in the presence and absence of glutathione. In contrast, hOR-expressing cells are similarly sensitive as the parental cells when oxidative DNA damage and micronuclei are induced by a mechanism independent of hOR, i.e., exposure to bromate. The results identify hOR as a potential source of endogenous oxidative DNA damage and subsequent genetic instability in mammalian cells.

Influence of pulmonary arterial endothelial cells on quinone redox status: effect of hyperoxia-induced NAD(P)H:quinone oxidoreductase 1

The objective of this study was to examine the impact of chronic hyperoxic exposure (95% O2for 48 h) on intact bovine pulmonary arterial endothelial cell redox metabolism of 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ). DQ or durohydroquinone (DQH2) was added to normoxic or hyperoxia-exposed cells in air-saturated medium, and the medium DQ concentrations were measured over 30 min. DQ disappeared from the medium when DQ was added and appeared in the medium when DQH2was added, such that after ∼15 min, a steady-state DQ concentration was approached that was ∼4.5 times lower for the hyperoxia-exposed than the normoxic cells. The rate of DQ-mediated reduction of the cell membrane-impermeant redox indicator, potassium ferricyanide [Fe(CN)[Formula: see text]], was also approximately twofold faster for the hyperoxia-exposed cells. Inhibitor studies and mathematical modeling suggested that in both normoxic and hyperoxia-exposed cells, NAD(P)H:quinone oxidoreductase 1 (NQO1) was the dominant DQ reductase and mitochondrial electron transport complex III the dominant DQH2oxidase involved and that the difference between the net effects of the cells on DQ redox status could be attributed primarily to a twofold increase in the maximum NQO1-mediated DQ reduction rate in the hyperoxia-exposed cells. Accordingly, NQO1 protein and total activity were higher in hyperoxia-exposed than normoxic cell cytosolic fractions. One outcome for hyperoxia-exposed cells was enhanced protection from cell-mediated DQ redox cycling. This study demonstrates that exposure to chronic hyperoxia increases the capacity of pulmonary arterial endothelial cells to reduce DQ to DQH2via a hyperoxia-induced increase in NQO1 protein and total activity.

BIOACTIVATION OF THE NONTRICYCLIC ANTIDEPRESSANT NEFAZODONE TO A REACTIVE QUINONE-IMINE SPECIES IN HUMAN LIVER MICROSOMES AND RECOMBINANT CYTOCHROME P450 3A4

The therapeutic benefits of the antidepressant nefazodone have been hampered by several cases of acute hepatotoxicity/liver failure. Although the mechanism of hepatotoxicity remains unknown, it is possible that reactive metabolites of nefazodone play a causative role. Studies were initiated to determine whether nefazodone undergoes bioactivation in human liver microsomes to electrophilic intermediates. Following incubation of nefazodone with microsomes or recombinant P4503A4 in the presence of sulfydryl nucleophiles, conjugates derived from the addition of thiol to a monohydroxylated nefazodone metabolite were observed. Product ion spectra suggested that hydroxylation and sulfydryl conjugation occurred on the 3-chlorophenylpiperazine-ring, consistent with a bioactivation pathway involving initial formation of p-hydroxynefazodone, followed by its two-electron oxidation to the reactive quinone-imine intermediate. The formation of novel N-dearylated nefazodone metabolites was also discernible in these incubations, and 2-chloro-1,4-benzoquinone, a by-product of N-dearylation, was trapped with glutathione to afford the corresponding hydroquinone-sulfydryl adduct. Nefazodone also displayed NADPH-, time-, and concentration-dependent inactivation of P4503A4 activity, suggesting that reactive metabolites derived from nefazodone bioactivation are capable of covalently modifying P4503A4. A causative role for 2-chloro-1,4-benzoquinone and/or the quinone-imine intermediate(s) in nefazodone hepatotoxicity is speculated. Although the antianxiety agent buspirone, which contains a pyrimidine ring in place of the 3-chlorophenyl-ring, also generated p-hydroxybuspirone in liver microsomes, no sulfydryl conjugates of this metabolite were observed. This finding is consistent with the proposal that two-electron oxidation of p-hydroxybuspirone to the corresponding quinone-imine is less favorable due to differences in the protonation state at physiological pH and due to weaker resonance stabilization of the oxidation products as predicted from ab initio measurements.

Hepatotoxicity of the thiazolidinediones

Troglitazone, the first of the thiazolidinediones, caused severe hepatotoxicity including liver failure in several patients. It appears, however, that the thiazolidinediones as a class are not as hepatotoxic as troglitazone. Comparative data at comparable dates of usage indicate that pioglitazone and rosiglitazone are not significant hepatotoxins. This is further supported by experimental data that demonstrate that troglitazone, alone among the thiazolidinediones, is toxic in hepatocyte cell culture. All of the thiazolidinediones cause ALT elevations; however, ALT monitoring for hepatotoxicity does not appear to prevent serious liver disease nor reduce patient risk.

Cataract formation by a semiquinone metabolite of acetaminophen in mice: possible involvement of Ca(2+)and calpain activation

Acetaminophen, an analgesic/antipyretic, is metabolized by hepatic cytochrome P450 to N -acetyl- p -benzoquinone imine (NAPQI), which is transported by blood circulation to the eye and induces anterior cortical cataract in mice. In this study we injected NAPQI into the anterior chamber of mouse eye and investigated time-dependent cellular responses in the lens. After a lag period of about 2 hr following NAPQI injection, lens opacification as determined by measurement of light scattering by the lens became evident and progressively increased thereafter. There was no difference in the profile of opacity development between a P450-inducer responsive mouse strain and a non-responsive strain. During the lag period, a marked increase in free intracellular Ca(2+)in the lens epithelium was observed at 1 hr by confocal fluorescence microscopy with a Ca(2+)probe. Concurrent with the free Ca(2+)increase, there was a 300% rise in the activity of the non-lysosomal neutral protease calpain in the lens at 1 hr after NAPQI injection. Evidence indicated degradation of vimentin in the lens in which calpain activity was enhanced. Co-injection of calpain inhibitors (N-Ac-Leu-Leu-norleucinol and N-Ac-Leu-Leu-methioninal) with NAPQI protected animals completely from cataract development, although a rise in free intracellular Ca(2+)in the lens epithelium was still observed. Lenses from the protected mice did not exhibit enhanced calpain activity. These results suggest the following sequence of events as a possible mechanism of NAPQI-induced cataract. NAPQI introduced in the anterior chamber of the eye enters the lens epithelial cells and disturbs Ca(2+)homeostasis with a resultant rise in free intracellular Ca(2+)which in turn activates calpain in the epithelium. The neutral protease then degrades cellular proteins (e.g. cytoskeletal proteins) and initiates anterior cortical cataract formation.

The membrane permeability transition in liver mitochondria of the great green goby Zosterisessor ophiocephalus (Pallas)

Liver mitochondria from the great green goby Zosterisessor ophiocephalus (Pallas) normally exhibit bioenergetic variables (membrane potential 165+/−7 mV; respiratory control ratio 6.6+/−0.4; ADP/O ratio 1.85+/−0.8; means +/− s.e.m., N=6) and activities of physiological transport systems (phosphate/proton symporter, adenine nucleotide antiporter, Ca(2+) electrophoretic uniporter) comparable with those of rat liver mitochondria. When incubated in the presence of Ca(2+) and an inducer agent such as phosphate, these mitochondria undergo a complete collapse of membrane potential accompanied by a large-amplitude swelling of the matrix, influx of sucrose from the incubation medium, release of endogenous Mg(2+) and K(+) (approximately 90% of the total) and of preaccumulated Ca(2+) and oxidation of endogenous pyridine nucleotides. All these phenomena, which are completely eliminated by cyclosporin A and inhibited with different efficacies by Mg(2+) and spermine, demonstrate that the induction of the permeability transition in this type of mitochondria has characteristics similar to those described in rat liver mitochondria. In contrast, the requirement for very high Ca(2+) concentrations (greater than 100 micromol l(−1) for the induction of the permeability transition represents a very important difference that distinguishes this phenomenon in fish and mammalian mitochondria.

Protective effect of ethanol against acetaminophen-induced hepatotoxicity in mice: role of NADH:quinone reductase

The role of NAD(P)H:quinone reductase (QR; EC 1.6.99.2) in the alcohol-derived protective effect against hepatotoxicity caused by acetaminophen (APAP) was studied. In mice pretreated with dicoumarol (30 mg/kg), an inhibitor of QR, hepatic necrosis caused by APAP (400 mg/kg) was potentiated. Hepatocellular injuries induced by APAP, as assessed by liver histology, serum aminotransferase activities, hepatic glutathione (reduced and oxidized) contents, and liver microsomal aminopyrine N-demethylase activities, all were potentiated by pretreatment of mice with dicoumarol. Even in mice given APAP and ethanol (4 g/kg), in which APAP-inducible hepatic necrosis was abolished, the dicoumarol pretreatment again produced moderate hepatotoxicity and reversed the protective effect of ethanol. In mice pretreated with dicoumarol and ethanol, levels of APAP in blood and bile fluid between 90 and 240 min were higher than those in mice given ethanol. However, the biliary contents of sulfate and glucuronide conjugates of APAP were much lower than those in the ethanol group, particularly at early time points. In contrast, the biliary level of APAP-cysteine conjugate, which in the ethanol group was at its basal level, was increased maximally in the dicoumarol-pretreated mice. In the mice given dicoumarol and ethanol, the biliary APAP-cysteine conjugate level was increased moderately. These results suggest that ethanol inhibited not only the microsomal (CYP2E1 mediated) formation of a toxic quinone metabolite from APAP, but also accelerated the conversion of the toxic quinone metabolite produced back to APAP by stimulating cytoplasmic QR activity. In the presence of dicoumarol, however, QR activity was inhibited, and conversion of the toxic quinone metabolite back to APAP became inhibited and diminished the alcohol-dependent protective effect against APAP-induced hepatic injury.

Metabolites of acetaminophen trigger Ca2+ release from liver microsomes

Release of mitochondrial calcium is believed to play a key role in the toxicity of acetaminophen in biological systems. Elevated cytosolic Ca2+ may also result from activation of calcium releasing channels. The major metabolites of acetaminophen, benzoquinone imine and 1,4-benzoquinone, induced Ca2+ release in isolated rat liver microsomes. The 1,4-benzoquinone-induced release of calcium was suppressed by ryanodine and fully inhibited by reduced glutathione. Concentrations of 1,4-benzoquinone that induced Ca2+ release did not affect the activity of the microsomal Ca2+, Mg2+-APTase. The binding of [3H]ryanodine to liver microsomes, however, was significantly decreased by 1,4-benzoquinone, suggesting a direct interaction of this metabolite with the ryanodine-binding protein (ryanodine receptor). These results suggest that cellular Ca2+ levels may be elevated by acetaminophen by pathways involving, in part, activation of Ca2+ releasing channels such as the ryanodine receptor.

Protective effect of nifedipine against cytotoxicity and intracellular calcium alterations induced by acetaminophen in rat hepatocyte cultures

Alteration of calcium homeostasis has been proposed to play a major role in cell necrosis induced by a variety of chemical agents such as acetaminophen (APAP). In this study, a potential protective effect of the dihydropyridine calcium channel blocking agent, nifedipine, was investigated in vitro on acetaminophen-induced hepatocyte damage. Rat hepatocytes were exposed during 20 hours to various concentrations of APAP (0.50 to 4.00 mM). The following metabolic and functional parameters were investigated: -lactate dehydrogenase (LDH) release as an indicator of plasma membrane integrity, -cell viability evaluated by the colorimetric MTT assay, and intracellular calcium concentration as evaluated by two fluorimetric methods: a scanning laser cytometer using indo-1-AM as fluorescent probe and a fluorescence plate reader using fluo-3-AM as calcium indicator. Incubation of hepatocytes with APAP alone in the range 0.50 to 4.00mM resulted in a dose-response relationship with regard to LDH release (243% to 750% of control) and to the loss of cell viability (0 to 67% of control). Moreover these results were correlated with a significant increase in cytosolic calcium content (189 to 406 nM). Nifedipine treatment prior to APAP exposure, partially prevented LDH release, the plasma membrane blebbing, and thereby the loss of viability. In addition, intracellular calcium level progressively returned within the limits of the control values with increasing concentrations of nifedipine. It can be concluded that, in vitro conditions, nifedipine pretreatment exhibits a preventive effect against acetaminophen hepatocyte injury.

The role of redox cycling versus arylation in quinone-induced mitochondrial dysfunction: a mechanistic approach in classifying reactive toxicants

In an attempt to distinguish between the mechanisms by which electrophilic and redox cycling quinones induce the cyclosporine A (CyA)-sensitive mitochondrial membrane permeability transition, the ability of a series of quinones that span a broad range of electrophilic and redox cycling reactivities has been examined. The order of potency of quinone-induced Ca2+ release was 1,4-naphthoquinone (NQ) > 1,4-benzoquinone (BQ) > 2-methyl-1,4-naphthoquinone (MQ) > 2,3-dimethoxy-1,4-naphthoquinone (DiOMeNQ) > 2,3-dimethyl-1,4-naphthoquinone (DiMeNQ). Quinones with predominantly redox cycling reactivity, NQ ( < or = 4 microM), MQ, DiOMeNQ and DiMeNQ, induced the CyA-sensitive membrane permeability transition. In contrast, NQ ( > 4 microM) and BQ, induced rapid and complete Ca2+ release and membrane depolarization, but not swelling. Furthermore, BQ and NQ ( > 4 microM)-induced effects were not prevented by CyA. Therefore, we maintain that, unlike MQ, DiOMeNQ, DiMeNQ and NQ ( < or = 4 microM), effects of BQ and NQ( > 4 microM) on calcium flux and membrane potential are manifest via a mechanism independent of altering the regulation of the cyclosporine A-sensitive PTP. These findings suggest that stereoelectronic descriptors for soft electrophilicity and one electron reduction potential may be useful in differentiating and predicting mechanisms of quinone toxicity.

Inhibition by spermine of the inner membrane permeability transition of isolated rat heart mitochondria

The effect of spermine on the permeability transition of the inner mitochondrial membrane of isolated rat heart mitochondria was evaluated. The permeability transition was triggered using a series of agents (t-butyl hydroperoxide, phenylarsine oxide, carboxyatractylate, and elevated Ca2+ and inorganic phosphate concentrations), and was monitored via Ca(2+)-release, mitochondrial swelling and pyridine nucleotide oxidation. By all three criteria, spermine inhibited the transition. A C50 of 0.38 +/- 0.06 (SD) mM was measured for inhibition.

Glutathione depletion, lipid peroxidation, DNA double-strand breaks and the cytotoxicity of 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone in rat renal cortical cells

The mechanisms involved in the cytotoxicity of 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone, a model compound for hydroquinone derived mercapturic acids, were investigated in rat renal proximal tubule cells. 2-Bromo-3-(N-acetylcystein-S-yl)hydroquinone induced a time- and concentration-dependent decrease in cell viability and in the levels of cellular glutathione. Antioxidants such as N,N'-diphenyl-p-phenylene diamine and ascorbic acid and the iron chelator desferrioxamine very efficiently protected the cells from 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone without influencing glutathione depletion. The acetoxymethyl ester of the Ca2+ chelator Quin-2, the inhibitor of the Ca(2+)- and Mg(2+)-dependent endonucleases, aurintricarboxylic acid and the poly(ADP-ribose)-polymerase inhibitor 3-aminobenzamide also ameliorated 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone cytotoxicity. Moreover, 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone depleted Ca2+ from isolated kidney mitochondria, increased the amount of malondialdehyde in rat kidney cells and induced DNA double-strand breaks in renal cells in culture. These results suggest that renal cells oxidize 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone to the corresponding quinone; this soft electrophile reacts rapidly with glutathione, thus depleting cellular glutathione concentrations as indicated by the tentative identification of a 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone thioether in the incubation medium of renal cells treated with the mercapturate. As a result of the massive glutathione depletion, peroxidative mechanisms then cause an elevation of the cytosolic concentrations of ionized calcium; impairment of the ability of the mitochondria to sequester Ca2+ plays an important role in the elevation of the Ca2+ concentration. Finally, activation of Ca(2+)- and Mg(2+)-dependent endonucleases results in DNA damage and cell death.

Prevention of Ca(2+)-induced or thromboxane B2-induced hepatocyte plasma membrane bleb formation by thromboxane receptor antagonists

Isolated hepatocytes incubated in the presence of either Ca2+ ionophore A23187 or thromboxane B2 develop many plasma membrane blebs which are a characteristic feature of toxic or ischaemic cell injury. When hepatocytes are incubated in the presence of both Ca2+ ionophore A23187 and any one of three thromboxane receptor antagonists (SK and F 88046, B.M. 13505, B.M. 13177), bleb formation is strongly inhibited. Hepatocytes incubated in the presence of both thromboxane B2 and any one of the three thromboxane receptor antagonists are also well protected from the formation of blebs. Treatment of isolated hepatocytes with Ca2+ ionophore A23187 is known to stimulate the production of thromboxanes. The data presented are consistent with thromboxane B2 acting as an intermediary in a proposed mechanism of cell injury and death in which elevated cytosolic free Ca2+ levels activate phospholipase A2 and the arachidonate cascade.

Glutathionyl- and hydroxyl radical formation coupled to the redox transitions of 1,4-naphthoquinone bioreductive alkylating agents during glutathione two-electron reductive addition

The kinetic parameters of the redox transitions subsequent to the two-electron transfer implied in the glutathione (GSH) reductive addition to 2- and 6-hydroxymethyl-1,4-naphthoquinone bioalkylating agents were examined in terms of autoxidation, GSH consumption in the arylation reaction, oxidation of the thiol to glutathione disulfide (GSSG), and free radical formation detected by the spin-trapping electron spin resonance method. The position of the hydroxymethyl substituent in either the benzenoid or the quinonoid ring differentially influenced the initial rates of hydroquinone autoxidation as well as thiol oxidation. Thus, GSSG- and hydrogen peroxide formation during the GSH reductive addition to 6-hydroxymethyl-1,4-naphthoquinone proceeded at rates substantially higher than those observed with the 2-hydroxymethyl derivative. The distribution and concentration of molecular end products, however, was the same for both quinones, regardless of the position of the hydroxymethyl substituent. The [O2]consumed/[GSSG]formed ratio was above unity in both cases, thus indicating the occurrence of autoxidation reactions other than those involved during GSSG formation. EPR studies using the spin probe 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) suggested that the oxidation of GSH coupled to the above redox transitions involved the formation of radicals of differing structure, such as hydroxyl and thiyl radicals. These were identified as the corresponding DMPO adducts. The detection of either DMPO adduct depended on the concentration of GSH in the reaction mixture: the hydroxyl radical adduct of DMPO prevailed at low GSH concentrations, whereas the thiyl radical adduct of DMPO prevailed at high GSH concentrations. The production of the former adduct was sensitive to catalase, whereas that of the latter was sensitive to superoxide dismutase as well as to catalase. The relevance of free radical formation coupled to thiol oxidation is discussed in terms of the thermodynamic and kinetic properties of the reactions involved as well as in terms of potential implications in quinone cytotoxicity.

Bioreductive activation of quinones: a mixed blessing

Quinones can be metabolized by various routes: substitution or reductive addition with nucleophilic compounds (mainly glutathione and protein thiol groups), one-electron reduction (mainly by NADPH: cytochrome P-450 reductase) and two-electron reduction (by D,T-diaphorase). During reduction semiquinone radicals and hydroquinones are formed, which can transfer electrons to molecular oxygen, resulting in the formation of reactive oxygen intermediates and back-formation of the parent quinone (redox cycling). Reaction of semiquinones and reactive oxygen intermediates with DNA and other macromolecules can lead to acute cytotoxicity and/or to mutagenicity and carcinogenicity. The enhanced DNA-alkylating properties of certain hydroquinones are exploited in the bioreductive alkylating quinones. Acute cytotoxicity of quinones appears to be related to glutathione depletion and to interaction with mitochondria and subsequent disturbance of cellular energy homoeostasis and calcium homoeostasis. These effects can to a certain extent be predicted from the electron-withdrawing and electron-donating effects of the substituents on the quinone nucleus of the molecule. Prediction of cytostatic potential remains much more complicated, because reduction of the quinones and the reactivity of the reduction products with DNA are modulated by the prevailing oxygen tension and by the prevalence of reducing enzymes in tumour cells.

Oxidation of mitochondrial pyridine nucleotides by aglycone derivatives of adriamycin

Adriamycin (AdM) and related anthracyclines are potent antineoplastic agents, the clinical utility of which is limited by severe cardiotoxicity. Aglycone derivatives of AdM have recently been reported to trigger the release of Ca2+ from isolated, preloaded rat heart mitochondria and to modify mitochondrial sulfhydryl (-SH) groups. Both mitochondrial Ca2+ retention and -SH status are sensitive to mitochondrial NAD(P)+/NAD(P)H ratios. This investigation examined the effects of AdM and its aglycone derivatives on the pyridine nucleotide redox status of isolated, intact heart mitochondria with the following results. (i) AdM aglycones induced the slow, Ca2(+)-independent oxidation of mitochondrial NAD(P)H. Oxidation was proportional to aglycone concentration between 5 and 60 microM. (ii) In terms of potency, 7-deoxy AdM aglycone greater than or equal to 7-hydroxy AdM aglycone much greater than AdM. (iii) Inhibitor data suggested that NAD(P)H oxidation reflects the rotenone-insensitive reduction of AdM aglycone and subsequent electron transfer to O2 generating superoxide. (iv) NAD(P)H oxidation mediated by AdM aglycone could be distinguished from the Ca2(+)-dependent NAD(P)H oxidation associated with mitochondrial Ca2+ release. This communication is the first to describe redox interactions of AdM with intact mitochondria.

Molecular mechanisms of quinone cytotoxicity

Quinones are probably found in all respiring animal and plant cells. They are widely used as anticancer, antibacterial or antimalarial drugs and as fungicides. Toxicity can arise as a result of their use as well as by the metabolism of other drugs and various environmental toxins or dietary constituents. In rapidly dividing cells such as tumor cells, cytotoxicity has been attributed to DNA modification. However the molecular basis for the initiation of quinone cytotoxicity in resting or non-dividing cells has been attributed to the alkylation of essential protein thiol or amine groups and/or the oxidation of essential protein thiols by activated oxygen species and/or GSSG. Oxidative stress arises when the quinone is reduced by reductases to a semiquinone radical which reduces oxygen to superoxide radicals and reforms the quinone. This futile redox cycling and oxygen activation forms cytotoxic levels of hydrogen peroxide and GSSG is retained by the cell and causes cytotoxic mixed protein disulfide formation. Most quinones form GSH conjugates which also undergo futile redox cycling and oxygen activation. Prior depletion of cell GSH markedly increases the cell's susceptibility to alkylating quinones but can protect the cell against certain redox cycling quinones. Cytotoxicity induced by hydroquinones in isolated hepatocytes can be attributed to quinones formed by autoxidation. The higher redox potential benzoquinones and naphthoquinones are the most cytotoxic presumably because of their higher electrophilicty and thiol reactivity and/or because the quinones or GSH conjugates are more readily reduced to semiquinones which activate oxygen.

Prevention of thromboxane B2-induced hepatocyte plasma membrane bleb formation by certain prostaglandins and a proteinase inhibitor

Isolated hepatocytes incubated in the presence of thromboxane B2 developed many plasma membrane blebs which are a characteristic feature of toxic or ischaemic cell injury. When hepatocytes were incubated in the presence of both thromboxane B2 and the non-lysosomal proteinase inhibitor, leupeptin, were also well protected from the formation of blebs. This implies that thromboxane B2 is able to activate non-lysosomal proteinases which appear to attack certain cytoskeletal proteins. The data presented are consistent with thromboxane B2 acting as an intermediary in a proposed mechanism of cell injury and death in which elevated cytosolic free Ca2+ levels activate phospholipase A2 and the arachidonic acid cascade.

2,5-Di(tert-butyl)-1,4-benzohydroquinone--a novel mobilizer of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool

Isolated hepatocytes and the isolated perfused rat liver have been used to study the alterations of cytosolic free Ca2+ concentration ([Ca2+]i) produced by 2,5-di(tert-butyl)-1,4-benzohydroquinone (tBuBHQ), a potent inhibitor of hepatic microsomal Ca2+ sequestration (Moore, G.A., McConkey, D.J., Kass, G.E.N., O'Brien, P.J. and Orrenius, S. FEBS Lett., 224, 331-336), (1987). Addition of tBuBHQ to isolated hepatocytes caused a rapid increase in [Ca2+]i which was similar in magnitude to the [Ca2+]i elevation induced by the Ca2+ mobilizing hormone, vasopressin. In contrast with vasopressin which caused a Ca2+ transient, tBuBHQ elevated [Ca2+]i to a new steady state that was maintained for up to 15-20 min. When vasopressin was administered during the tBuBHQ-induced period of elevated [Ca2+]i, [Ca2+]i rapidly returned to basal levels. Similarly, if vasopressin was administered just prior to tBuBHQ, the resultant tBuBHQ-dependent change in [Ca2+]i was transient, and not sustained. The hydroquinone mobilized the same intracellular Ca2+ pool as inositol 1,4,5-trisphosphate, but tBuBHQ did not produce any detectable inositol polyphosphate accumulation. tBuBHQ stimulated glucose release from perifused hepatocytes, mimicking the effect of vasopressin. In the perfused liver, tBuBHQ infusion produced a single, slow and prolonged release of Ca2+ into the perfusate and inhibition of subsequent vasopressin-induced Ca2+ effluxes. Inhibition of the response to vasopressin was reversed over time, and closely correlated with the extent of inhibition of both Ca2+ sequestration and (Ca2+-Mg2+)-ATPase activity in microsomes isolated from the isolated perfused liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged high intracellular free calcium concentrations induced by ATP are not immediately cytotoxic in isolated rat hepatocytes. Changes in biochemical parameters implicated in cell toxicity

Isolated rat hepatocytes were incubated with ATP to induce high intracellular free Ca2+ concentrations as determined with the Quin-2 method. Immediately after addition of ATP, the intracellular concentration of Ca2+ rose from 200 nM to more than 2.5 microM. It stayed at this value during the first 1/2 h; the rise was absolutely dependent on extracellular Ca2+. After the first 1/2 h the Ca2+ concentration decreased to 1-2 microM (5-10 times the control value). These high intracellular free Ca2+ concentrations did not lead to an immediate loss of cell viability. Only after 2 h of incubation a substantial number of cells lost viability. This was preceded by a decrease in cellular NADH (greater than 40%) and accompanied by a sharp increase in the intracellular Ca2+ concentration. Under these conditions the NADPH concentration was not affected. Cellular GSH was decreased to 30% of the initial value, but no lipid peroxidation or protein-thiol depletion was observed. Intracellular ATP, ADP and AMP were increased in the presence of extracellular ATP. Ca2+-dependent proteases seemed not to be involved in cell death. These observations are consistent with a collapse of mitochondrial functions as a final trigger of cell death.

Fluorescence-based viability assay for studies of reactive drug intermediates

Studies of drug toxicity, toxicologic structure-function relationships, screening of idiosyncratic drug reactions, and a variety of cytotoxic events and cellular functions in immunology and cell biology require the sensitive and rapid processing of often large numbers of cell samples. This report describes the development of a high-sensitivity, high-throughput viability assay based on (a) the carboxyfluorescein derivative 2'-7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) as a vital dye, (b) instrumentation capable of processing multiple small (less than 100 cells) samples, and (c) a 96-well unidirectional vacuum filtration plate. Double staining of cultured peripheral blood mononuclear cells with BCECF and propidium iodide (PI) showed no overlap between PI+ (nonviable) and BCECF+ (viable) cells by flow cytometric analysis. Optimal conditions were developed for dye loading and minimizing physical cell damage and fluorescence quench during the assay procedure. The ratio of BCECF fluorescence to internal standard fluorescent particles was linear from 40 to greater than 20,000 cells with a signal:noise ratio of approximately 3 at 40 cells/well. Sulfamethoxazole hydroxylamine (SMX-HA) was used as a model toxic drug metabolite to explore the validity of the BCECF procedure. SMX-HA, but not its parent compound sulfamethoxazole, resulted in a dose dependent loss of cellular fluorescence and the parallel accumulation of PI+ nonviable cells. When compared to the currently used tetrazolium dye reduction viability assay, the BCECF method was 3-fold more sensitive, greater than 10-fold faster, and required 1/10-1/100 the cell numbers.

DT-diaphorase-catalysed reduction of 1,4-naphthoquinone derivatives and glutathionyl-quinone conjugates. Effect of substituents on autoxidation rates

DT-diaphorase catalysed the reduction of 1,4-naphthoquinones with hydroxy, methyl, methoxy and glutathionyl substituents at the expense of reducing equivalents from NADPH. The initial rates of quinone reduction did not correlate with either the half-wave reduction potential (E1/2) value (determined by h.p.l.c. with electrochemical detection against an Ag/AgCl reference electrode) or the partition coefficient of the quinones. After their reduction by DT-diaphorase the 1,4-naphthoquinone derivatives autoxidized at distinct rates, the extent of which was influenced by the nature of the substituents. Thus for the 1,4-naphthoquinone series the following order of rate of autoxidation was found: 5-hydroxy-1,4-naphthoquinone greater than 3-glutathionyl-1,4-naphthoquinone greater than 5-hydroxy-3-glutathionyl-1,4-naphthoquinone greater than 1,4-naphthoquinone greater than 2-hydroxy-1,4-naphthoquinone. For the 2-methyl-1,4-naphthoquinone (menadione) series the following order was observed: 5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-2-methyl-1,4-naphthoquinone greater than 2-methyl-1,4-naphthoquinone greater than 3-hydroxy-2-methyl-1,4-naphthoquinone. The autoxidized naphthohydroquinone derivatives were re-reduced by DT-diaphorase, thus closing a cycle of enzymic reduction in equilibrium autoxidation. This was expressed as an excess of NADPH oxidized over the initial concentration of quinone present as well as H2O2 formation. These findings demonstrate that glutathionyl conjugates of 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone and those of their respective 5-hydroxy derivatives are able to act as substrates for DT-diaphorase and that they also autoxidize at rates higher than those for the unsubstituted parent compounds. These results are discussed in terms of the cellular role of DT-diaphorase in the reduction of hydroxy- or glutathionyl-substituted naphthoquinones as well as the further conjugation of these hydroquinones with glucuronide or sulphate within the cellular milieu, thereby facilitating their disposal from the cells.

Redox and addition chemistry of quinoid compounds and its biological implications

The overall biological activity of quinones is a function of the physico-chemical properties of these compounds, which manifest themselves in a critical bimolecular reaction with bioconstituents. Attempts have been made to characterize this bimolecular reaction as a function of the redox properties of quinones in relation to hydrophobic or hydrophilic environments. The inborn physico-chemical properties of quinones are discussed on the basis of their reduction potential and dissociation constants, as well as the effect of environmental factors on these properties. Emphasis is given on the effect of methyl-, methoxy-, hydroxy-, and glutathionyl substituents on the reduction potential of quinones and the subsequent electron transfer processes. The redox chemistry of quinoid compounds is surveyed in terms of a) reactions involving only electron transfer, as those accomplished during the enzymic reduction of quinones and the non-enzymic interaction with redox couples generating semiquinones, and b) nucleophilic addition reactions. The addition of nucleophiles, entailing either oxidation or reduction of the quinone, are exemplified in reactions with oxygen- or sulfur nucleophiles, respectively. The former yields quinone epoxides, whereas the latter yields thioether-hydroquinone adducts as primary molecular products. The subsequent chemistry of these products is examined in terms of enzymic reduction, autoxidation, cross-oxidation, disproportionation, and free radical interactions. The detailed chemical mechanisms by which quinoid compounds exert cytotoxic, mutagenic and carcinogenic effects are considered individually in relation to redox cycling, alterations of thiol balance and Ca++ homeostasis, and covalent binding.

Role of sulfhydryl groups in benzoquinone-induced Ca2+ release by rat liver mitochondria

Incubation of rat liver mitochondria with benzoquinone derivatives in the presence of succinate plus rotenone has been shown to cause NAD(P)H oxidation followed by Ca2+ release. Further investigation revealed: (1)p-Benzoquinone-induced Ca2+ release was not initiated by a collapse of the mitochondrial membrane potential. However, Ca2+ release and subsequent Ca2+ cycling caused limited increased membrane permeability. (2) p-Benzoquinone-induced NAD(P)H oxidation and Ca2+ release were prevented by isocitrate, 3-hydroxybutyrate, and glutamate but not by pyruvate or 2-oxoglutarate. (3) Inhibition of pyruvate and 2-oxoglutarate dehydrogenases by p-benzoquinone was attributed to arylation of the SH groups of the cofactors, CoA and lipoic acid. Isocitrate dehydrogenase was also inhibited by p-benzoquinone, but the cofactors NAD(P)H and Mn2+ protected the enzyme. Glutamate dehydrogenase was not inhibited by p-benzoquinone. (4) Arylation of mitochondrial protein thiols by p-benzoquinone was associated with an inhibition of state 3 respiration, which was attributed to the inactivation of the phosphate translocase. In contrast, state 4 respiration, and the F1.F0-ATPase and ATP/ADP translocase activities were not inhibited. It was concluded that inhibition of mitochondrial NAD(P)H dehydrogenases by arylation of critical thiol groups will decrease the NAD(P)+-reducing capacity, and possibly lower the NAD(P)H/NAD(P)+ redox status in favor of Ca2+ release.

Reductive addition of glutathione to p-benzoquinone, 2-hydroxy-p-benzoquinone, and p-benzoquinone epoxides. Effect of the hydroxy- and glutathionyl substituents on p-benzohydroquinone autoxidation

The reductive addition of GSH to p-benzoquinones, 2-hydroxy-p-benzoquinone, and 2,3-epoxy-p-benzoquinones with different degree of methyl substitution was studied in terms of absorption spectral changes and autoxidation reactions. The nucleophilic addition of GSH to p-benzoquinone yields a glutathionyl-p-benzohydroquinone product with maximal absorption at lambda 303nm. This compound autoxidizes slowly--but at a rate 8-fold higher than the parent hydroquinone--to glutathionyl-p-benzoquinone, which reveals maximal absorption at lambda 367 nm. The autoxidation of the glutathionyl derivative is accompanied by O2 consumption and H2O2 formation. The nucleophilic addition of GSH to either 2-hydroxy-p-benzoquinone or 2,3-epoxy-p-benzoquinone yields the same primary molecular product, 2-hydroxy-5-glutathionyl-p-benzohydroquinone, a compound that shows maximal absorption at lambda 300 nm and autoxidizes at rates substantially higher (44-fold) than the parent glutathionyl hydroquinone lacking a -OH substituent. The autoxidation product, 2-hydroxy-5-glutathionyl-p-benzoquinone, reveals maximal absorbance at lambda 343 nm as well as a resolved absorption band at longer wavelengths (lambda 520 nm), the latter contributed by the -OH substituent. The glutathionyl substituent exerted only minor changes in the reduction potential of the quinones, whereas the -OH substituent lowered significantly the half-wave reduction potential, as measured in aqueous solutions. The rate of autoxidation was markedly enhanced by both substituents as follows: hydroxy-glutathionyl-p-benzohydroquinone much greater than hydroxy-p-benzohydroquinone much greater than glutathionyl-p-benzohydroquinone greater than p-benzohydroquinone. Superoxide dismutase enhanced the rate of autoxidation of p-benzohydroquinone and its glutathionyl adduct, whereas it inhibited autoxidation of the hydroxy derivatives with or without glutathionyl substitution. The biochemical significance of these results is discussed in terms of the pro-oxidant character of the reductive addition of GSH to p-benzoquinones, alpha-hydroxyquinones, and quinone epoxides.