The toxicity of acetaminophen and N-acetyl-p-benzoquinone imine in isolated hepatocytes is associated with thiol depletion and increased cytosolic Ca2+

Toxicity of the cyanobacterium Nodularia spumigena Mertens

The bloom forming cyanobacterium (blue-green alga) Nodularia spumigena produced a peptide hepatotoxin with an LD50 of 70 micrograms/kg i.p. in mice. The livers of lethally poisoned mice were haemorrhagic and enlarged, the weight doubling to about 10% of total body weight. Histologically there was centrilobular to midzonal disruption and lysis of hepatocytes resulting in haemorrhage and formation of blood lakes. Death occurred approximately 1 hr after i.p. injection. By 30 min significant increases had occurred in the plasma levels of lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase and glucose paralleling degeneration and necrosis of centrilobular hepatocytes. In vitro the toxin caused rapid dose-dependent deformation of freshly isolated rat hepatocytes, which was accompanied by the activation of phosphorylase a; 125 ng/ml of toxin being sufficient to cause these changes in 10(6) cells. This work demonstrates that, both in vivo and in vitro, Nodularia toxin shares many similarities in its action to the well characterized peptide toxins of another cyanobacterium, Microcystis aeruginosa.

Ca2+ release from mitochondria induced by prooxidants

A variety of chemically different prooxidants causes Ca2+ release from mitochondria. The prooxidant-induced Ca2+ release occurs from intact mitochondria via a route which is physiologically relevant and may be regulated by protein ADP-ribosylation. When the released Ca2+ is excessively cycled by mitochondria they are damaged. This leads to uncoupling, a decreased ATP supply, and a decreased ability of mitochondria to retain Ca2+. Excessive Ca2+ cycling by mitochondria will deprive cells of ATP. As a result, Ca2+ ATPases of the endoplasmic (sarcoplasmic) reticulum and the plasma membrane are stopped. The rising cytosolic Ca2+ level cannot be counterbalanced due to damage of mitochondria which, under normoxic conditions, act as safety device against increased cytosolic Ca2+. It is proposed that prooxidants are toxic because they impair the ability of mitochondria to retain Ca2+.

Cytochrome P-450-mediated oxidation of substrates by electron-transfer; role of oxygen radicals and of 1- and 2-electron oxidation of paracetamol

The mechanism by which the hepatic cytochrome P-450 (Cyt. P-450) containing mixed-function oxidase system oxidizes the analgesic drug paracetamol (PAR) to a hepatotoxic metabolite was studied. Since previous studies excluded the possibility of oxygenation of PAR, three other mechanisms, namely direct 1-electron oxidation by a Cyt. P-450-ferrous-dioxygen complex under concomitant formation of H2O2 to N-acetyl-p-semiquinone imine (NAPSQI), direct 2-electron oxidation by a Cyt. P-450-ferric-oxene complex to N-acetyl-p-benzoquinone imine (NAPQI) and indirect oxidation by active oxygen species released from Cyt. P-450, were considered. Indirect oxidation by active oxygen species was not involved, as active oxygen scavengers such as superoxide dismutase, catalase and DMSO did not affect the oxidation of PAR in hepatic microsomes. No reaction products characteristic for a direct 1-electron oxidation of PAR by Cyt. P-450 were observed: neither NAPSQI radical formation was detectable by ESR, nor PAR-dimer formation, nor stimulation of the microsomal H2O2 production was found to occur. In fact, PAR inhibited the spontaneous microsomal H2O2 formation. Studies on the reactions of NAPSQI with glutathione (GSH) revealed that NAPSQI hardly conjugated with GSH to a 3-glutathionyl-paracetamol conjugate (PAR-GSH) conjugate. The reactions of the elusive reactive metabolite formed during microsomal oxidation of PAR in the presence of GSH closely resembled those of synthetic NAPQI: both PAR-GSH and oxidized glutathione (GSSG) formation occurred. Furthermore, in agreement with a 2-electron oxidation hypothesis, iodosobenzene-dependent oxidation of PAR by cyt. P-450 in the presence of GSH resulted in the formation of the PAR-GSH conjugate. It is concluded that bioactivation of PAR by the Cyt. P-450 containing mixed-function oxidase system consists of a direct 2-electron oxidation to NAPQI.

Acetaminophen hepatotoxicity in aging rats

Severity of liver damage 24 hr after i.p. administration of acetaminophen in doses of 0.4 and 0.8 g/kg was evaluated in male Fischer 344 rats at 4, 14 and 25 months of age. Both doses of acetaminophen produced significant elevations of serum alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) activities in 4-month-old rats. Enzyme release was somewhat diminished in old age. Hepatic glutathione (GSH) and microsomal cytochrome P-450 concentrations were decreased in rats that received 0.8 g/kg of acetaminophen. The decreases occurred in young-adult and middle-aged rats, but not in old rats. The results demonstrated that old age does not enhance the hepatotoxic effects of acetaminophen in male Fischer 344 rats.

Histochemical and biochemical observations on the cytotoxicity of paracetamol and its effects on glycogen metabolism in rat liver

The effects of paracetamol overdose on glycogen metabolism in rat liver have been investigated and related to its cytotoxicity. Paracetamol was administered to male rats by gavaging after a 24-h fast and refeeding was not permitted. An early (9-12-h) increase in histochemically demonstrable glycogen phosphorylase alpha activity in perivenous hepatocytes preceded major loss of membrane integrity as assessed by serum glutamate-pyruvate transaminase (SGPT) activity and uptake of trypan blue during perfusion. These changes occurred only after a decrease in the concentration of reduced glutathione, which is generally observed about 4 h after paracetamol treatment. The activation of glycogen phosphorylase in perivenous hepatocytes occurred concurrently with an increase in glycogen content of periportal hepatocytes, indicating a clear heterogeneity in the response of the two-cell populations to the hepatotoxin. The use of trypan blue perfusion together with histochemical techniques allowed changes in glycogen content and phosphorylase alpha activity of individual hepatocytes to be assessed with reference to the extent of membrane damage evident. The relevance of the results to possible mechanisms of hepatotoxicity is discussed.

Correspondence of results from hepatocyte studies with in vivo response

Isolated hepatocyte systems are being examined in our laboratory for a number of applications, including alternatives to animal testing. This report summarizes findings from studies with chlorinated aliphatics, acetaminophen, nitrotoluenes, and cyanide and its antidotes that relate to in vivo toxicity and validation of these systems for cytotoxicity screening.

Early inhibition of the Na+/K+-ATPase ion pump during acetaminophen-induced hepatotoxicity in rat

The status of Na+ regulation was examined during early stages of alkylation insult to rat liver. Na+/K+-ATPase activity in plasma membranes declined by 52% within 3 hr of treatment with 850 mg/kg acetaminophen. This loss preceded the release of alanine aminotransferase (2880 +/- 1550 U/ml) and necrosis (2+) seen at 24 hr. Activities of 5'-nucleotidase and Mg2+-ATPase and recovery of plasma membranes were comparatively unchanged at 3 hr. Because damage to Na+/K+-ATPase appeared early in the pathogenesis of acetaminophen hepatotoxicity, loss of hepatocellular Na+ regulation could represent one of the critical molecular consequences of lethal alkylation by acetaminophen.

Early disturbance of calcium translocation across the plasma membrane in toxic liver injury

An increased influx and/or a decreased extrusion of calcium across the plasma membrane resulting in an increase in cytosolic-free calcium could play an important role in the initiation of irreversible cell injury. Therefore, the translocation of calcium across the plasma membrane was probed in the perfused rat liver using multiple indicator dilution methodology. The sucrose space corresponding to the extracellular space amounted to 0.35 +/- 0.13 ml per gm liver, and the water space corresponding to the extra- and intracellular spaces was 0.97 +/- 0.08 ml per gm. The calcium space was always slightly larger (0.42 +/- 0.10 ml per gm) than the sucrose space. The calcium space further increased during perfusion with the calcium ionophore A 23187, whereas the sucrose space remained unchanged. Two hours after administration to intact rats of acetaminophen (2 gm per kg) and carbon tetrachloride (2 ml per kg), respectively, the calcium space had increased markedly relative to the sucrose space and relative to the water space, indicating an increased accessibility of the cells to extracellular calcium. Similarly, reperfusion of livers after 90 min of ischemia was associated with an increase in calcium space relative to the sucrose and water spaces. These studies indicate that, in three models of acute liver injury, the net influx of calcium across the plasma membrane is increased early in the evolution of the injury before irreversible damage occurs.

Lipid peroxidation, protein thiols and calcium homeostasis in bromobenzene-induced liver damage

The mechanisms of bromobenzene hepatotoxicity in vivo were studied in mice. The relationships among glutathione (GSH) depletion, lipid peroxidation, loss of protein thiols, disturbed calcium homeostasis and liver necrosis were investigated. Liver necrosis (as estimated by the serum glutamate-pyruvate transaminase (SGPT) level) appeared between 9 and 12 hr and increased at 18 hr. Lipid peroxidation which was already detectable at 6 hr in some animals, increased thereafter showing a good correlation with the severity of liver necrosis. Despite a quite fast depletion of hepatic GSH, a significant decrease in protein thiols could be observed at 12-18 hr only. Loss of protein thiols in both whole liver and subcellular fractions (microsomes and mitochondria) was correlated with lipid peroxidation. Also a good inverse correlation was seen between lipid peroxidation and the calcium sequestration activity of liver microsomes and mitochondria. The treatment of mice with desferrioxamine (DFO) after bromobenzene-intoxication completely prevented lipid peroxidation, loss of protein thiols and liver necrosis in the animals sacrificed 15 hr after poisoning. When, however, the animals were examined at 24 hr, although the general correlation between lipid peroxidation and liver necrosis was held, in some animals (about 30% of the survivors) elevation of SGPT was observed in the virtual absence of lipid peroxidation. It seems likely therefore that the liver damage seen during the first phase of bromobenzene-intoxication is strictly related to lipid peroxidation. It is, however, possible that in some animals in which for some reason lipid peroxidation does not develop, another mechanism of liver necrosis unrelated to lipid peroxidation occurs at later times.

Reversible oxidation of glyceraldehyde 3-phosphate dehydrogenase thiols in human lung carcinoma cells by hydrogen peroxide

Human lung carcinoma cells (A549) were oxidatively stressed with mildly-toxic or non-toxic amounts of hydrogen peroxide (H2O2, 0.1 mM to 120 mM) for 5 min. Hydrogen peroxide exposure resulted in a dose dependent inhibition of binding (pH 7) of the thiol reagent iodoacetic acid (IAA) to a 38 kDa cell protein. Incubation of cells in saline for 60 min following H2O2 removal restored the ability of IAA to bind to the protein. Treatment with 20 mM dithiothreitol or 2 M urea also restored IAA binding, but 10% Triton X102 or 1 mM Brij 58 had no effect. Increasing to pH 11 during the IAA binding also increased thiol availability. Glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) has been identified as the protein undergoing thiol/disulfide redox status and enzymic activity changes.

The mechanism of prevention of paracetamol-induced hepatotoxicity by 3,5-dialkyl substitution. The roles of glutathione depletion and oxidative stress

Recently, we have reported that 3,5-dialkyl substitution of paracetamol, in contrast to 3-monoalkyl substitution, prevented the paracetamol-induced toxicity in freshly isolated rat hepatocytes without having any effect on its cytochrome P-450 mediated bioactivation to reactive N-acetyl-p-benzoquinone imines (NAPQI). In the present study the mechanism of this prevention of toxicity, with special emphasis on oxidative stress, was studied in more detail in freshly isolated rat hepatocytes, using paracetamol, 3-methyl-, 3,5-dimethyl-paracetamol, synthetic NAPQI and 3,5-dimethyl-NAPQI. 3-Methyl-paracetamol was found to induce glutathione (GSH) depletion, lipid-peroxidation and cytotoxicity in hepatocytes to the same extent as paracetamol. 3,5-Dimethyl-paracetamol, however, even when added in a ten-fold higher concentration when compared to paracetamol, did not induce any of these effects. Similar differences of toxicity were observed between NAPQI and 3,5-dimethyl-NAPQI; 3,5-dimethyl-NAPQI, in contrast to NAPQI, did not reduce protein thiol levels, did not induce GSH depletion, lipid-peroxidation nor cytotoxicity. Only after artificial depletion of GSH levels in the hepatocytes by DEM or BCNU, 3,5-dimethyl-NAPQI was cytotoxic. This effect was accompanied by depletion of protein thiol levels, but not by lipid-peroxidation. Addition of the disulfide reducing agent, dithiothreitol, prevented the artificially created cytotoxicity of 3,5-dimethyl-NAPQI. It is concluded that prevention of paracetamol-induced toxicity by 3,5-dialkyl substitution is primarily due to prevention of irreversible GSH-depletion, presumably caused by the inability of 3,5-dialkyl-NAPQI to conjugate with thiols. As a result, the GSH-dependent cellular defense mechanism against potential oxidative cellular injury by 3,5-dialkyl-NAPQI is left unimpaired. Our observations indicate that a compound, not capable of covalent binding to thiol groups of proteins, can induce toxicity solely as a result of protein thiol oxidation without inducing lipid-peroxidation.

Vitamin E protection against chemical-induced cell injury. I. Maintenance of cellular protein thiols as a cytoprotective mechanism

Vitamin E protection against chemical-induced toxicity to isolated hepatocytes was examined during an imbalance in the thiol redox system. Intracellular reduced glutathione (GSH) was depleted by two chemicals of distinct mechanisms of action: adriamycin, a cancer chemotherapeutic agent that undergoes redox cycling, producing reactive oxygen species that consume GSH, and ethacrynic acid, a direct depleter of GSH. The experimental system used both nonstressed vitamin E-adequate isolated rat hepatocytes and compromised hepatocytes subjected to physiologically induced stress, generated by incubation in calcium-free medium. At doses whereby intracellular GSH was near total depletion, cell injury induced by either chemical was found to follow the depletion of cellular alpha-tocopherol, regardless of the status of the GSH redox system. Changes in protein thiol contents of the cells closely paralleled the changes in alpha-tocopherol contents throughout the incubation period. Supplementation of the calcium-depleted hepatocytes with alpha-tocopheryl succinate (25 microM) markedly elevated their alpha-tocopherol content and prevented the toxicities of both drugs. The prevention of cell injury and the elevation in alpha-tocopherol contents were both associated with a prevention of the loss in cellular protein thiols in the near total absence of intracellular GSH. The mechanism of protection by vitamin E against chemical-induced toxicity to hepatocytes may therefore be an alpha-tocopherol-dependent maintenance of cellular protein thiols.

Paracetamol, 3-monoalkyl- and 3,5-dialkyl derivatives: comparison of their hepatotoxicity in mice

The effect of 3-monoalkyl and 3,5-dialkyl substitution (R = CH3, C2H5, and i-C3H7) on hepatotoxicity of the analgesic paracetamol was studied in vivo. To that purpose, varying doses of paracetamol and six alkyl-substituted derivatives were orally administered to male DAP mice. Paracetamol caused hepatotoxicity as judged from elevation of plasma transaminase activities and liver histopathology at a dose of 3.95 mmol/kg. All 3-monoalkyl-substituted derivatives of paracetamol caused centrilobular necrosis at oral doses of 4.40, 4.85, and 5.30 mmol/kg of 3-methyl-, 3-ethyl-, and 3-isopropyl derivatives, respectively. Oral dosage of the 3,5-dialkyl-substituted derivatives up to 6.25 mmol/kg did not result in hepatotoxicity. Since 3,5-dialkyl substitution of paracetamol does not reduce the analgesic activity, the observed prevention of paracetamol-induced hepatic necrosis by 3,5-dialkyl substitution may offer perspectives for the design of safer analgesics.

Ultrastructural changes during acute acetaminophen-induced hepatotoxicity in the mouse: a time and dose study

This study was undertaken to evaluate the early ultrastructural changes during the development of acetaminophen hepatotoxicity. Doses at or near the threshold for hepatotoxicity were selected to permit comparison of early reversible effects to those which ultimately progressed to necrosis in the absence of early agonal effects or drug-induced mortality. Both 300- and 600-mg/kg doses resulted in similar declines in hepatic glutathione levels to 14 and 22% of control values, respectively, by 2 hours, with more rapid recovery after the low dose. Plasma sorbitol dehydrogenase activity was elevated after 600 mg/kg but not after 300 mg/kg. During the first 2 hours after acetaminophen there was cytomegaly with rapid progression to necrosis after 600 mg/kg but minimal progression after 300 mg/kg. Ultrastructurally, vesiculation, vacuolation and mitochondrial and plasma membrane degeneration culminated in scattered single cell death by 4 hours and widespread centrilobular necrosis by 8 hours after 600 mg/kg. The time course of lesion development was slower after 300 mg/kg with damage restricted to the first two to three rows of centrilobular cells and limited numbers of isolated necrotic cells by 8 hours. By 18 to 24 hours livers of mice given 300 mg/kg appeared normal. Results are consistent with the endoplasmic reticulum being the site of acetaminophen activation and initial attack. However, early ultrastructural changes in mitochondria and plasma membrane observed after the high dose were not prominent after the low dose. This suggests that early acetaminophen damage to these organelles may play a critical role in acetaminophen hepatotoxicity.

A general theory of chemical cytotoxicity based on a molecular model of the living cell, the Bhopalator

To define the molecular processes underlying toxicological manifestations experimentally measured on the cellular level, it is essential to have available a molecular model of the living cell itself. The Bhopalator is a molecular model of the living cell formulated by integrating the three major branches of biology within a coherent theoretical framework - the Watson-Crick molecular genetics, the conformon theory of enzymic catalysis, and the theory of dissipative structures developed by I. Prigogine. According to this model, the living cell is a self-moving, self-thinking and self-reproducing machine (automaton) that receives information and energy from its environment, processes them according to the genetic programs stored in DNA, and generates output signals to environment in order to realize teleonomically designed functions. The Bhopalator suggests a set of general statements useful in toxicological research, and these statements have been utilized to provide possible answers to several fundamental questions raised by recent experimental findings on chemically-induced cell injury and death.

Cytosolic calcium after carbon tetrachloride, 1,1-dichloroethylene, and phenylephrine exposure. Studies in rat hepatocytes with phosphorylase a and quin2

Carbon tetrachloride (CCl4) and 1,1-dichloroethylene (DCE), both hepatotoxins, inhibit sequestration of Ca2+ by rat liver endoplasmic reticulum (ER) both in vivo and in vitro. It is possible that, as a result, cytosolic Ca2+ concentrations rise in liver cells. In experiments presented here, isolated hepatocytes were exposed to CCl4, DCE, and phenylephrine (PE), a non-hepatotoxic alpha 1-adrenergic agent that mobilizes Ca2+. Cytoplasmic Ca2+ concentrations were evaluated by two methods: indirectly by assaying the activity of glycogen phosphorylase a, and directly by monitoring the fluorescence of quin2. In primary hepatocyte cultures, CCl4, DCE, and PE exposure increased the activity of phosphorylase a at 5 min from 39 +/- 2 to 130 +/- 12, 80 +/- 13, and 97 +/- 10 nmoles PO4(3-)/mg protein/min respectively. In rat hepatocyte suspensions loaded with quin2 and exposed to CCl4, DCE, or PE, cytosolic Ca2+ concentrations were elevated within 20 sec to 0.83 +/- 0.13, 0.59 +/- 0.06 and 0.99 +/- 0.14 microM Ca2+ respectively. Basal Ca2+ levels in these cells averaged 0.25 +/- 0.03 microM. Thus, CCl4 and PE apparently increased cytosolic Ca2+ levels to approximately the same extent, whereas DCE was somewhat less effective. The durations of the effects of CCl4 and PE were examined further by determining their time courses of elevated phosphorylase a activity. In hepatocyte cultures, increased phosphorylase a activity persisted through at least 60 min following CCl4 exposure. In contrast, phosphorylase a activity returned to basal levels by 20 min after PE. Increases in cytoplasmic Ca2+ levels that are sustained rather than transient may distinguish these hepatotoxic chlorinated aliphatic hydrocarbons from non-toxic hormonal agents.

Species differences in the hepatotoxicity of paracetamol are due to differences in the rate of conversion to its cytotoxic metabolite

The cytotoxicity of paracetamol and of its putative toxic metabolite, N-acetyl-p-benzo-quinoneimine (NABQI) have been investigated in hepatocytes from hamster, mouse, rat and human liver. Whereas paracetamol readily caused cell blebbing and a loss of viability in hepatocytes from mouse and hamster, human and rat hepatocytes were much more resistant to these effects. In marked contrast, there were no significant differences in the sensitivity of the cells from any species to the toxic effects of NABQI. Glutathione depletion by NABQI and paracetamol correlated very well with the toxic effects of these compounds. It is concluded that species differences in sensitivity to the hepatotoxicity of paracetamol are due almost entirely to differences in the rate of formation of NABQI, and not to any intrinsic differences in sensitivity or in any difference in the fate of NABQI once formed. Further, man appears to be relatively resistant to the hepatotoxic effects of paracetamol, and the results in hepatocytes were confirmed by both in vitro and in vivo analyses.

The use of N-acetylcysteine long after an acetaminophen overdose in mice

N-Acetylcysteine (NAC), given to mice 5 h after an LD50 dose of acetaminophen, decreased 24-h survival to 33%. The reduction in survival observed for late NAC is significantly lower than the survival observed for the LD50 dose alone (53%), and is in sharp contrast to the 100% survival reported for the early use of NAC.

Aziridine biotransformation by microsomes and lethality to hepatocytes isolated from rat

To clarify the relationship of aziridine biotransformation to their cytotoxic activities, the metabolism of optical isomers of typical cytotoxic and non-cytotoxic aziridines was studied in isolated hepatocytes, rat liver microsomes, mitochondria and L-1210 mouse leukemia cells. Cytotoxic 1-methyl-2-beta-naphthylaziridine (NAZ) gave nitrosomethane as one of the bioactivation products in isolated hepatocytes and simultaneously induced a marked decrease in cellular ATP followed by cell lethality. NAZ itself did not directly affect the respiratory function of mitochondria in isolated hepatocytes or in buffer solution, however, it inhibited the mitochondrial activity in the presence of microsomes in the buffer solution. Nitroso-t-butane or nitrosomethane dimer, used as a substitute for extremely labile nitrosomethane, strongly inhibited the respiration of mitochondria. On the other hand, optical isomers of 2-aziridinecarboxylic acid (AZC) which did not give nitrosomethane in isolated hepatocytes or microsomes also did not show cytotoxicity. Thus, the cytotoxicity of NAZ seems to be induced by bioactivation via cellular oxidases with the nitrosomethane generated being a major toxic component. This may occur with most of the cytotoxic aziridine derivatives.

Injury to hepatocytes induced by a peptide toxin from the cyanobacterium Microcystis aeruginosa

The freshwater, bloom forming cyanobacterium (blue-green alga) Microcystis aeruginosa produces a peptide hepatotoxin which causes death accompanied by liver necrosis. We show here that the time and dose-dependent blebbing of isolated hepatocytes is accompanied by the activation of phosphorylase a, with no changes in cyclic AMP levels, and by glutathione (acid-soluble thiols) depletion. These results suggest that the disruption of cytoskeletal structures is accompanied by disturbances in cellular calcium homeostasis and by decreased protection against oxidative damage to the cells.

Cellular effects of N(4-ethoxyphenyl)p-benzoquinone imine, a p-phenetidine metabolite formed during peroxidase reactions

The interaction of N-(4-ethoxyphenyl)p-benzoquinone imine (NEPBQI), a metabolite formed during peroxidase catalyzed metabolism of p-phenetidine, with GSH and its effects in isolated rat hepatocytes were investigated. When reacted with GSH NEPBQI formed both a mono- and a diglutathione conjugate as well as GSSG. Formation of glutathione conjugates and GSSG also occurred when NEPBQI was added to isolated hepatocytes. The formation of GSSG was, however, only detectable if the hepatocytes had been pretreated with the GSSG reductase inhibitor BCNU (1,3-bis-(2-chloroethyl-1-nitrosourea). Similarly, NEPBQI caused a rapid decrease in cellular free protein thiols when added to hepatocytes, however this was expressed at higher concentrations than for effects on GSH. The protein thiol decrease was correlated with protein binding of NEPBQI. NEPBQI was also shown to be toxic to isolated hepatocytes. At a concentration of 400 microM extensive bleb formation was followed by loss of cell membrane integrity and cell death. To assess further the subcellular metabolism of NEPBQI microsomes and cytosol was used. NEPBQI was found to be preferentially reduced by cytochrome P-450 reductase in the microsomes whereas DT-diaphorase catalyzed its reduction in cytosol. NEPBQI did not undergo significant redox cycling since no formation of O2 was observed. Thus, the cytotoxic effect of NEPBQI appears to be due to protein arylation rather than redox cycling.

Mechanisms of petroleum hydrocarbon toxicity: destruction of liver microsomal and mitochondrial calcium pump activities by a Prudhoe Bay crude oil

Administration of Prudhoe Bay crude oil (PBCO) to rats resulted in an abrupt drop in liver mitochondrial and microsomal ATP-dependent calcium uptake activity. Also, in vitro incubations of either mitochondria or microsomes in the presence of a dimethyl sulfoxide (DMSO) extract of PBCO resulted in a dose-dependent inhibition of calcium influx. The release of calcium from calcium-loaded mitochondria and microsomes was also observed in the presence of the PBCO extract. At concentrations which effect calcium sequestration, the PBCO extract produced swelling of mitochondria. Microsomal ATPase activity in the presence or absence of calcium was unaffected by PBCO. The results indicate that increased permeability of the membranes to calcium is a contributory factor in the inhibition of calcium uptake by PBCO.

Effect of N-hydroxyparacetamol on cell cycle progression

N-hydroxyparacetamol treatment of rat kidney cells gave rise to a dose-dependent decrease in DNA synthesis. A concentration of 1.0 mM N-hydroxyparacetamol at pH 7.2 decreased the level of DNA synthesis to 13.0 +/- 2.3% of the control value after 1 hr incubation. This compound also caused a perturbation of cell cycle progression. A concentration of 0.44 mM N-hydroxyparacetamol induced G1/S and S phase blocks. These delays became evident at approximately 12 hr after treatment and persisted until about 15 hr when cells started to recover. It seems unlikely that N-hydroxyparacetamol inhibits DNA synthesis and perturbs cycle progression through alterations to DNA structure as such, since this compound failed to alter the migration pattern of naked plasmid DNA.

tert-Butylhydroperoxide-induced toxicity in isolated hepatocytes: contribution of thiol oxidation and lipid peroxidation

Incubation of isolated rat hepatocytes with tert-butylhydroperoxide resulted in marked cytotoxicity preceded by intracellular glutathione depletion and extensive lipid peroxidation. Addition of antioxidants delayed, but did not prevent, this toxicity. A significant decrease in protein-free sulfhydryl groups also occurred in the presence of tert-butylhydroperoxide; direct oxidation of protein thiols and mixed disulfide formation with glutathione were responsible for this decrease. The involvement of protein thiol depletion in tert-butylhydroperoxide-induced cytotoxicity is suggested by our observation that administration of dithiothreitol, which caused re-reduction of the oxidized sulfhydryl groups and mixed disulfides, efficiently protected the cells from toxicity. Moreover, depletion of intracellular glutathione by pretreatment of the hepatocytes with diethyl maleate accelerated and enhanced the depletion of protein thiols induced by tert-butylhydroperoxide and potentiated cell toxicity even in the absence of lipid peroxidation.

Protection of rat hepatocytes from tert-butyl hydroperoxide-induced injury by catechol

Metabolism of tert-butyl hydroperoxide (TBHP, 2.0 mM) by glutathione peroxidase within isolated rat hepatocytes caused a rapid oxidation of intracellular reduced glutathione and ultimately NADPH through glutathione reductase. TBHP also caused the formation of surface blebs in the hepatocyte plasma membrane followed by the leakage of cytosolic enzymes, such as lactate dehydrogenase, into the incubation medium. Catechol (0.1 mM) protected hepatocytes from the cytotoxic effects of TBHP but did not prevent the rapid oxidation of glutathione indicating normal metabolism of TBHP through glutathione reductase. In contrast, addition of catechol to the hepatocyte incubations prevented TBHP-induced depletion of intracellular NADPH and increased the total NADP+ + NADPH concentration without altering significantly the intracellular NADP+ content or the NADPH/NADP + NADPH ratio. Catechol did not alter TBHP stimulation of the pentose phosphate pathway. Hepatocytes incubated with sublethal concentrations of TBHP (1.0 mM) did not leak lactate dehydrogenase into the medium but did lose intracellular potassium. In these experiments, TBHP caused a sustained increase in phosphorylase alpha activity suggesting that TBHP metabolism may be associated with a sustained increase in cytosolic free Ca2+. In the presence of catechol, phosphorylase alpha activity was increased by 5 min but returned toward control by 20 min. These data suggest that catechol may be protecting hepatocytes from TBHP-induced injury by preventing a sustained rise in cytosolic free Ca2+ concentration.

Cytotoxicity of phenazine methosulfate in isolated rat hepatocytes is associated with superoxide anion production, thiol oxidation and alterations in intracellular calcium ion homeostasis

The metabolism of phenazine methosulfate (PMS) by isolated rat hepatocytes is associated with superoxide anion production, and with a substantial decrease in intracellular levels of reduced glutathione, most of which is oxidized to GSSG. A marked loss of protein-free sulfhydryl groups also occurs when intracellular glutathione is depleted, and cytotoxicity follows. These effects are associated with the inhibition of the plasma membrane Ca2+-ATPase and with intracellular accumulation of calcium ion which is preferentially sequestered in mitochondria. Maintenance of protein sulfhydryl groups in the reduced state by dithiothreitol (DTT) prevents the alterations in intracellular calcium homeostasis and protects against toxicity.

Ca2+ homeostasis and cytotoxicity in isolated hepatocytes: studies with extracellular adenosine 5'-triphosphate

The incubation of isolated rat hepatocytes with extracellular adenosine 5'-triphosphate (ATP) resulted in an inhibition of Ca2+ efflux. The ATP-induced Ca2+ accumulation as determined by the increase in phosphorylase a activity and the Ca2+-sensitive fluorescent indicator (2-[(2-bis-[carboxymethyl]-amino-5-methylphenoxy)-methyl]-6-methoxy-8- bis-[carboxymethyl]aminoquinoline-tetrakis-[acetoxymethyl]ester) (Quin 2-AM) was associated with both the hydrolysis of ATP and the phosphorylation of a 110 kDa protein. No significant alteration in the intracellular ATP level was observed. The appearance of surface blebs and cytotoxicity followed the rise in cytosolic Ca2+, suggesting that the increased free Ca2+ may be responsible for the loss of viability. When a calmodulin inhibitor, 1-[bis(4-chlorophenyl)methyl]-3-[ 2-(2,4-dichlorophenyl)-2-[(2,4-dichlorophenyl)methoxy] ethyl]-1H- imidazolium chloride (calmidazolium), was included in the medium prior to ATP addition, bleb formation was reduced and the loss of viability was completely prevented, indicating that a Ca2+-calmodulin process may be involved in the initiation of cytotoxicity.

Cellular defense mechanisms against toxic substances

Recent studies of cellular defense mechanisms against toxic substances are reviewed with particular emphasis on the critical functions of reduced glutathione. Studies of the metabolism of paracetamol and of the redox active quinone menadione in isolated rat hepatocytes, are summarized in order to illustrate how multiple defense mechanisms are involved in the protection of the cell against the toxicity of these agents. Cytotoxicity with both agents occurs only after the cellular defense mechanisms have become exhausted.