Carbonyl-scavenging drugs & protection against carbonyl stress-associated cell injury

This mini-review highlights the chemical and cytoprotective properties of various hydralazine analogues that block the induction of cell death by acrolein, a highly toxic contributor to "carbonyl stress" during a diverse range of human diseases. Recent work on the action of hydralazine against various carbonyl-mediated pathologies is also reviewed.

Optimisation of the comet genotoxicity assay in freshly isolated murine hepatocytes: detection of strong in vitro DNA damaging properties for styrene

While the comet assay is used to detect DNA damage in isolated cells following exposure to chemicals in vitro, few publications report the use of the procedure in liver cells isolated from mice. Our initial efforts to use the assay to assess DNA damage in mouse hepatocytes maintained on collagen-coated dishes were hampered by high levels of baseline damage in controls, which appeared to result from mechanical damage sustained during the dislodgement of adherent cells in the early stages of the assay protocol. Here we describe an efficient version of the comet assay in cultured mouse hepatocytes that involves careful recovery of cells using a "scraping" buffer supplemented with 10% high purity grade DMSO. Use of this buffer strongly diminished the frequency of false positives. Using the industrial reagent styrene as a positive control in the optimised procedure, non-cytotoxic concentrations of this substance (2.5-10 mM) significantly increased mean comet tail length, area, and moment. Co-incubation with the CYP inhibitor SKF-525A strongly attenuated these effects of styrene. Collectively, these findings confirm this method is highly suitable for the detection of DNA damage by bioactivation-dependent compounds in freshly isolated mouse hepatocytes.

Extensive protein carbonylation precedes acrolein-mediated cell death in mouse hepatocytes

Allyl alcohol hepatotoxicity is mediated by an alcohol dehydrogenase-derived biotranformation product, acrolein. This highly reactive alpha,beta-unsaturated aldehyde readily alkylates model proteins in vitro, forming, among other products, Michael addition adducts that possess a free carbonyl group. Whether such damage accompanies acrolein-mediated toxicity in cells is unknown. In this work we established that allyl alcohol toxicity in mouse hepatocytes involves extensive carbonylation of a wide range of proteins, and that the severity of such damage to a subset of 18-50 kDa proteins closely correlated with the degree of cell death. In addition to abolishing cytotoxicity and glutathione depletion, the alcohol dehydrogenase inhibitor 4-methyl pyrazole strongly attenuated protein carbonylation. Conversely, cyanamide, an aldehyde dehydrogenase inhibitor, enhanced cytotoxicity and protein carbonylation. Since protein carbonylation clearly preceded the loss of membrane integrity, it may be associated with the toxic process leading to cell death.

Cell-free protein synthesis inhibition assay for the cyanobacterial toxin cylindrospermopsin

The cyanobacterial toxin cylindrospermopsin (CYN) is known to be a potent inhibitor of protein synthesis. This paper describes the use of a rabbit reticulocyte lysate translation system as a protein synthesis inhibition assay for CYN. A dose response curve for protein synthesis inhibition by CYN was constructed and was modeled to a sigmoidal dose response curve with variable slope (R2 = 0.98). In this assay, CYN has an IC50 of 120 nM [95% confidence limits (Cl) = 111-130 nM] with a detection limit in the region of 50 nM in the assay solution. Application of the assay allows quantification of toxin samples within the range 0.5-3.0 microM (200-1200 micrograms/L) CYN. To assess the usefulness of this assay, a range of toxic and nontoxic Cylindrospermopsis raciborskii extracts, including both laboratory strains and environmental samples, were assayed by protein synthesis inhibition. These CYN quantifications were then compared to quantifications obtained by high performance liquid chromatography (HPLC) and HPLC-tandem mass spectrometry (HPLCMS-MS). The results demonstrate that the protein synthesis inhibition assay correlates well with both HPLCMS-MS (r2 = 0.99) and HPLC (r2 = 0.97) quantifications. We conclude that this is an accurate and rapid assay for the measurement of cylindrospermopsin in cyanobacterial extracts.

Clofibrate pretreatment in mice confers resistance against hepatic lipid peroxidation

Pretreatment with peroxisome proliferators protects mice against various hepatotoxicants. Since our previous work suggested that the hepatoprotection may involve an increased ability to cope with oxidative stress, the present work directly addressed this possibility. Several observations indicated a heightened defense against oxidative stress accompanies the hepatoprotection produced by clofibrate. Firstly, the carbonyl content of hepatic proteins from clofibrate-pretreated mice was 40% lower than those from vehicle-treated controls. Secondly, liver homogenates from clofibrate-pretreated mice produced less thiobarbituric acid reactive substances upon incubation under aerobic conditions or exposure to ferrous sulfate. This effect was not due to lower levels of peroxidation-prone polyunsaturated fatty acids in clofibrate-treated livers. Thirdly, in vitro experiments indicated that the antioxidant factor in liver homogenates from clofibrate-pretreated mice was not glutathione. Rather, since it was inactivated by proteases and heat treatment, we concluded that a protein is involved. Collectively, our results suggest that a resistance to lipid peroxidation develops in mouse liver during exposure to clofibrate. The identity of the putative antioxidant protein and its contribution to the protection against liver toxicity observed in this and other laboratories awaits future investigation.

The antihypertensive hydralazine is an efficient scavenger of acrolein

Recent work indicates the highly toxic alpha,beta-unsaturated aldehyde acrolein is formed during the peroxidation of polyunsaturated lipids, raising the possibility that it functions as a 'toxicological second messenger' during oxidative cell injury. Acrolein reacts rapidly with proteins, forming adducts that retain carbonyl groups. Damage by this route may thus contribute to the burden of carbonylated proteins in tissues. This work evaluated several amine compounds with known aldehyde-scavenging properties for their ability to attenuate protein carbonylation by acrolein. The compounds tested were: (i) the glycoxidation inhibitors, aminoguanidine and carnosine; (ii) the antihypertensive, hydralazine; and (iii) the classic carbonyl reagent, methoxyamine. Each compound attenuated carbonylation of a model protein, bovine serum albumin, during reactions with acrolein at neutral pH and 37 degrees C. However, the most efficient agent was hydralazine, which strongly suppressed carbonylation under these conditions. Study of the rate of reaction between acrolein and the various amines in a protein-free buffered system buttressed these findings, since hydralazine reacted with acrolein at rates 2-3 times faster than its reaction with the other scavengers. Hydralazine also protected isolated mouse hepatocytes against cell killing by allyl alcohol, which undergoes in situ alcohol dehydrogenase-catalysed conversion to acrolein.

Clofibrate-induced in vitro hepatoprotection against acetaminophen is not due to altered glutathione homeostasis

Prior induction of peroxisome proliferation protects mice against the in vivo hepatotoxicity of acetaminophen and various other bioactivation-dependent toxicants. The mechanisms underlying such chemoresistance are poorly understood, although they have been suggested to involve alterations in glutathione homeostasis. To clarify the role of glutathione in this phenomenon, we isolated hepatocytes from mice in which hepatic peroxisome proliferation had been induced with clofibrate. The cells were incubated with a range of acetaminophen concentrations and the extent of cell killing after up to 8 h was assessed by measuring lactate dehydrogenase leakage from the cells. Hepatocytes from clofibrate-pretreated mice were much less susceptible to acetaminophen than cells from vehicle-treated controls. However, the extent of glutathione depletion during exposure to acetaminophen was similar in both cell types, as were rates of excretion of the product of glutathione-mediated detoxication of acetaminophen's quinoneimine metabolite, 3-glutathionyl-acetaminophen. The glutathione-replenishing ability of clofibrate-pretreated cells after a brief exposure to diethyl maleate also resembled that of control cells. More importantly, prior depletion of glutathione by diethyl maleate did not abolish the resistance of clofibrate-pretreated cells to acetaminophen. Taken together, these findings indicate that although glutathione-dependent pathways may contribute to hepatoprotection during peroxisome proliferation, the resistance phenomenon is not due exclusively to this mechanism.

Role of cytochrome P450 3A in the metabolism of mefloquine in human and animal hepatocytes

We studied mefloquine metabolism in cells and microsomes isolated from human and animal (monkey, dog, rat) livers. In both hepatocytes and microsomes, mefloquine underwent conversion to two major metabolites, carboxymefloquine and hydroxymefloquine. In human cells and microsomes these metabolites only were formed, as already demonstrated in vivo, while in other species several unidentified metabolites were also detected. After a 48 hr incubation with human and rat hepatocytes, metabolites accounted for 55-65% of the initial drug concentration, whereas in monkey and dog hepatocytes, mefloquine was entirely metabolized after 15 and 39 hrs, respectively. The consumption of mefloquine was less extensive in microsomes, and unchanged drug represented 60% (monkey) to 85-100% (human, dog, rat) of the total radioactivity after 5 hr incubations. The involvement of the cytochrome P450 3A subfamily in mefloquine biotransformation was suggested by several lines of evidence. Firstly, mefloquine metabolism was strongly increased in hepatic microsomes from dexamethasone-pretreated rats, and also in human and rat hepatocytes after prior treatment with a cytochrome P450 3A inducer. Secondly, mefloquine biotransformation in rifampycin-induced human hepatocytes was inhibited in a concentration-dependent manner by the cytochrome P450 3A inhibitor ketoconazole and thirdly, a strong correlation was found between erythromycin-N-demethylase activity (mediated by cytochrome P450 3A) and mefloquine metabolism in human microsomes (r=0.81, P < 0.05, N=13). Collectively, these findings concerning the role of cytochrome P450 3A in mefloquine metabolism may have important in vivo consequences especially with regard to the choice of agents used in multidrug antimalarial regimens.

Internal hazards: baseline DNA damage by endogenous products of normal metabolism

Recent improvements in the ability to detect chemically modified bases in DNA have revealed that not only does the genetic material incur damage by foreign chemicals, but that it also sustains injury by reactive products of normal physiological processes. This review summarises current understanding of the DNA-damaging potential of various substances of endogenous origin, including oxidants, lipid peroxidation products, alkylating agents, estrogens, chlorinating agents, reactive nitrogen species, and certain intermediates of various metabolic pathways. The strengths and weaknesses of the existing database for DNA damage by each class of substance are discussed, as are future strategies for resolving the difficult question of whether endogenous chemicals are significant contributors to spontaneous mutagenesis and cancer development in vivo.

Mutations at G:C base pairs predominate after replication of peroxyl radical-damaged pSP189 plasmids in human cells

The mutagenicity of peroxyl radicals, important participants in lipid peroxidation cascades, was investigated using a plasmid-based mutational assay system. Double-stranded pSP189 plasmids were incubated with a range of concentrations of the water-soluble peroxyl radical generator 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH). Following replication in human Ad293 cells, the plasmids were screened for supF mutations in indicator bacteria. Exposure to peroxyl radicals caused strand nicking and a decrease in transfection efficiency, which was accompanied by a significant increase in supF mutants. Each of these effects was abolished in the presence of the water-soluble vitamin E analogue Trolox. Automated sequencing of 76 AAPH-induced mutant plasmids revealed that substitutions at G:C base pairs were the most common changes, accounting for 85.5% of all identified mutations. Of these, most comprised G:C-->T:A transversions (53.5%), with lesser contributions by G:C-->A:T transitions (23.9%) and G:C-->C:G transversions (22.5%). Collectively, these data confirm our previous findings concerning the spectrum of mutations produced upon bacterial replication of peroxyl radical-damaged phage DNA and extend them by showing that such damage has mutagenic consequences during replication in more complex eukaryotic systems.

Genotoxic lipid peroxidation products: their DNA damaging properties and role in formation of endogenous DNA adducts

The peroxidation of polyunsaturated lipids generates a range of substances that possess DNA damaging potential. This includes lipid hydroperoxides and various species that contain unpaired electrons, such as the alkoxyl and peroxyl radicals. In addition, a range of genotoxic carbonyl-containing compounds are formed, such as malondialdehyde, various 4-hydroxy-2-alkenals such as 4-hydroxynonenal and a number of 2-alkenals. It has previously been assumed that the antioxidants and electrophile scavenging enzymes existing in mammalian cells effectively protect the genetic material against these substances. However, thanks to recent analytical advances in the detection of low levels of DNA adducts, it is now evident that DNA adducts formed from a range of lipid peroxidation products are abundant in both rodent and human genomes. This suggests that the cellular defence system is not 100% efficient and that a proportion of endogenously produced lipid peroxidation products escape detoxification and cause DNA damage. This review surveys the genotoxic properties of the major classes of lipid peroxidation products, focusing on their chemistry of DNA adduction, the mutagenic properties of such damage and the evidence that it occurs in intact biological systems. Furthermore, avenues of future research that will clarify the significance of such damage to spontaneous mutagenesis and carcinogenesis are proposed and discussed.

Genotoxicity of acyl glucuronide metabolites formed from clofibric acid and gemfibrozil: a novel role for phase-II-mediated bioactivation in the hepatocarcinogenicity of the parent aglycones?

Glucuronides formed from carboxylate-containing xenobiotics are more chemically reactive than most Phase II conjugates. However, while they have been shown to form protein adducts, their reactions with DNA have received little attention. We thus used the M13 forward mutational assay to assess the genotoxicity of acyl glucuronides formed from two widely used fibrate hypolipidemics, clofibric acid and gemfibrozil. Single-stranded M13mp19 bacteriophage DNA was incubated in pH 7.4 buffer for 16 h in the presence of 0, 1, 2.5, and 5 mM concentrations of each glucuronide as well as the respective aglycones. The modified DNA was then transfected into SOS-induced competent Escherichia coli JM105 cells and the transfection efficiency was determined after phage growth overnight at 37 degrees C. Significantly, both acyl glucuronides, but not the aglycones, caused a concentration-dependent decrease in the transfection efficiency of the DNA, with a greater than 80% decrease in phage survival produced by the 5 mM concentrations of the glucuronides. No increase in lacZa mutations accompanied the loss of phage survival. We propose that these genotoxic effects involve reactions with nucleophilic centers in DNA via a Schiff base mechanism that is analogous to the glycosylation of DNA by endogenous sugars. Since strand nicking is known to accompany such damage, we also analyzed glucuronide-treated pSP189 plasmids for strand breakages via agarose gel electrophoresis. Both clofibric acid and gemfibrozil glucuronides produced significant concentration-related strand nicking and exhibited over 10-fold greater reactivity than the endogenous glycosylating agent, glucose 6-phosphate. On the basis of these findings, the possibility that this novel bioactivation route participates in the carcinogenicity of the fibrate hypolipidemics deserves investigation.

Formation of novel C1-oxidised abasic sites in alkylperoxyl radical-damaged plasmid DNA

We have recently shown that peroxyl radicals react with DNA to form alkali-labile sites. To further characterise these lesions, we studied their susceptibility to digestion by repair endonucleases that recognise different types of abasic sites. We found that peroxyl radical-damaged pSP189 plasmids were resistant to cleavage by T4 endonuclease V, an enzyme that incises DNA at "regular" and C4-oxidised abasic residues. In contrast, the DNA was digested by exonuclease III, an enzyme that recognises "regular" and C1-oxidised abasic sites. The presence of Trolox during exposure to peroxyl radicals reduced subsequent DNA cleavage by exonuclease III, while prior incubation of damaged plasmids with methoxyamine potentiated digestion by this enzyme. These findings suggest that peroxyl radical-induced DNA damage involves the generation of novel C1-oxidised deoxyribose residues.

Role of G-->T transversions in the mutagenicity of alkylperoxyl radicals: induction of alkali-labile sites in bacteriophage M13mp19

The mutagenicity of peroxyl radicals, ubiquitous products of lipid peroxidation, was assessed using an in vitro M13 forward mutational assay. Single-stranded M13mp19 plasmids were incubated with a range of concentrations of the azo initiator 2,2'-azobis(2-amidinopropane) hydrochloride, and then transfected into competent, SOS-induced Escherichia coli JM105 cells. Incubation with peroxyl radicals produced a concentration-dependent decrease in phage survival, with a 500 microM concentration of the azo initiator reducing the transfection efficiency by more than 90% while inducing a corresponding 6-fold increase in lacZ alpha mutation frequencies. Peroxyl radical-induced mutagenesis was completely prevented by the peroxyl radical scavenger Trolox. Automated DNA sequence analysis of the lacZ alpha gene of 100 peroxyl radical-induced mutants revealed that the most frequent sequence changes were base pair substitutions (92/95), with G-->T transversions predominating (73/92). Alkaline treatment prior to transfection diminished the mutagenicity of damaged plasmids to a level resembling that of unmodified DNA. While abasic sites might account for the sensitivity to alkaline cleavage, the possibility that unidentified nonabasic alkaline-labile lesions also contribute to peroxyl radical mutagenesis cannot be excluded. Collectively, these findings raise the possibility that DNA damage caused by a major class of endogenous radicals contributes to one of the most common spontaneous mutational events, the G-->T transversion.

Diminished susceptibility to proteolysis after protein modification by the lipid peroxidation product malondialdehyde: inhibitory role for crosslinked and noncrosslinked adducted proteins

The lipid peroxidation product malondialdehyde forms adducts with proteins that are detected during routine assays for protein carbonylation. To test whether this damage alters the susceptibility of a protein to proteolysis, we treated bovine serum albumin with various concentrations of malondialdehyde and examined its susceptibility to digestion by alpha-chymotrypsin. In keeping with findings concerning the consequences of protein damage by other carbonyl products of lipid peroxidation, we found that malondialdehyde-modified protein was resistant to proteolysis. Since significant protein crosslinking occurred during modification with malondialdehyde, we investigated the possibility that crosslinked proteins were acting as proteolytic inhibitors. Malondialdehyde-modified proteins were resolved into crosslinked and noncrosslinked forms and the effectiveness of both species as proteolytic antagonists was examined. While both forms of malondialdehyde-adducted proteins were more potent proteolytic inhibitors than unmodified albumin, there were no significant differences in inhibitory potency between crosslinked and noncrosslinked proteins. Our findings suggest that malondialdehyde-modification produces protease-resistant proteins without an obligatory role for crosslinking.

Introduction of carbonyl groups into proteins by the lipid peroxidation product, malondialdehyde

Incubation of model proteins with the toxic lipid peroxidation product malondialdehyde resulted in a time- and concentration-dependent increase in carbonyl contents. Carbonyl groups were detected either spectrophotometrically or immunochemically after derivatization with 2,4-dinitrophenylhydrazine. Although significant adduction occurred when modifications were performed at pH 7.0, carbonyl formation was most extensive when modifications were carried out at pH 4.0 or pH 5.0. Similarly, formation of intermolecular crosslinks was most extensive when reactions were carried out under mildly acidic conditions. Our results raise the possibility that malondialdehyde adducts contribute to the carbonyl content of proteins recovered from mammalian tissues.

Abasic sites stimulate double-stranded DNA cleavage mediated by topoisomerase II. DNA lesions as endogenous topoisomerase II poisons

Several clinically relevant anticancer drugs induce genomic mutations and cell death by increasing topoisomerase II-mediated DNA breakage. To determine whether endogenous DNA damage also affects this cleavage event, the effects of abasic sites (the most commonly formed spontaneous DNA lesion) on topoisomerase II activity were investigated. The presence of 3 abasic sites/plasmid stimulated enzyme-mediated DNA breakage > 6-fold, primarily by enhancing the forward rate of cleavage. This corresponds to a potency that is > 2000-fold higher than that of the anticancer drug, etoposide. These findings suggest that abasic sites represent endogenous topoisomerase II poisons and imply that anticancer drugs mimic the cleavage-enhancing actions of naturally occurring DNA lesions.

Site-specific mutagenesis by a propanodeoxyguanosine adduct carried on an M13 genome

The spectrum of mutations induced upon in vivo replication of an M13 genome containing a site-specifically located propanodeoxyguanosine (PdG) adduct was determined. PdG was used as a model for the major deoxyguanosine adduct produced on reaction of DNA with the endogenous genotoxin malondialdehyde. PdG was introduced at position 6256 of M13MB102 by ligating the oligodeoxynucleotide 5'-GGT(PdG)TCCG-3' into an 8-base gap in the (-)-strand of duplex M13MB102. Replication of the adducted strand was maximized by incorporation of uracil into the unadducted (+)-strand. Following replication of dG-containing and PdG-containing M13MB102 genomes in Escherichia coli JM105, frameshift mutations were detected as phenotypic changes in the lacZ alpha marker gene. Base pair substitutions were detected by differential hybridization using 32P-labeled 13-mers bearing different bases opposite position 6256. Neither frameshift nor base pair substitution mutations were detected following replication of PdG-adducted genomes in non-SOS-induced JM105. However, PdG-->T transversions and PdG-->A transitions were detected following transformation of PdG-adducted M13MB102 into SOS-induced JM105. Both types of mutations were detected at comparable frequencies, and the total mutation frequency was approximately 2%. The results indicate that PdG is an efficient premutagenic lesion in E. coli strains in which the SOS response is induced.

A thermospray liquid chromatography/mass spectrometry method for analysis of human urine for the major malondialdehyde-guanine adduct

A method is described for detection and quantitation of the major malondialdehyde-guanine adduct (M1G) based on thermospray liquid chromatography/mass spectrometry. A stable isotope analog of M1G ([2H2]M1G) was used as an internal standard. Thermospray mass spectra of M1G and [2H2]M1G showed intense protonated molecular (MH+) ions that were suitable for use in quantitation of M1G. M1G was purified from human urine and reduced with NaBH4 to a dihydro derivative that was cleanly separated from the contaminants in the urine. The detection limit of reduced M1G by thermospray liquid chromatography/mass spectrometry in the selected ion monitoring mode was 250 fmol on column. Six human urine samples were analyzed, and the concentrations of M1G were below the limit of detection of the assay (500 fmol/mL).

Level of cytosolic free calcium during acetaminophen toxicity in mouse hepatocytes

It has been suggested that elevated cytosolic free calcium plays a key role in acetaminophen-induced cell death. The present study has examined the effect of a toxic concentration of acetaminophen on cytosolic free calcium in single mouse hepatocytes, using the dye fura-2 and video imaging fluorescence microscopy. Cytosolic free calcium was calculated from the ratio of emitted fluorescence at greater than 475 nm produced by excitation at 340 and 380 nm, using a double-intensified silicon target camera and digital image processing. In the presence of 5 mM acetaminophen, cell death did not occur for 2 hr, but the toxic lesion that ultimately killed the cells occurred as early as 1 hr. If cytosolic free calcium plays an important role in these toxic events, it would be expected to increase during this period. However, during a 2-hr exposure, cytosolic free calcium concentration in cells exposed to acetaminophen was not different from control. In hepatocytes incubated for longer than 2 hr, the calcium concentration increased shortly before loss of cell viability (i.e., as a late event), consistent with an influx of calcium through a damaged cell membrane. This late increase in calcium occurred well after the appearance of cell surface blebs. The data suggest that there is no sustained change in cytosolic free calcium in acetaminophen injury either before or during the time when irreversible toxic events occur in hepatocytes.

Acetaminophen toxicity results in site-specific mitochondrial damage in isolated mouse hepatocytes

Exposure of isolated mouse hepatocytes to a toxic concentration of acetaminophen (5 mM) resulted in damage to the mitochondrial respiratory apparatus. The nature of this damage was investigated by measuring respiration stimulated by site-specific substrates in digitonin-permeabilized hepatocytes after acetaminophen exposure. Respiration stimulated by succinate at energy-coupling site 2 was most sensitive to inhibition and was decreased by 47% after 1 h. Respiration supported by NADH-linked substrates (site 1) was also decreased but to a lesser extent, while there was no decrease in the rate of ascorbate + N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD)-supported respiration (site 3). The loss of mitochondrial respiratory function was accompanied by a decrease in ATP levels and ATP/ADP ratios in the cytosolic compartment and was preceded by a loss of reduced glutathione in both the cytosol and mitochondria. All these effects occurred well before the loss of cell membrane integrity. The putative toxic metabolite of acetaminophen, N-acetyl-p-benzoquinonimine (NAPQI), produced a similar pattern of respiratory dysfunction in isolated hepatic mitochondria. Respiration stimulated by succinate- and NADH-linked substrates was very sensitive to 50 microM NAPQI, while ascorbate + TMPD-supported respiration was unaffected. The interaction between NAPQI and the respiratory chain was further investigated using submitochondrial particles. Succinate dehydrogenase (associated with respiratory complex II) was found to be very sensitive to NAPQI, while NADH dehydrogenase (respiratory complex I) was inhibited to a lesser extent. Our results indicate that a loss of the ability to utilize succinate- and NADH-linked substrates due to attack of the respiratory chain by NAPQI causes a disruption of energy homeostasis in acetaminophen hepatotoxicity.

Mitochondrial dysfunction in paracetamol hepatotoxicity: in vitro studies in isolated mouse hepatocytes

The effect of paracetamol intoxication on mitochondrial function was studied in isolated mouse hepatocytes. Inhibition of cellular respiration as well as a lowering of cellular ATP contents and ATP/ADP ratios was associated with exposure to toxic concentrations of paracetamol. Significantly, inhibition of 3-hydroxybutyrate- and lactate/pyruvate-supported respiration, as well as the reduction in cellular ATP levels and ATP/ADP ratios, preceded the appearance of plasma membrane damage, as assessed by LDH leakage. N-Acetylcysteine reduced the extent of plasma membrane damage induced by paracetamol and protected against the impairment of cellular respiration. This suggests that respiratory dysfunction was a consequence of the oxidation of paracetamol to its reactive metabolite within the liver cell. These findings indicate that paracetamol toxicity results in an impairment of mitochondrial function which precedes the loss of plasma membrane integrity.

Paracetamol-induced stimulation of glycogenolysis in isolated mouse hepatocytes is not directly associated with cell death

Paracetamol intoxication in vivo is known to be accompanied by depletion of hepatic glycogen stores. We have demonstrated a dose-dependent stimulation of glycogenolysis by paracetamol in glycogen-rich hepatocytes isolated from the mouse. Concentrations of paracetamol that produced plasma membrane damage were also found to activate glycogen phosphorylase a and deplete cellular glycogen contents. However, paracetamol-mediated stimulation of glycogenolysis could be dissociated from the events associated with paracetamol-induced cell killing. Both N-acetylcysteine and 2,4-dichloro-6-phenylphenoxyethylamine markedly reduced the extent of hepatocellular plasma membrane damage induced by paracetamol, yet neither agent prevented the activation of phosphorylase a nor the depletion of glycogen. These findings suggest that the hepatic glycogen depletion that accompanies paracetamol intoxication in vivo is due, at least in part, to a direct effect of the drug on the liver.

Effect of acetaminophen hepatotoxicity on hepatic mitochondrial and microsomal calcium contents in mice

The effect of toxic doses of acetaminophen on hepatic intracellular calcium compartmentation were studied in mice. No effects on the calcium contents of the mitochondria, microsomes or cytosol were observed 4 h after the administration of 175 and 375 mg/kg acetaminophen when compared to saline-treated controls. However, doses of 500 and 750 mg/kg of acetaminophen increased mitochondrial calcium contents at this time. Also, the 750 mg/kg dose caused marked alterations in the calcium contents of microsomal and cytosolic compartments. The time-course of the onset of these effects was examined using a 500 mg/kg dose. No changes in either mitochondrial, microsomal or cytosolic calcium contents were observed in the livers of mice treated with acetaminophen compared to saline-treated controls at either 1 or 2 h after dose administration. However, at 3, 4 and 24 h after acetaminophen, mitochondrial and cytosolic calcium contents were significantly increased above control values. The increases in mitochondrial and cytosolic calcium contents observed in the acetaminophen-intoxicated mouse liver appear to occur at the same time as the appearance of plasma membrane damage, as measured by sorbitol dehydrogenase leakage. The data suggest that a perturbation in hepatic calcium compartmentation is not an early event in acetaminophen-induced hepatotoxicity in the mouse.