Structural and chemical requirements for hydroxychlorobiphenyls to uncouple rat liver mitochondria and potentiation of uncoupling with aroclor 1254

De novo assembly of the ringed seal (Pusa hispida) blubber transcriptome: A tool that enables identification of molecular health indicators associated with PCB exposure

The ringed seal, Pusa hispida, is a keystone species in the Arctic marine ecosystem, and is proving a useful marine mammal for linking polychlorinated biphenyl (PCB) exposure to toxic injury. We report here the first de novo assembled transcriptome for the ringed seal (342,863 transcripts, of which 53% were annotated), which we then applied to a population of ringed seals exposed to a local PCB source in Arctic Labrador, Canada. We found an indication of energy metabolism imbalance in local ringed seals (n=4), and identified five significant gene transcript targets: plasminogen receptor (Plg-R(KT)), solute carrier family 25 member 43 receptor (Slc25a43), ankyrin repeat domain-containing protein 26-like receptor (Ankrd26), HIS30 (not yet annotated) and HIS16 (not yet annotated) that may represent indicators of PCB exposure and effects in marine mammals. The abundance profiles of these five gene targets were validated in blubber samples collected from 43 ringed seals using a qPCR assay. The mRNA transcript levels for all five gene targets, (Plg-R(KT), r²=0.43), (Slc25a43, r²=0.51), (Ankrd26, r²=0.43), (HIS30, r²=0.39) and (HIS16, r²=0.31) correlated with increasing levels of blubber PCBs. Results from the present study contribute to our understanding of PCB associated effects in marine mammals, and provide new tools for future molecular and toxicology work in pinnipeds.

Polychlorinated biphenyl 126 exposure in L6 myotubes alters glucose metabolism: a pilot study

Polychlorinated biphenyls (PCBs) are increasingly recognized as metabolic disruptors. Due to its mass, skeletal muscle is the major site of glucose disposal. While muscle mitochondrial dysfunction and oxidative stress have been shown to play a central role in metabolic disease development, no studies to date have investigated the effect of PCB exposure on muscle energy metabolism and oxidative stress. In this pilot study, we tested the effect of exposure to PCB126 in L6 myotubes (from 1 to 2500 nM for 24 h) on mitochondrial function, glucose metabolism, and oxidative stress. Exposure to PCB126 had no apparent effect on resting, maximal, and proton leak-dependent oxygen consumption rate in intact L6 myotubes. However, basal glucose uptake and glycolysis were inhibited by 20-30 % in L6 myotubes exposed to PCB126. Exposure to PCB126 did not appear to alter skeletal muscle anti-oxidant defense or oxidative stress. In conclusion, our study shows for the first time that exposure to a dioxin-like PCB adversely affects skeletal muscle glucose metabolism. Given the importance of skeletal muscle in the maintenance of glucose homeostasis, PCB126 could play an important role in the development of metabolic disorders.

Modeling and predicting pKa values of mono-hydroxylated polychlorinated biphenyls (HO-PCBs) and polybrominated diphenyl ethers (HO-PBDEs) by local molecular descriptors

Hydroxylated polychlorinated biphenyls (HO-PCBs) and polybrominated diphenyl ethers (HO-PBDEs) are attracting considerable concerns because of their multiple endocrine-disrupting effects and wide existence in environment and organisms. The hydroxyl groups enable these chemicals to be ionizable, and dissociation constant, pKa, becomes an important parameter for investigating their environmental behavior and biological activities. In this study, a new pKa prediction model was developed using local molecular descriptors. The dataset contains 21 experimental pKa values of HO-PCBs and HO-PBDEs. The optimized geometries by ab initio HF/6-31G(∗∗) algorithm were used to calculate the site-specific molecular readiness to accept or donate electron charges. The developed model obtained good statistical performance, which significantly outperformed commercial software ACD and SPARC. Mechanism analysis indicates that pKa values increase with the charge-limited donor energy EQocc on hydroxyl oxygen atom and decrease with the energy-limited acceptor charge QEvac on hydroxyl hydrogen atom. The regression model was also applied to calculate pKa values for all 837 mono-hydroxylated PCBs and PBDEs in each class, aiming to provide basic data for the ecological risk assessment of these chemicals.

Polychlorinated biphenyls in prospectively collected serum and Parkinson's disease risk

Evidence suggests possible Parkinson's disease (PD)-relevant neural effects of exposure to polychlorinated biphenyls. Limited epidemiological evidence suggests that polychlorinated biphenyl exposure may increase PD risk, but no studies have involved biomarkers of polychlorinated biphenyl exposure before PD onset. We examined the prospective association between serum polychlorinated biphenyls and PD. We conducted a nested case-control study within the Finnish Mobile Clinic Health Examination Survey with serum samples collected during 1968-1972 and analyzed in 2005-2007 for polychlorinated biphenyls. Incident PD cases were identified through the Social Insurance Institution's registry and were confirmed by medical record review (n = 101). Controls (n = 349) were matched on age, sex, municipality, and vital status. We used logistic regression to estimate adjusted odds ratios. There was no evidence of increasing risk of PD with increasing polychlorinated biphenyl exposure in adjusted analyses. Instead, there was a trend toward lower odds of PD with increasing serum polychlorinated biphenyl concentrations, which was most pronounced for the sum of all measured polychlorinated biphenyl congeners and the sum of dioxin-like congeners. Compared with that of those in the lowest quintile, the odds ratio of PD among those in the highest quintile of total polychlorinated biphenyls was 0.29 (95% confidence interval, 0.12-0.70; P trend = .02) and for dioxin-like congeners was 0.34 (95% confidence interval, 0.13-0.90; P trend = .05). These results do not support an increased risk of PD from polychlorinated biphenyl exposure and instead suggest a possible protective effect of polychlorinated biphenyl exposure.

pK(a) values of the monohydroxylated polychlorinated biphenyls (OH-PCBs), polybrominated biphenyls (OH-PBBs), polychlorinated diphenyl ethers (OH-PCDEs), and polybrominated diphenyl ethers (OH-PBDEs)

The SPARC software program aqueous pK(a) prediction module was validated against corresponding experimental acidity constants for chlorinated and brominated phenols and the limited experimental aqueous pK(a) data sets for monohydroxylated polychlorinated biphenyls (OH-PCBs), polychlorinated diphenyl ethers (OH-PCDEs), and polybrominated diphenyl ethers (OH-PBDEs). pK(a) values were then estimated for all 837 monohydroxylated mono- through nona-halogenated congeners in each of the OH-PCB, OH-PCDE, and OH-PBDE classes, as well as for the monohydroxylated polybrominated biphenyls (OH-PBBs), giving a total of 3348 compounds. Large intrahomolog pK(a) variation by up to six units is expected within each contaminant class, with pK(a) values ranging from about 4 to 11 dependent on the degree and pattern of halogenation. Increasing halogenation generally decreased the average pK(a) within each homolog group. Significant intrahomolog differences in pK(a) values exist between OH-PCB, OH-PBB, OH-PCDE, and OH-PBDE congeners, including large acidity constant variation between isomers with equivalent halogenation patterns but varying location of the hydroxy moiety. Congener specific pH dependent investigations into the partitioning and degradation behaviors of these compounds are necessary, including greater consideration of analyte ionization effects during their extraction and analysis.

Detection of hydroxylated polychlorinated biphenyls (OH-PCBs) in the abiotic environment: surface water and precipitation from Ontario, Canada

Hydroxylated PCBs (OH-PCBs) are well-known metabolites of PCBs in organisms, but there has been no direct study of their presence in the abiotic environment. In this study, OH-PCBs were determined in samples of rain, snow, and surface waters from sites in Ontario, Canada. OH-PCBs were quantified by gas chromatography with high-resolution mass spectrometry (GC-HRMS)in order to provide complete characterization of all OH-PCB homolog groups. OH-PCBs and PCBs were detected in all the samples analyzed, although half of the sigmaOH-PCBs could not be identified even with 71 individual congener standards. Total concentrations of OH-PCBs (sigmaOH-PCBs) in water ranged from 0.87 to 130 pg/L and from 230 to 990 pg/g in particulate organic matter. Total fluxes of those compounds in snow and rain were from < 1 to 100 pg/m2 and from < 1 to 44 pg/ m2/day, respectively. Higher sigmaOH-PCB fluxes in rain were found in southern Ontario than in a remote north-central Ontario site possibly reflecting greater sources of precursor PCBs near urban areas. Relatively higher sigmaOH-PCB concentrations were found in surface waters from sites near sewage treatment plant (STP) outfalls in the cities of Toronto (130 pg/L) and Hamilton (35 pg/L) than in offshore samples from Lake Ontario (1.6 pg/L). The results indicate that STPs are one of the sources of OH-PCBs for lake waters in this region. Similar homolog and congener profiles in rain and offshore surface water samples suggest that atmospheric deposition is the predominant source at offshore sites. This is the first report to detect the OH-PCBs in the abiotic environment.

Polychlorinated biphenyls affect gene expression in the rabbit preimplantation embryo

Polychlorinated biphenyls (PCBs) have been shown to be embryotoxic. The mechanism(s) of action is not clearly understood. The toxic effects could be either direct or indirect. Furthermore, PCB congeners vary in their toxic potential. They can be classified in coplanar PCBs binding to the transcription factor aryl hydrocarbon receptor (AhR), which induce subsequent changes in gene expression, and noncoplanar PCBs exhibiting AhR-independent effects. In order to investigate possible mechanisms, 5 and 6 days old preimplantation rabbit embryos were exposed in vitro to low levels of coplanar (PCB 77, 126, and 169) or noncoplanar PCBs (PCB 28, 52, 101, 118, 138, 153, and 180). The PCB effects were studied by semiquantitative RT-PCR analysis of AhR target genes (cytochrome P450 (CYP) 1A1, 1A2, UDP-glucuronosyl transferase 1, glutathione S-transferase pi1 and aldehyde dehydrogenase) and dioxin-responsive genes (IL 1beta, PAI 2, Cox 2, TGFalpha, EGF, erbB 1-4, c-fos, c-jun, HSP 90, cyclophilin 40), and by differential display (DD) RT-PCR. CYP 1B1 mRNA and AhR protein were localized by in situ hybridization and immunohistochemistry, respectively. From the AhR target genes studied only CYP 1B1, and cyclooxygenase 2 showed an increase in mRNA levels after coplanar and noncoplanar PCB. Interleukin 1beta and plasminogen activator inhibitor 2 were downregulated. CYP 1B1 mRNA showed a stage specific inducibility at day 6, but not at day 5. By DD RT-PCR we identified six new genes previously not reported to be regulated by PCBs. The mRNAs encoding the subunits 1, 2, 4, and 5 of the NADH ubiquinone oxidoreductase and beta-globin showed a decrease, whereas trichohyalin mRNA was increased after PCB exposure. Coplanar and noncoplanar PCB congeners elicited similar responses on the mRNA levels of the studied genes. Exposure to coplanar PCBs did not result in the AhR being translocated to the nucleus. Our results show that (i). PCBs induce changes in gene expression in rabbit day 5 and 6 preimplantation embryos and imply (ii). that the transcriptional changes observed were not mediated by the nuclear AhR.

Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca(2+) buffering in rat cerebellum by hydroxylated polychlorinated biphenyls

Our previous structure-activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca(2+) homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]phorbol ester ([3H]PDBu) binding in cerebellar granule cells, and Ca(2+) sequestration as determined by 45Ca(2+) uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [3H]PDBu binding in a concentration-dependent manner and the order of potency as determined by E(50) (concentration that increases control activity by 50%) is 2',4',6'-trichloro-4-biphenylol (32 +/- 4 microM), 2',5'-dichloro-4-biphenylol (70 +/- 9 microM), 2,2',4',5,5'-pentachloro-4-biphenylol (80 +/- 7 microM) and 2,2',5'-trichloro-4-biphenylol (93 +/- 14 microM). All the selected hydroxy-PCBs inhibited microsomal 45Ca(2+) uptake to a different extent. Among the hydroxy-PCBs selected, 2',4',6'-trichloro-4-biphenylol is the most active in increasing [3H]PDBu binding as well as inhibiting microsomal 45Ca(2+) uptake. 3,5-Dichloro-4-biphenylol and 3,4',5-trichloro-4-biphenylol did not increase [3H]PDBu binding, but inhibited microsomal 45Ca(2+) uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

Aroclor 1254 inhibits the mitochondrial permeability transition and release of cytochrome c: a possible mechanism for its in vivo toxicity

The mitochondrial permeability transition (MPT) occurs in several forms of necrotic cell death induced by various insults, including oxidative stress, ischemia/reperfusion injury Ca(2+)-ionophore toxicity, and apoptosis. In fact, the release of an apoptogenic factor such as cytochrome c is often associated with the opening of the transition pore. The present study shows that Aroclor 1254, a mixture of polychlorinated biphenyls that was banned in the U.S. in 1977 but is still present in the environment, inhibits the MPT in a dose-dependent manner in a concentration range of 1 to 25 nmol/mg protein. The compound prevents key phenomena associated with the MPT, including colloid-osmotic swelling, the collapse of membrane potential, nonspecific bidirectional traffic of solutes through the transition pore, and the oxidation of pyridine nucleotides. In contrast, Aroclor 1254 does not inhibit uptake of Ca(2+) or P(i). The effects of Aroclor 1254 are evident both in sucrose-based media and in saline and are observed when the compound is added before the opening of the pore. Aroclor 1254 prevents MPT induction provoked by a variety of agents, including phosphate, menadione, tert-butylhydroperoxide, and atractyloside. Aroclor 1254 also inhibits the specific release of cytochrome c, a correlate of MPT induction. These effects reveal a possible toxicological mechanism of action of this compound. The possibility that its effect on mitochondrial function is linked to its action as a tumor promoter is discussed.

Use of a combined human liver microsome-estrogen receptor binding assay to assess potential estrogen modulating activity of PCB metabolites

Polychlorinated biphenyls (PCBs) are metabolized by hydroxylation; some of these hydroxylated metabolites exhibit estrogen-like activity in animal models. Because PCBs may have effects on human health, it is of interest to determine if human tissues also metabolize PCBs to potentially estrogenic metabolites. In this study metabolites of seven PCBs with different degrees and positions of chlorination, generated by human liver microsomal reaction mixtures (MRM) have been examined, and their affinity for human recombinant estrogen receptor-alpha (ER) has been tested before and after HPLC fractionation. Two of the three MRMs with di-chloro-biphenyls (BPs, 2,5BP and 3,4BP), one of the three MRMs with tetra-BPs (2,6,2',6'BP), and one hexa-BP (2,4,6,2',4',6'BP) generated metabolites that competed for ER. HPLC of the ER-binding MRMs generated fractions that also exhibited ER-binding. This study shows the usefulness of combining in vitro metabolism and an ER-binding assay in initial identification of PCBs with estrogen-modulating potential.

In vitro metabolism of 3,3',4,4'-tetrachlorobiphenyl in relation to ethoxyresorufin-O-deethylase activity in liver microsomes of some wildlife species and rat

A qualitative study was performed of the capacity of hepatic microsomes of several wildlife species to metabolize 3,3',4,4'-tetrachlorobiphenyl (TCB). Hepatic microsomes of species environmentally exposed to polychlorinated biphenyls (PCBs): harbour porpoise (Phocoena phocoena), harbour seal (Phoca vitulina), common tern (Sterna hirundo), and hepatic microsomes from species experimentally exposed to PCBs: eider duck (Somateria mollissima), rainbow trout (Salmo gairdneri), flounder (Platichthys flesus), and Wistar rat, were incubated with 14C-labelled TCB ([14C]TCB). The mammals and birds were able to metabolize TCB at a rate that correlated with their ethoxyresorufin-O-deethylase (EROD) activity. No [14C]TCB metabolism was observed in the fish, despite elevated EROD activity in the trout. HPLC analysis of diisopropylether extracts of the microsomal incubations indicated the presence of 4-OH-, 5-OH-, and 6-OH-tetrachlorobiphenyl metabolites and a yet unidentified metabolite. The ratio of the different hydroxy metabolites formed varied for the various species.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Effects of hydroxylated polychlorinated biphenyls on mouse liver mitochondrial oxidative phosphorylation

The effects of three tetrachlorobiphenylols [2',3',4',5'-tetrachloro-2-biphenylol (1); 2',3',4',5'-tetrachloro-4- biphenylol (2); and 2',3',4',5'-tetrachloro-3-biphenylol (3)]; three monochlorobiphenylols [5-chloro-2-biphenylol (5), 3-chloro-2-biphenylol (6); and 2-chloro-4-biphenylol (7)] and a tetrachlorobiphenyldiol [3,3',5,5'-tetrachloro-4,4'-biphenyldiol (4) on respiration, adenosine triphosphatase (ATPase) activity, and swelling in isolated mouse liver mitochondria have been investigated. Tetrachlorobiphenylols (1-3) and the tetrachlorobiphenyldiol (4) inhibited state-3 respiration in a concentration-dependent manner with succinate as substrate (flavin adenine dinucleotide [FAD]-linked) and the tetrachlorobiphenyldiol (4) caused a more pronounced inhibitory effect on state-3 respiration than the other congeners. The monochlorobiphenylols 5-7 were less active as inhibitors of state-3 mitochondrial respiration and significant effects were observed only at higher concentration (greater than or equal to 0.4 microM). However, in the presence of the nicotinamide adenine dinucleotide (NAD)-linked substrates (glutamate plus malate), hydroxylated PCBs (1-7) significantly inhibited mitochondrial state-3 respiration in a concentration-dependent manner. Compounds 5, 6, and 7 uncoupled mitochondrial oxidative phosphorylation only in the presence of FAD-linked substrate as evidenced by increased oxygen consumption during state-4 respiratory transition, stimulating ATPase activity, releasing oligomycin-inhibited respiration, and inducing mitochondrial swelling (5, 6, and 7). Tetrachlorobiphenylols 1, 2, and 3 had no effect on mitochondrial ATPase activity while the tetrachlorobiphenyldiol, 4, decreased the enzyme activity. The possible inhibitory site of electron transport by these compounds and their toxicologic significance is discussed.

3,4,3',4'-tetrachlorobiphenyl-induced effects in the rat liver. II. Electron microscopic autoradiographic localization of 3H-TCB

Recent results (3) indicate that 200 mg 3,4,3',4'-tetrachlorobiphenyl induces hepatomegaly accompanied by significant decreases in serum and hepatic retinoid content and hepatocyte morphologic alterations of proliferated and vesiculated endoplasmic reticulum and megamitochondria with paracrystalline inclusions. There was also an associated change in the number, size, and distribution of lipid droplets in hepatocytes and fat-storing cells. Electron microscopic autoradiographic techniques were utilized to determine the cellular and subcellular distribution of 3H-3,4,3',4'-tetrachlorobiphenyl (3H-TCB) in the adult rat liver and determine if there is any relationship between subcellular morphologic change and radiolabel localization. Adult female WAG/Rij rats received a single intraperitoneal injection of 200 mg TCB/kg containing 1.85 mCi of 3H-TCB and were sacrificed at 1, 3, 7, and 14 days following exposure. The vast majority of 3H-TCB-derived radioactivity was located in the hepatocyte at all time points examined, ranging from 79-86% of the total number of autoradiographic grains counted over the liver cells. Sequential order of radiolabel localization per liver cell type at 1, 3, and 7 days was hepatocyte much greater than Kupffer cell greater than fat-storing cell greater than endothelial cell. At day 14, the sequential order of radiolabel localization per liver cell type was hepatocyte much greater than fat-storing cell greater than Kupffer cell greater than endothelial cell, which indicates that there was some shift movement of label over time. The lipid droplet, mitochondria, and endoplasmic reticulum were the subcellular structures or organelles of hepatocytes having the highest number of 3H-TCB-derived grains at all time periods examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.