Quantitative structure toxicity relationships for phenols in isolated rat hepatocytes

How many drugs are catecholics

By examination of the 8659 drugs recorded in the Comprehensive Medicinal Chemistry (CMC) database, 78 catecholics (including five pyrogallolics) were identified, of which 17 are currently prescribed by FDA. Through analyzing the substituent patterns, ClogPs and O-H bond dissociation enthalpies (BDEs) of the catecholic drugs, some molecular features that may benefit circumventing the toxicity of catecholics were revealed: i) strong electron-donating substituents are excluded; ii) ClogP 3; iii) an energy penalty exists for quinone formation. Besides, the present analyses also suggest that the clinical usage and dosage of currently prescribed catecholic drugs are of importance in designing or screening catecholic antioxidants.

A microcalorimetric method for studying the toxic effect of different diphenol species on the growth of Escherichia coli

A microcalorimetric technique based on bacterial heat-output was explored to evaluate the toxic effect of different diphenol species on the growth of Escherichia coli (E. coli). Power-time curves of the growth metabolism for E. coli in the presence of different diphenol species were studied using a multi-channel microcalorimetric system with an ampoule method at 37 degrees C. The growth rate constant (k), generation time (t(G)), inhibitory ratio (I), half-inhibitory concentration (IC(50)) and the total thermal effect (Q(T)) for E. coli were obtained. The results show that catechol and hydroquinone are more toxic to E. coli than resorcinol. In all cases, the growth rate constants of E. coli (in log phase) decreased as the concentrations of these diphenols increased. Among these diphenols species, catechol was found to be the most poisonous species at an IC(50) of 323.5 micro g/mL against E. coli. Hydroquinone exhibited moderate virulence with an IC(50) of 1196 micro g/mL and resorcinol had the lowest toxicity with an IC(50) of 2113 micro g/mL. The microcalorimetric bioassay can be a quantitative, inexpensive, and versatile method for acute cellular toxicity study.

Structure–activity relationships for halobenzene induced cytotoxicity in rat and human hepatoctyes

Halobenzenes are ubiquitous environmental contaminants, which are hepatotoxic in both rodents and humans. The molecular mechanism of halobenzene hepatotoxicity was investigated using Quantitative structure-activity relationships (QSAR) and accelerated cytotoxicity mechanism screening (ACMS) techniques in rat and human hepatocytes. The usefulness of isolated hepatocytes for prediciting in vivo xenobiotic toxicity was reassessed by correlating the LC(50) of 12 halobenzene congeners in phenobarbital (PB) induced rat hepatocytes in vitro determined by ACMS to the hepatotoxicities reported in vivo in PB-induced male Sprague-Dawely (SD) rats. A high correlation (r(2)=0.90) confirmed the application of hepatocytes as a "gold standard" for toxicity testing in vitro. QSARs were derived to determine the physico-chemcial variables that govern halobenzene toxicity in PB-induced rat, normal rat and human hepatocytes. We found that toxicity in normal rat and normal human hepatocytes both strongly correlate with hydrophobicity (logP), ease of oxidation (E(HOMO), energy of the highest molecular orbital) and on the asymmetric charge distribution according to arrangement of halogen substituents (dipole moment, mu). This suggests that halobenzene interaction with cytochrome P450 for oxidation is the metabolic activating path for toxicity and is similar in both species. In PB-induced rat hepatocytes the QSAR derivation is changed, where halobenzene toxicity strongly correlates to logP and dipole moment, but not E(HOMO). The changed QSAR suggests that oxidation is no longer the rate-limiting step in the cytotoxic mechanism when CYP2B/3A levels are increased, confirming CYP450 oxidation as the metabolic activating step under normal conditions.

Bioaccumulation of paraquat by Lumbriculus variegatus in the presence of dissolved natural organic matter and impact on energy costs, biotransformation and antioxidative enzymes

Dissolved organic matter from natural sources (DNOM) is omnipresent in aquatic ecosystems. Besides affecting bioavailability of substances including xenobiotics, it directly influences physico-chemistry of the habitat and there is increasing evidence for it is interaction with organisms. We investigated direct and interacting effects of DNOM from three sources, Lake Valkea-Kotinen, Svartberget Brook, and Lake Fuchskuhle with the herbicide paraquat on the oligochaete worm Lumbriculus variegatus. Bioavailability of paraquat to L. variegates as well as activities of antioxidative enzymes catalase (CAT) and peroxidase (POD) and biotransformation enzyme soluble glutathione S-transferase (sGST) were assessed without and in the presence of DNOM. Furthermore, metabolic heat dissipation due to the exposure was quantified. Uptake of paraquat into the worms was concentration dependently reduced by DNOM, and with differences concerning the DNOM sources. sGST and CAT responded with increased activities to DNOM (5 and 25 mg C l-1) and paraquat (5.0, 50, and 500 microg l-1) separately. Paraquat at 5.0 microg l-1 and DNOM in combination caused increased activities of sGST, especially at 5 mgC l-1, but inhibition of CAT activities. The latter probably occurred due to saturation of the enzyme. Changes in enzyme activities were independent from the source of DNOM. Increasing DNOM concentrations raised metabolic heat dissipation in L. variegatus with maximum at 3h of exposure. In the combined treatments, metabolic heat dissipation changed more due to the source of DNOM than due to the bioavailability of paraquat.

Adaptation of the psychrotrophArthrobacter chlorophenolicusA6 to growth temperature and the presence of phenols by changes in the anteiso/iso ratio of branched fatty acids

Arthrobacter chlorophenolicus is a previously described Gram-positive bacterium capable of degrading high concentrations of several phenolic compounds under optimal mesophilic (28 degrees C) as well as psychrophilic (5 degrees C) conditions. However, the exact mechanisms by which this organism is able to tolerate such extremes in temperature and high levels of toxic compounds are currently not known. In this study, we monitored changes in the fatty acid composition of the cell membrane under different extreme growth conditions. Arthrobacter chlorophenolicus adapts to differences in temperature and phenol concentrations by altering the anteiso/iso ratio of fatty acids in the cell membrane to different extents. According to the different physico-chemical properties of those two species of branched fatty acids, the bacteria showed an increased amount of anteiso fatty acids when grown under psychrophilic conditions to decrease the viscosity of their membranes. On the other hand, at higher growth temperatures as well as in the presence of toxic concentrations of phenol, 4-chlorophenol and 4-nitrophenol, the cells adapted their membrane by a dose-dependent decrease in the anteiso/iso ratio, leading to a more rigid membrane and counteracting the fluidity increase caused by the higher temperature and the organic solvents.

Drug-induced mitochondrial toxicity

Mitochondria play a critical role in generating most of the cell's energy as ATP. They are also involved in other metabolic processes such as urea generation, haem synthesis and fatty acid beta-oxidation. Disruption of mitochondrial function by drugs can result in cell death by necrosis or can signal cell death by apoptosis (e.g., following cytochrome c release). Drugs that injure mitochondria usually do so by inhibiting respiratory complexes of the electron chain; inhibiting or uncoupling oxidative phosphorylation; inducing mitochondrial oxidative stress; or inhibiting DNA replication, transcription or translation. It is important to test for mitochondrial toxicity early in drug development as impairment of mitochondrial function can induce various pathological conditions that are life threatening or can increase the progression of existing mitochondrial diseases.

Dinitrophenol-induced mitochondrial uncoupling in vivo triggers respiratory adaptation in HepG2 cells

Here, we show that 3 days of mitochondrial uncoupling, induced by low concentrations of dinitrophenol (10 and 50 microM) in cultured human HepG2 cells, triggers cellular metabolic adaptation towards oxidative metabolism. Chronic respiratory uncoupling of HepG2 cells induced an increase in cellular oxygen consumption, oxidative capacity and cytochrome c oxidase activity. This was associated with an upregulation of COXIV and ANT3 gene expression, two nuclear genes that encode mitochondrial proteins involved in oxidative phosphorylation. Glucose consumption, lactate and pyruvate production and growth rate were unaffected, indicating that metabolic adaptation of HepG2 cells undergoing chronic respiratory uncoupling allows continuous and efficient mitochondrial ATP production without the need to increase glycolytic activity. In contrast, 3 days of dinitrophenol treatment did not change the oxidative capacity of human 143B.TK(-) cells, but it increased glucose consumption, lactate and pyruvate production. Despite a large increase in glycolytic metabolism, the growth rate of 143B.TK(-) cells was significantly reduced by dinitrophenol-induced mitochondrial uncoupling. We propose that chronic respiratory uncoupling may constitute an internal bioenergetic signal, which would initiate a coordinated increase in nuclear respiratory gene expression, which ultimately drives mitochondrial metabolic adaptation within cells.

The protection of bioenergetic functions in mitochondria by new synthetic chromanols

alpha-Tocopherol is the most important lipophilic antioxidant of the chromanol type protecting biomembranes from lipid peroxidation (LPO). Therefore, alpha-tocopherol and its derivatives are frequently used in the therapy or prevention of oxygen radical-derived diseases. In the present study, novel chromanol-type antioxidants (twin-chromanol, cis- and trans-oxachromanol) as well as the well-known short-chain analogue of alpha-tocopherol, pentamethyl-chromanol, were tested for their antioxidative potency in rat heart mitochondria (RHM). Our experiments revealed that the bioenergetic parameters of mitochondria were not deteriorated in the presence of chromanols (up to 50 nmol/mg protein). Exposure of RHM to cumene hydroperoxide and Fe2+ (final concentrations 50 microM each), inducing LPO, significantly affected their bioenergetic parameters which were determined in the presence of glutamate and malate (substrates of mitochondrial complex I). Alterations of the bioenergetic parameters were partially prevented in a concentration-dependent manner by preincubating RHM with antioxidants before adding the radical-generating system. In the lower concentration range, twin-chromanol turned out to be more efficient than pentamethyl-chromanol, both being far more protective than cis- and trans-oxachromanol. Measurement of protein-bound SH groups and thiobarbituric acid-reactive substances revealed that this protective effect was due to their antioxidative action. Furthermore, HPLC measurements of alpha-tocopherol and alpha-tocopheryl quinone in rat liver mitochondria demonstrated an alpha-tocopherol-sparing effect of twin-chromanol. In conclusion, new chromanol-type antioxidants, especially twin-chromanol, were able to improve bioenergetic and biochemical parameters of mitochondria exposed to oxidative stress.

Quantitative structure-activity relationships in prooxidant cytotoxicity of polyphenols: role of potential of phenoxyl radical/phenol redox couple

The aim of this work was to characterize the role of the potential of phenoxyl radical/phenol redox couple, E(7)(2), in the cytotoxicity of polyphenols. The cytotoxicity of polyphenols in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK), and human promyelocytic leukemia cells (line HL-60) was partly inhibited by catalase, by the antioxidant N,N'-diphenyl-p-phenylene diamine and desferrioxamine, and potentiated by 1,3-bis-(2-chloro-ethyl)-1-nitrosourea, thus showing its prooxidant character. Dapsone, an inhibitor of myeloperoxidase, did not affect the cytotoxicity of polyphenols in HL-60 cells, whereas dicumarol, an inhibitor of DT-diaphorase, showed controversial effects on their cytotoxicity in FLK cells. Inhibitors of cytochromes P-450, alpha-naphthoflavone and izoniazide, decreased the cytotoxicity of several polyphenols, whereas 3,5-dinitrocatechol, an inhibitor of catechol-o-methyltransferase (COMT), increased it. The cytotoxicity of 13 polyhydroxybenzenes was described by the equations: logcL50 (microM) = -0.67 + 5.46E(7)(2) (V) - 0.16 logD (FLK), and logcL50 (microM) = -1.39 + 6.90E(7)(2) (V) - 0.20logD (HL-60), where cL50 is compound concentration for 50% cell survival, and D is octanol/water distribution coefficient at pH 7.0. The flavonoids comprise a separate series of compounds with lower cytotoxicity. The correlations obtained quantitatively confirm the parallelism between the polyphenol cytotoxicity and the rates of their single-electron oxidation, and point to the leading role of formation of the reactive oxygen species in their cytotoxicity. Depending on the examined system, this parallelism may be distorted due to the cytochrome P-450 and COMT-catalyzed transformation of polyphenols.

Interpretation of the reactivity of peroxidase compound II with phenols and anilines using the Marcus equation

The catalytic cycle of heme peroxidases involves three processes: the formation of compound I, its conversion to compound II and regeneration of the native enzyme. Each of the processes consists of a reversible binding stage followed by an irreversible transformation stage. Our group has proposed a continuous, sensitive and reliable chronometric method for measuring the steady-state rate of peroxidase activity. Furthermore, we have derived an analytical expression for the steady-state rate and simplified it, taking into consideration the experimental values of the rate constants of some stages previously determined by other authors in stopped-flow assays. We determined the value of the constant for the transformation of a series of phenols and anilines by compound II, and found that it involves a deprotonation step and an electron transfer step. Study of the solvent deuterium isotope effect on the oxidation of phenol revealed the non-rate-limiting character of the deprotonation step in a proton inventory study. Usage of the Marcus equation showed that the electronic transfer step is rate-limiting in both cases, while phenols and anilines were oxidised at different rates for the same potentials. This can be attributed to the shorter electron-tunnelling distance for electron transfer to the iron ion in the phenols than in the anilines.

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.

Mechanism of cytotoxicity of catechols and a naphthalenediol in PC12-AC cells: the connection between extracellular autoxidation and molecular electronic structure

ortho-Hydroxyphenols (catechols) form a common structural unit in naturally occurring antioxidants such as polyphenols. They also show pro-oxidant characteristics which depend on their particular structure. Here we examined the acetylated versions of three catechols and a naphthalenediol for cytotoxicity to adrenal PC12-AC cells. We found that the three catechols H1 (a p-methoxycatechol), H2 (a catechol analog of alpha-tocopherol), and H4 (a dioxymethylene-substituted catechol) strongly upregulate glutathione (GSH) in 24 h, whereas 1,4-dipropyl-2,3-naphthalenediol (DPND) does not. Upregulation of GSH is primarily caused by oxidative stress in the form of hydrogen peroxide generation, and both GSH upregulation and the rate of H(2)O(2) generation correlate well with the cytotoxicity. The major source of H(2)O(2) is autoxidation in the extracellular space, which results from transport of the (deacetylated) hydroquinone form outside the cell, rather than internal redox cycling. DPND is much less cytotoxic than any of the catechols. We show that this is because it cannot form a naphthoquinone due to the energy penalty associated with the loss of aromaticity in the benzene ring adjacent to the diol functional group. The relevance of these results to the design of antioxidants is discussed.

Application of quantitative structure–toxicity relationships for acute NSAID cytotoxicity in rat hepatocytes

Non-steroidal anti-inflammatory agents (NSAIDs) are widely used for pain relief. However, they have been associated with harmful and sometimes fatal side effects. Usually, the target organs are the GI tract and liver. In this study, we have investigated the physicochemical requirements of 21 NSAIDs for glucuronidation and cytotoxicity by quantitative structure-toxicity relationships (QSTRs) in isolated rat hepatocytes. Furthermore, we have investigated the contrast in physicochemical variables that correlated with NSAID-induced hepatocyte cytotoxicity when glucuronidation was inhibited with borneol. The competitive inhibition of hepatocyte p-nitrophenol glucuronidation by NSAIDs was determined by HPLC. Glucuronidation-inhibited hepatocytes were more susceptible to NSAID-induced cytotoxicity. Also, we found a parabolic correlation between lipophilicity and the inhibition of glucuronidation for a subset of NSAIDs. For NSAIDs with a benzoic acid moiety, cytotoxicity also correlated parabolically with lipophilicity, but correlated linearly with the HOMO-LUMO gap, and the first-order valence connectivity index. The cytotoxicity of NSAIDs with a phenylacetic acid (or propionic acid) substructure also correlated with lipophilicity, but not with the HOMO-LUMO gap. Our findings indicated that the inhibition of glucuronidation resulted in increased NSAID cytotoxicity, suggesting that acyl-glucuronide metabolites were acutely less cytotoxic. Also, comparative QSTRs revealed that benzoic acid NSAIDs may form cytotoxic radical metabolites (parameterized by the HOMO-LUMO gap) or alter mitochondrial respiration (parameterized by the connectivity index), whereas phenylacetic acid derived NSAIDs may form different cytotoxic metabolites, since they did not correlate with these parameters. In summary, we have used QSTRs as a tool to distinguish the cytotoxic mechanism of two groups of NSAIDs, which, if analyzed together as one group, did not reveal such mechanism-based differences.

High performance solid-phase analytical derivatization of phenols for gas chromatography-mass spectrometry

The solid-phase analytical derivatization of phenols with pentafluoropyridine is performed. Fourteen phenols including chlorophenols and alkylphenols, could be efficiently adsorbed on a strong anion-exchange solid phase, Oasis MAX. The phenols adsorbed on Oasis MAX as phenolate ions were desorbed after derivatization with pentafluoropyridine. After optimization of the adsorption and derivatization, we established a procedure for the determination of the phenols in water samples by means of GC-MS. Under the optimized conditions, calibration curves were linear in the range of 10-1000 ng/l for the alkylphenols (100-10000 ng/l for nonylphenol) and 50-1000 ng/l for the others. By processing 100 ml samples, the method detection limits (MDLs) were in the range of 0.45-2.3 ng/l for the alkylphenols (8.5 ng/l for nonylphenol) and 2.4-16 ng/l for the others. Compared with the biphasic reaction system, the signal-to-noise ratios obtained by the solid-phase analytical derivatization were significantly higher. This is ascribed to the fact that coexisting neutral and acidic compounds are efficiently removed from the sample solution by this solid-phase analytical derivatization system.

Triphenyltin as a potential human endocrine disruptor

Organotin compounds have been implicated as reproductive toxicants and endocrine disruptors primarily through studies in aquatic organisms, with little information available in mammals. Among the organotins, aryltins have been less studied than alkyltins. Extensive data is available on mammalian developmental and reproductive toxicity of one aryltin compound, triphenyltin (TPT), from toxicity studies conducted in connection with the registration of triphenyltin hydroxide (TPTH) as a pesticide and supporting publications from the open literature. Indications of adverse functional and morphological effects on the reproductive tract of rats were reported in a dose range of 1.4-20 mg/kg/d. Gonadal histopathology (both ovaries and testes) and infertility were affected at the higher doses, while reproductive-tract cancer, smaller litter sizes, and reproductive organ weights were affected at the lower end of the dose range. In vitro studies indicate that TPT can directly activate androgen receptor-mediated transcription and inhibit enzymes that are involved in steroid hormone metabolism. These data suggest that the aryltin TPT can be active as a reproductive toxicant in mammals and may be a human endocrine disruptor.

Molecular cytotoxic mechanisms of anticancer hydroxychalcones

Chalcones are being considered as anticancer agents as they are natural compounds that are particularly cytotoxic towards K562 leukemia or melanoma cells. In this study, we have investigated phloretin, isoliquiritigenin, and 10 other hydroxylated chalcones for their cytotoxic mechanisms towards isolated rat hepatocytes. All hydroxychalcones partly depleted hepatocyte GSH and oxidized GSH to GSSG. These chalcones also caused a collapse of mitochondrial membrane potential and increased oxygen uptake. Furthermore, glycolytic or citric acid cycle substrates prevented cytotoxicity and mitochondrial membrane potential collapse. The highest pKa chalcones were the most effective at collapsing the mitochondrial membrane potential which suggests that the cytotoxic activity of hydroxychalcones are likely because of their ability to uncouple mitochondria.

QSARS for toxicity to the bacterium Sinorhizobium meliloti

In the present study, structure-activity relationship (QSAR) models for the prediction of the toxicity to the bacterium Sinorhizobium meliloti have been developed, based on a data set of 140 compounds. The data set is highly heterogeneous both in terms of chemistry and mechanisms of toxic action. For deriving QSARs, chemicals were divided into groups according to mechanism of action and chemical structure. The QSARs derived are considered to be of moderate statistical quality. A baseline effect (relationship between the toxicity and logP), which can be related to non-polar narcosis, was observed. To explain toxicity greater than the baseline toxicity, other structural descriptors were used. The development of models for non-polar and polar narcosis had some success. It appeared that the toxicity of compounds acting by more specific mechanisms of toxic action is difficult to predict. A global QSAR was also developed, which had square of the correlation coefficient r2 = 0.53. A QSAR with reasonable statistical parameters was developed for the aliphatic compounds in the data set (r2 = 0.83). QSARs could not be obtained for the aromatic compounds as a group.

Quantitative structure toxicity relationships for catechols in isolated rat hepatocytes

One- and two-parameter quantitative structure toxicity relationship (QSTR) equations were obtained to describe the cytotoxicity of isolated rat hepatocytes induced by 23 catechols in which LD(50) represents the catechol concentration required to induce 50% cytotoxicity in 2 h. A QSTR equation logLD(50) (microM = - 0.464(+/-0.065) log P + 3.724(+/-0.114) (n = 20, r(2) = 0.740, s(y,x) = 0.372, P < 1 x 10(-6), outliers: 4-methoxycatechol, 3-methoxycatechol, L-dopa) was derived where logP represents octanol/water partitioning. Outliers were determined by adopting a statistical method to standardize the identification of outliers. When pK(a1), the first ionization constant, was considered as a contributing parameter a two-parameter QSTR equation was derived: logLD(50) (microM = - 0.343(+/-0.058) log P - 0.116(+/-0.041) pK(a1)+4.389 (+/-0.315) (n = 22, r(2) = 0.738, s(y,x) = 0.375, P < 0.01, outlier: 4-methoxycatechol). Replacing logP with logD(7.4), the partition coefficient at pH 7.4, improved the first correlation by limiting the outlier to 4-methoxycatechol: logLD(50) (microM)=-0.252(+/-0.039) logD(7.4)+3.168(+/-0.090) (n = 22, r(2) = 0.671, s(y,x) = 0.420, P < 1 x 10(-5). In this study, 4-methoxycatechol (readily autooxidizable) was found to be an outlier for all QSTR equations derived. These findings point to lipophilicity and pK(a1) as two important characteristics of catechols that can be used to predict their cytotoxicity towards isolated rat hepatocytes. The catechols with the higher lipophilicity/distribution coefficient, the lower degree of ionization and the higher pK(a(catechol)) were more toxic towards hepatocytes than the other catechols.