Chronotoxicity as related to chronobiology

This review focuses primarily on the complexities of chronotoxicity and chronopharmacology (time-of-day effects on the metabolism of environmental chemicals and therapeutic agents as related to chronobiology). The nature of the melatonin signal may modify the function of the hepatic endoplasmic reticulum resulting in variations in the metabolism of xenobiotic chemicals. Concepts are explored for modification of exposure limits and/or Threshold Limit Values (TLVs) of industrial chemicals in risk assessment and health effects of workers on rotating shifts. The TLVs of chemicals may be changed during work shift schedules to minimize adverse health effects among workers.

The effect of nutrition on chemical toxicity

The toxicity of chemicals, and of reactive oxygen species (ROS), are both affected by nutrition and diet. Calorific excess (continuous feeding), or deficiency (fasting), may increase production of ROS, which are also formed by interaction of toxic chemicals with cytochromes P450 (CYP2E or futile cycling). Both ROS (GSH reductase and peroxidase) and toxic chemicals (S-transferases) are detoxified by GSH enzymes; ROS are scavenged by a system comprising GSH, ascorbic acid and tocopherols, which may be regenerated by NADPH. Dietary protein is necessary for GSH or enzyme replacement, lipids are required for polyunsaturated fatty acids (PUFAs) and prostanoid biosynthesis, lipotropes and phospholipids for synthesis of endoplasmic reticulum, and folate is needed for dug metabolizing activity. Among required minerals, Se is necessary as the essential component of the antioxidant enzyme, glutathione peroxidase. Other dietary factors considered are the natural toxicants, gossypol, lathyrogens, glucosinolates, and saponins, and toxicants from food spoilage, food intoxication and food processing.

The micro gas-liquid chromatographic analysis of 4-(3',3'-dimethylallyl)-1,2-diphenylpyrazolidine-3,5-dione (feprazone) in human biosamples

A gas-liquid chromatographic method for the determination of feprazone in various biological matrixes, employing a choice of detector options, is described. After rapid, micro-scale extraction of the sample with n-butyl acetate at physiological pH, the solution was injected directly onto the chromatograph. Separation was with either an OV7 column and flame ionisation or electron capture detection, or with a carbowax high polymer column and nitrogen specific detection. When 100 microl of plasma was extracted the limit of accurate measurement was 2 mg 1(-1) for F.I.D. and N.P.D. and 0.5 mg 1(-1) with E.C. detection. Quantification was by comparison with a range of plasma calibrators carried throughout the procedure, and determination of peak height ratios against an internal standard incorporated into the extracting solvent. The CV of the assay throughout the concentration range normally encountered in patients undergoing feprazone treatment, ranged between 2.4 and 7.8% for the various detector options. The analytical method has been applied to samples collected both from patients and normal volunteers undergoing treatment with a range of feprazone maintenance doses.

Multiple organ dysfunction syndrome

Multiple organ dysfunction syndrome, including acute respiratory distress syndrome (ARDS) and renal failure, is described, its clinical features outlined, its origins in tissue oxidative stress following severe infections, surgical trauma, ionizing radiation, high-dosage drugs and chemicals, severe hemorrhage, etc., are defined, and its prevention and treatment prescribed.

Quantitative structure-activity relationships in a series of endogenous and synthetic steroids exhibiting induction of CYP3A activity and hepatomegaly associated with increased DNA synthesis

The results of a quantitative structure-activity relationship (QSAR) study on a total of 14 steroids exhibiting induction of a CYP3A-associated activity and increase in liver weight/DNA synthesis is reported. It is found that different, but related, structural descriptors correlate with increase in ethylmorphine N-demethylase activity (r=0.92) and with the increase in liver weight (r=0.78) and DNA synthesis (r=0.78). Although there is a strong correlation between increase in liver weight and DNA content (r=0.999), neither of these correlated with ethylmorphine N-demethylase activity. These findings are discussed in the light of CYP3A induction, substrate specificity and inhibition; a proposed model of human CYP3A4 based on sequence homology with CYP102, a bacterial P450 of known crystal structure, demonstrates the possible mode of interaction between substrates and inhibitors within the putative active site.

A combined COMPACT and HazardExpert study of 40 chemicals for which information on mutagenicity and carcinogenicity is known, including the results of human epidemiological studies

The COMPACT approach for defining structural criteria for substrates and inducers of cytochrome P450 (CYP) enzymes which mediate the formation of reactive intermediates is discussed in the context of prediction of potential carcinogenicity. This is broadened to encompass structural studies on mammalian P450s, including those relevant to genetic polymorphism in man. The use of the COMPACT system, in parallel with the structure alert program HazardExpert (now incorporated into the Pallas system), for evaluating human carcinogenicity data is reported, as an example of the possible employment of a battery of short-term test procedures for safety evaluation. In particular, the importance of using the log P value (as a measure of compound lipophilicity) to assess the likelihood of a potentially toxic compound reaching the site of activation, is emphasized by the finding that most procarcinogens requiring metabolic activation by P450s are lipophilic in nature.

Cytochromes P450 and species differences in xenobiotic metabolism and activation of carcinogen

The importance of cytochrome P450 isoforms to species differences in the metabolism of foreign compounds and activation of procarcinogens has been identified. The possible range of P450 isozymes in significant variations in toxicity exhibited by experimental rodent species may have a relevance to chemical risk assessment, especially as human P450s are likely to show changes in the way they metabolize xenobiotics. Consequently, in the safety evaluation of chemicals, we should be cautious in extrapolating results from experimental animal models to humans. This paper focuses on examples in which species differences in P450s lead to significant alterations in carcinogenic response, and includes a discussion of the current procedures for toxicity screening, with an emphasis on short-term tests.

Further evaluation of COMPACT, the molecular orbital approach for the prospective safety evaluation of chemicals

The molecular dimensions and electronic structures of the first group of 100 US NCI/NTP miscellaneous chemicals, evaluated for potential carcinogenicity by computer-optimized molecular parametric analysis for chemical toxicity (COMPACT) have been re-determined. Using improved criteria for cytochrome P450 (CYP) substrate specificity, re-defined for CYP1 as having a COMPACT radius [square root of (deltaE - 9.5)2 + (a/d(2) - 7.8)2] of < 6.5, and for CYP2E as having a collision diameter of 6.5 angstroms or less and deltaE < 15.5, the likely substrates of CYP1 and CYP2E, which are regarded as potential carcinogens, have been identified. In addition, log P values have been taken into account; those chemicals with log P < 0 are non-lipophilic substrates unlikely to reach the activating cytochrome enzymes, and have been regarded as non-carcinogens. The second group of 100 US NCI/NTP chemicals have also now been categorized by COMPACT into CYP1 and CYP2E substrates, and their potential carcinogenicities evaluated. Of the 203 chemicals in the 2 groups, those positive in the rodent two-species life-span carcinogenicity study (rodent assay) were 53%, those positive in the Ames test (mutagenicity) were 48%, and those positive in the COMPACT programme (carcinogenicity, mutagenicity, cytotoxicity) were 54%. Concordance between the COMPACT prediction of carcinogenicity/cytotoxicity and rodent two species life-span carcinogenicity data for the 203 chemicals is 69%, and correlation of COMPACT with Ames test data is 61%. The sensitivity of COMPACT for predicting rodent carcinogenicity is 72%, whereas the sensitivity of the Ames test for predicting carcinogenicity for the 203 chemicals was only 57%. The degree (severity) of rodent carcinogenicity also showed correlation with the COMPACT predictive evaluations of the chemicals.

Changes in drug-metabolizing enzymes of rats in ciprofibrate-induced hepatic nodules

1. Premalignant rat liver nodules produced in the resistant hepatocyte model, by exposure to carcinogenic chemicals (diethyl nitrosamine and 2-acetamidofluorene), and partial hepatectomy, exhibit decreased xenobiotic hydroxylase activities and increased conjugase activities, which are considered responsible for increased resistance to xenobiotic toxicity. 2. However, premalignant rat liver nodules generated by feeding the hypolipidaemic, peroxisomal proliferating drug, ciprofibrate, in a hypolipidaemic model, exhibit decreased hydroxylase activities but decreased conjugase activities also. 3. It is considered that reactive oxygen species (ROS) are generated in both the resistant hepatocyte model and in the hypolipidaemic model, resulting in lipid peroxidation, loss of haem, cytochromes and hydroxylase activities. 4. However, whereas there is a rebounding compensation of conjugase enzymes in the resistant hepatocyte model, this does not occur with the hypolipidaemic model, as peroxidation is probably persistent and the conjugases are continuously destroyed.

Molecular modelling of CYP2E1 enzymes from rat, mouse and man: an explanation for species differences in butadiene metabolism and potential carcinogenicity, and rationalization of CYP2E substrate specificity

Molecular modelling of substrates of cytochrome P4502E1 (CYP2E1) within the putative active site region of CYP2E1 constructed from the CYP102 crystal structure is reported. Structural characteristics of CYP2E1 substrates, such as molecular size, energy levels and polarity, calculated via molecular orbital procedures provide correlations with toxicity and carcinogenicity; and species differences in CYP2E1-mediated metabolism are rationalized in terms of interactions with putative active site amino acid residues, including Thr-437 and Phe-181. In particular, the activation of buta-1,3-diene can be explained by active site modelling with CYP2E1 enzymes sequenced from rat, mouse and man, where there is a non-conservative change T437H between rodent and human isozymes, together with a conservative change I438V between mouse and rat CYP2E1.

Personal reflections on 50 years of study of benzene toxicology

The metabolism of benzene is reviewed, and the objectives of a quantitative balance study begun in 1945 are outlined; problems of toxicology and metabolism research of some 50 years ago are considered. The quantitative metabolism of 14C-benzene in the rabbit is annotated and compared with that of unlabeled benzene quantified by nonisotopic methods. The anomalies of phenylmercapturic acid and trans-trans-muconic acid as metabolites of benzene are examined in detail by isotopic and nonisotopic methods; these compounds are true but minor metabolites of benzene. Oxygen radicals are involved in both the metabolism of benzene and its toxicity; the roles of CYP2E1, the redox cycling of quinone metabolites, glutathione oxidation, and oxidative stress in the unique radiomimetic, hematopoietic toxicity of benzene are discussed. Differences between the toxicity of benzene and the halobenzenes are related to fundamental differences in their electronic structures and to the consequent pathways of metabolic activation and detoxication.

COMPACT and molecular structure in toxicity assessment: a prospective evaluation of 30 chemicals currently being tested for rodent carcinogenicity by the NCI/NTP

A new series of 30 miscellaneous National Toxicology Program chemicals has been evaluated prospectively for carcinogenicity and overt toxicity by COMPACT (Computer Optimised Molecular Parametric Analysis for Chemical Toxicity. CYP1A and CYP2E1). Evaluations were also made by Hazardexpert, and for metal ion redox potentials; and these, together with COMPACT, were compared with results from the Ames test for mutagenicity in Salmonella, the micronucleus test, and 90-day subchronic rodent pathology. Seven of the 30 chemicals (nitromethane, chloroprene, xylenesulphonic acid, furfuryl alcohol, anthraquinone, emodin, cinnamaldehyde) were positive for potential carcinogenicity in the COMPACT evaluation; xylenesulphonic acid and furfuryl alcohol were only equivocally positive. Four of the 30 chemicals-scopolamine, D&C Yellow No. 11, citral, cinnamaldehyde-were positive by Hazardexpert; 6 of 30-D&C Yellow No. 11, 1-chloro-2-propanol, anthraquinone, emodin, sodium nitrite, cinnamaldehyde-were positive in the Ames test; 2 of 30-phenolphthalein and emodin-were positive in the in vivo cytogenetics test; and 3 of 30-molybdenum trioxide, gallium arsenide, vanadium pentoxide-were metal compounds with redox potentials of the metal/metal ion indicative of possible carcinogenicity. The overall prediction for carcinogenicity was positive for 12 of 30 chemicals: nitromethane, chloroprene, D&C Yellow No. 11, molybdenum trioxide, 1-chloro-2-propanol, furfuryl alcohol, gallium arsenide, anthraquinone, emodin, sodium nitrite, cinnamaldehyde, vanadium pentoxide). This overall prediction has been made on the basis of the results of the computer tests and from consideration of the information from bacterial mutagenicity, together with likely lipid solubility and pathways of metabolism and elimination.

Spontaneous development of fatty liver in ferrets in a toxicology study

Ferrets were maintained for 12 months on different diets (A, meat and biscuit; B, all meat; C, meat and fish; D, high fibre) to ascertain the cause of spontaneous development of fatty liver. High hepatic triglyceride contents resulted on diets B = C > D; whereas ferrets on diet A (control) showed no accumulation of lipid in liver. Serum triglyceride and total cholesterol were unchanged by diet. These ferrets (F0 generation) were mated with ferrets on the same diet and the offspring (F1 generation), maintained on the same diets as the parents, were killed at 12 months and the livers studied similarly. Histology showed that hepatic lipid accumulation in the F1 generation was identical with that in the same dietary groups of the F0 generation; liver glutathione (GSH) reductase and thiobarbituric acid-reacting substances (an index of lipid peroxidation) were increased in ferrets maintained on diets B, C and D, liver GSH concentration and GSH peroxidase activities were unchanged. Other ferrets fed a high-fat diet (diet A plus 20% w/w beef suet) for 18 days exhibited hepatic lipid accumulation and decreased hepatic cyanide-insensitive palmitoyl CoA oxidation (-30%), but hepatic lauric acid hydroxylation and carnitine acyl transferase activities were unchanged. These data indicate that ferrets on high-fat diets show no increased rates of liver fatty acid oxidation, as seen in rats, but instead accumulate triglyceride in the liver with some degree of lipid peroxidation.

Chemical toxicity and reactive oxygen species

Reactive oxygen species (ROS) are cytotoxic, causing inflammatory disease, including tissue necrosis, organ failure, atherosclerosis, infertility, birth defects, premature aging, mutations and malignancy. ROS are produced in the metabolism of drugs and industrial chemicals by (i) one-electron peroxidase oxidations to form cation radicals, (ii) cytochrome P450 metabolism to free radical products, (iii) stabilisation of the ROS-generator, CYP2E1, and (iv) futile cycling of other cytochromes P450. ROS production initiates inflammation which unless quenched may result in chronic inflammatory disease states, e.g. hepatitis, nephritis, myositis, scleroderma, lupus erythematosus, multiple system organ failure. Quenching of ROS is affected by the redox buffer, glutathione (GSH), and the antioxidants, ascorbic acid, tocopherols, retinoids, in conjunction with the redox enzymes, GSH reductase, GSH peroxidase, catalase and superoxide dismutase. Many industrial workers with symptoms of systemic inflammation, resulting from exposure to toxic chemicals, are diagnosed as having rheumatoid arthritis, virus infections, or other microbial lesions, largely because many physicians are unaware that exposure to certain chemicals can initiate inflammatory disease states.

Molecular orbital-generated QSARs in a homologous series of alkoxyresorufins and studies of their interactive docking with P450s

1. Molecular and electronic structural parameters have been determined, by molecular orbital (MO) calculations, for a homologous series of 8 alkoxyresorufins (methoxy- to octoxy-). 2. Quantitative structure-activity relationships (QSARs) between these structural parameters and the rates of metabolism of the alkoxyresorufins in hepatic microsomes from the 3-methylcholanthrene (MC)-, and phenobarbital (PB)-pretreated mouse, and the beta-naphthoflavone (beta NF)-pretreated rat have been established. 3. The most significant single relationship is between beta NF-induction of cytochrome P4501 (CYP1A) and the total nucleophilic superdelocalizability (sigma SN) for the eight compounds in the series. 4. For double regressions, the electronic charge on the alkoxy oxygen, Q(O), or alpha-carbon Q(C), is important when combined with the hydrophobic substituent constant (pi). 5. These findings indicate that the rates of metabolism of these alkoxyresorufins are dependent upon their ability to cross cellular membranes, to fit the relevant CYP1A binding site, and on their ability to accept electrons from a donor nucleophilic species. 6. A different set of parameters correlated with CYP2B activity, namely, parameters of overall shape, which indicates that the way in which the alkoxyresorufins fit the CYP2B site, determines their differences in specificity. 7. Computer graphic interactive docking studies of the alkoxyresorufins with their affinity-specific cytochromes P450, namely, methoxy- with CYP1A2; ethoxy- with CYP1A1; pentoxy- with CYP2B1; and benzyloxy- with CYP3A, have also been undertaken to show the specific interactions of the alkoxyresorufins with the binding sites of the individual P450s.

Quantitative structure-activity relationships and COMPACT analysis of a series of food mutagens

Quantitative structure-activity relationships between chemical structure and Ames mutagenicity for a group of 24 food mutagens, including 17 cooked-food heterocyclic amines, have been determined. For the TA98 strain of Salmonella typhimurium (frameshift mutagens) the best correlation of mutagenicity is with molecular diameter (R = 0.91), while for the TA100 strain (base-pair mutations) the best correlation is with delta E, the energy difference between the lowest unoccupied and highest occupied molecular orbitals. High mutagenicity is related to high values of molecular diameter, hence to planarity and to high values of the COMPACT ratio ([area/depth2]/delta E). High mutagenicity is also related to low values of delta E. Consequently, highly mutagenic and potentially carcinogenic food chemicals can readily be identified as substrates of cytochrome P4501 (CYP1) and may therefore be detected by the COMPACT procedure. Highly mutagenic compounds also exhibit high values of dipole moment.

A quantitative structure-activity relationship study on a series of 10 para-substituted toluenes binding to cytochrome P4502B4 (CYP2B4), and their hydroxylation rates

Molecular structural and molecular orbital calculations (AM1 method) are reported on a series of 10 para-substituted toluene derivatives and this structural information has been used to rationalize the differences between both rates of hydroxylation catalysed by cytochrome P4502B4 and binding to the same cytochrome P450, via the generation of quantitative structure-activity relationships (QSARs). It was found that the rate constant for hydroxylation can be described by a two-variable expression involving the dipole moment and volume of the solvent-accessible molecular surface (r = 0.98), whereas binding free energies are well characterized by combinations of molecular volume and various electronic frontier orbital parameters (r = 0.98 and 0.99). This study represents an advance on a previous evaluation by White and McCarthy (Arch Biochem Biophys 246: 19-32, 1986) who used empirical physico-chemical parameters to obtain similar results which were generally of lower statistical significance to those of the present work. The QSAR expressions suggest that both binding to P450 and metabolism for this series of compounds are dependent on the relative ability of the molecules to desolvate and occupy the heme binding site, together with electronic properties of the whole molecule and of the methyl group which undergoes hydroxylation.

The genotoxicity of benzanthracenes: a quantitative structure-activity study

Molecular orbital (MO) evaluations of a series of 14 methyl-substituted benz[alpha]anthracenes, calculated by the complete neglect of differential overlap (CNDO/2) method, are reported. By quantitative structure-activity relationship (QSAR) analysis, the carcinogenic and mutagenic potencies of these compounds have been shown to be correlated with their electronic structures, namely, with the magnitude of the energy of the lowest unoccupied molecular orbital (LUMO). The log mutagenicity potencies for the series of 14 benz[alpha]anthracenes are negatively dependent on E(LUMO), with a correlation coefficient of 0.82, which is increased to 0.90 by inclusion in the QSAR of a second variable, namely Q3H, the electronic density in the highest occupied molecular orbital, E(HOMO), of carbon-3. E(LUMO) is also negatively correlated with mouse carcinogenicity of the benzanthracenes, with a correlation coefficient of 0.88 for tumour incidence, and of 0.83 for log carcinogenicity index. The carcinogenicity and mutagenicity of the individual members of this series of polycyclic aromatic hydrocarbons are discussed in terms of the relationships between molecular structure, electron density, metabolic activation and covalent binding of reactive intermediates.

Computer graphics analysis of the interaction of alkoxy methylenedioxybenzenes with cytochromes P4501

A quantitative structure-activity relationship (QSAR) in a homologous series of alkoxy methylenedioxybenzenes (MDBs) is reported. Measurements of molecular dimensions from computer-generated space-filling structures have provided values for the shape parameter area/depth2. These have been shown to correlate with the extent of inhibition of ethoxyresorufin O-deethylase activity by a series of MDBs. The implication of this is that the MDB nucleus fits the cytochrome P4501 substrate binding site and that this ability decreases with increase in the alkyl chain length of the alkoxy substituent. These findings are in agreement with previous results relating to the spatial dimensions of the cytochrome P4501 binding site, showing that substrate specificity can be rationalized in terms of overall molecular shape.

A retrospective evaluation of COMPACT predictions of the outcome of NTP rodent carcinogenicity testing

The carcinogenic potentials of 40 National Toxicology Program chemicals previously predicted by Computer Optimised Molecular Parametric Analysis for Chemical Toxicity (COMPACT), based on the identification of potential substrates of cytochromes P4501A and 2E (CYP1A and CYP2E), have been compared with new rodent carcinogenicity results. The COMPACT predictions have also been compared with published Ames mutagenicity data and with our own Hazardexpert predictions for carcinogenicity. Concordance evaluations between rodent carcinogenicity (1/4 segments positive) and predictions by COMPACT or Hazardexpert were 64% for COMPACT (CYP1A only), 72% for COMPACT (CYP1A plus CYP2E), 70% for Hazardexpert alone, and 86% for COMPACT (CYP1A plus CYP2E) plus Hazardexpert. Sensitivities of the predictions were for COMPACT, 75%; Hazardexpert, 60%; and Ames, 54%. Positive predictivities were for COMPACT, 75%; Hazardexpert, 78%; and Ames 81%. Negative predictivites were for COMPACT, 62%; Hazardexpert, 52%; and Ames, 42%.

Safety evaluations of food chemicals by "COMPACT". 1. A study of some acyclic terpenes

A group of 19 acyclic terpenes have been evaluated for potential toxicity/carcinogenicity by molecular orbital determinations of their spatial and electronic parameters, and hence prediction of their metabolic activation or detoxication by the cytochrome P-450 (CYP) superfamily of mixed-function oxidase enzymes. Previous studies have characterized the spatial dimensions of the CYP1A1, 1A2 and 2E1 enzymes, which are known to activate mutagens and carcinogens and to be involved in other mechanisms of toxicity. None of the terpenes was found to have shape or electronic parameters appropriate for metabolic activation by CYP1A1 or 1A2, and hence they are unlikely to be carcinogenic or mutagenic. Furthermore, none of these chemicals had spatial parameters critical for substrates of CYP2E, and they are therefore unlikely to induce the formation of reactive oxygen species (ROS) or to initiate or promote malignancy or toxicity by mechanisms involving ROS. However, citral, and others of these terpenes, are known to undergo metabolism to carboxylic acids that may induce CYP4, and are therefore possible inducers of hepatic peroxisomal proliferation at high dosage, which may have implications for possible hepatotoxicity.

Effect of high fat diet on liver microsomal oxygenations in ferret

1. Ferret on a high fat diet accumulated large amounts of fat in its liver and had blood acetoacetate and beta-hydroxybutyrate concentrations 250 and 375% of those in control animals. 2. The high fat diet alone increased ferret liver microsomal 7-ethoxyresorufin O-deethylase (EROD) activity by 90%, but had no effect on 7-methoxy-, 7-pentoxy-, or 7-benzyloxy-resorufin, O-dealkylase activities. Administration of 3-methylcholanthrene (MC) increased only liver EROD activity, by 5- to 6-fold, in ferret on both high fat and control diets. Induction of EROD, but not MROD activity, in ferret on the high fat diet indicates that P4501A1, but not P4501A2, is induced. 3. Activation of 3H-paracetamol, measured by covalent tissue binding to ferret liver microsomal fractions, was increased three-fold in ferret on the high fat diet, nine-fold by MC administered to ferret on a control diet, and 13-fold by MC given to ferret on the high fat diet. Similar results were obtained with activation of the cooked-food amine, Glu-P-1, by ferret liver microsomes. 4. Western blots with antibodies to rat liver P450s showed that ferret liver contains proteins orthologous with rat liver P4504A1 and bifunctional protein. However, whereas clofibrate, similar to high fat diets, induced these two proteins in rat liver, no increase of these proteins occurred in liver of ferret fed a high fat diet. Western blots also showed that ferret liver contains no P4501A1 or 1A2, and although these two proteins were induced by MC, no induction occurred when ferret was fed the high fat diet alone. Ferret liver microsomes also contain a protein recognized by rat anti-P4502E1 but of a lower molecular weight. 5. Immunosorbent (ELISA) analyses of ferret liver for P4501A1 and 4A1 showed that the high fat diet increased a protein orthologous to rat P4501A1 but did not increase any protein orthologous to rat P4504A1. 6. These findings indicate that the high fat diet does not induce ferret liver bifunctional protein or P4504A1 enzyme protein, but may enhance liver P4501A1 and 1A2 activities through the hyperketonaemia resulting from the high dietary fat. The conflicting P450 results, namely Glu-P-1 activation but no MROD activity for P4501A2, high EROD activity and ELISA quantification of P4501A1, but no positive Western blot, are probably due to differences in substrate specificity and immunological characteristics between rat and ferret enzymes.

The cytochromes P450 and mechanisms of chemical carcinogenesis

This article reviews mechanisms of chemical carcinogenesis, from metabolic activation and generation of reactive oxygen species by cytochromes P4511 and P4502E to DNA damage, activation of protein kinase C and ocogenes, hyperplasia, and proteoglycan changes in the cell glycocalyx and lysosomal enzymes which mediate invasion and metastasis.

Inhibition of rat hepatic aryl hydrocarbon hydroxylase activity by a series of 7-hydroxy coumarins: QSAR studies

1. Molecular orbital calculations, by the Modified Intermediate Neglect of Differential Overlap (MINDO/3) method, of a series of twenty-five 8-acyl-7-hydroxy coumarins show that the inhibition of aryl hydrocarbon hydroxylase (AHH) activities (cytochrome P4501, CYP1 activity, primarily CYP1A1) for 23 of these compounds is related to their structural parameters. The two remaining compounds are the only chlorinated derivatives; these are inactive towards the AHH system and were excluded from the quantitative structure-activity relationship (QSAR) analysis. 2. The results of multiple regression analyses show that AHH activity is dependent on the energy of the highest occupied molecular orbital, E(HOMO), in a single variable expression for the 23 compounds. However, a three-variable expression involving superdelocalizabilities provides a more significant correlation with biological activity. 3. The inactivity of the two chlorinated derivatives can be rationalized in terms of their low degree of molecular planarity, as estimated by the area/depth2 parameter, which presumably precludes them from interaction with CYP1.

The use of computers in the safety evaluation of drugs and other chemicals

The toxicity and carcinogenicity of drugs and other chemicals is, in most cases, mediated by highly reactive intermediates which are generated following metabolism catalysed by the enzymic apparatus of the exposed organisms. These reactive intermediates readily interact covalently with vital cellular components to provoke toxicity and carcinogenicity. The ubiquitous cytochrome P450-dependent mixed-function oxidases are the most important enzyme system in the activation of chemicals. This enzyme system comprises a number of families, each of which contains one or more subfamilies. The CYPIA and CYP2E subfamilies are the most closely associated with the production of reactive intermediates and, consequently, the manifestation of toxicity and carcinogenicity. A computer based molecular structure procedure (COMPACT) has been developed which, via a calculation of the molecular and electronic structure of the chemical, determines whether the chemical will interact with either of these two cytochrome P450 subfamilies and hence be metabolised to form reactive intermediates that manifest toxicity. As the basal levels of these two subfamilies are generally low, the ability of a chemical to induce them selectively, on repeated administration, is an important determinant of its toxic and carcinogenic potential. This inductive capability may be determined in short-term studies (ENACT) using only a small number of animals. Thus the combination of COMPACT and ENACT provides a rapid and inexpensive means for the preliminary screening of chemicals for toxicity and carcinogenicity before undertaking the long-term and expensive rodent lifetime bioassays.

Molecular modelling of cytochrome CYP1A1: a putative access channel explains differences in induction potency between the isomers benzo(a)pyrene and benzo(e)pyrene, and 2- and 4-acetylaminofluorene

The present studies were undertaken to provide a rationale for the observation that benzo(a)pyrene and 2-acetylaminofluorene induce the hepatic CYP1A1 protein, whereas their non-carcinogenic isomers benzo(e)pyrene and 4-acetylaminofluorene are, at best, relatively very weak inducers. Using amino acid sequence alignment, a molecular model of the CYP1A1 was constructed by analogy to CYP101, the bacterial protein for which the 3-dimensional structure is known from X-ray crystallographic analysis. The putative structure of the active site of the CYP1A1 protein shows the presence of two phenylalanine residues preferentially aligned in parallel orientation, presumably functioning as a 'sieve' for planar molecules, the established substrates of CYP1A1. The molecular dimensions of this putative access channel show a width and depth of 8.321 and 3.261 A, respectively. The width of 4-acetylaminofluorene, 8.794 A, and benzo(e)pyrene, 9.153 A, precludes their passage through this channel access in contrast to benzo(a)pyrene and 2-acetylaminofluorene having a width of 7.150 and 5.283 A, respectively, explaining their difference in CYP1A1 induction potential.

Interaction of a series of nitriles with the alcohol-inducible isoform of P450: computer analysis of structure-activity relationships

1. Structural studies are reported on a series of 20 nitriles of varying rates of P4502E-mediated oxidative metabolism. 2. Parameters of molecular and electronic structure have been calculated for the generation of quantitative structure-activity relationships (QSARs) with the rates of oxidative metabolism of the nitriles, and with their acute toxicity. 3. Correlations between molecular polarizability, excitation energy and biological activity are presented as a result of QSAR analysis.

Studies in surgical trauma: oxidative stress in ischaemia-reperfusion of rat liver

1. Hypovolaemic shock associated with surgical trauma has been studied in a rat liver ischaemia-reperfusion model by determination of oxidative stress, lipid peroxidation and tissue infiltration of polymorphonuclear leucocytes. 2. Liver ischaemia alone resulted in slight liver oedema and polymorphonuclear leucocyte infiltration, a slight increase in thiobarbituric acid-reacting substances (an index of lipid peroxidation) and decreases in liver reduced glutathione and total radical-trapping antioxidant parameter, indices of oxidative stress. Ischaemia plus 30 min of reperfusion further increased liver oedema, polymorphonuclear leucocyte infiltration and thiobarbituric-acid reacting substances, and further decreased liver reduced glutathione and total radical-trapping antioxidant parameter. 3. After 60 and 90 min of reperfusion, oedema (40% increase), polymorphonuclear leucocyte infiltration (40-fold increase) and thiobarbituric-acid reacting substances (20-fold increase) were maximal, and liver reduced glutathione (75-95% decrease) and total radical-trapping antioxidant parameter (85-90% decrease) were at a minimum. 4. All parameters were exacerbated by 24 h starvation. Liver reduced glutathione closely paralleled total radical-trapping antioxidant parameter, and ischaemia alone depleted both by 30% in fed rats and 50% in fasted rats. 5. Oxidative stress and lipid peroxidation were associated more with the period of reperfusion and polymorphonuclear leucocyte infiltration. Polymorphonuclear leucocyte infiltration into lung also occurred after 90 min of liver reperfusion. 6. Possible mechanisms of hepatic ischaemia-reperfusion-induced oxidative stress are discussed.

Hepatic disease, the gastrointestinal tract, and rheumatic disease

In previous years, this review concentrated on the relationship between pathology in the gastrointestinal tract and rheumatologic complaints associated with this pathology. This year, we have emphasized the relationship between hepatic disorders and rheumatologic complaints, although a resumé of recent literature pertaining to the gastrointestinal tract and its rheumatologic consequences is also presented. We believe it is necessary to divert our primary focus of attention because of recent developments in identifying the extrahepatic effects of hepatitis C infection and the current interest in abnormalities of drug metabolism in various rheumatic and autoimmune disorders. These recent developments bring us full circle in incriminating not only bacterial and dietary antigens in the pathogenesis of the spondyloarthropathies but also viruses and exogenous chemicals as potential etiologic agents in genetically predisposed hosts, resulting in the development of a variety of diseases, including glomerulonephritis, vasculitis, Sjögren's syndrome, and systemic lupus erythematosus.

Pharmacokinetics of single oral doses of feprazone in patients with rheumatoid arthritis or with impaired renal clearance

1. The pharmacokinetics of feprazone have been studied in 10 patients with rheumatoid arthritis (RA), and in a further six patients with renal impairment (RI) who were not suffering from rheumatoid disease. 2. For RA patients, the mean elimination half-life (t1/2) of feprazone after a single oral dose was 21 +/- 5 h (SD), the mean apparent clearance (Cl) was 0.012 +/- 0.009 l/h per kg, and the mean apparent volume of distribution (Vd) was 0.33 +/- 0.17 l/kg. Corresponding values for RI patients were 25 +/- 13 h, 0.016 +/- 0.011 l/h per kg, and 0.46 +/- 0.24 l/kg, respectively. 3. These results show no impairment of the elimination of feprazone in RA or RI patients; Vd and Cl are greater than in healthy young volunteers or elderly subjects, the AUC values are lower, but t1/2 values are similar in all groups. 4. It is suggested that the greater Cl and Vd, and lower AUC, in RA and RI patients may be due to renal insufficiency and decreased plasma protein binding of feprazone and its metabolite, or to induction of glucuronyl transferase activity by the prior medication, thus enhancing the formation of the major metabolite, the C(4)-glucuronide, and increasing drug elimination.

Role of tissue glutathione in prevention of surgical trauma

1. Surgical trauma has been associated with pre-anaesthesia fasting, anaesthetic toxicity, haemorrhage, hypovolaemic shock, and other pathological phenomena. Tissue glutathione (GSH), thiobarbituric acid-reacting substances (TBAR), and radical-trapping activity (RTA) have been determined at various time intervals after fasting, anaesthesia, and also after hepatic ischaemia and reperfusion as a model for haemorrhage and hypovolaemic shock. 2. Light ether anaesthesia of rats resulted in an immediate (5 min) and progressive decrease in liver and kidney total glutathione (GSH and GSSG), which was much greater in animals that had been fasted for 20 h. TBARs, a measure of lipid peroxidation, in rat liver and kidney increased as total GSH decreased. Fasting (20 h) alone decreased tissue GSH by 50%, and increased TBAR 100%; fasting plus 30 min of ether anaesthesia decreased tissue glutathione by 80 to 85%, and increased TBAR by some 600%. 3. Liver ischaemia alone decreased total liver GSH by 20% in the fed rat, and 50% in the fasted rat. Ischaemia, followed by reperfusion, decreased liver total GSH by 70% in the fed rat, and 90% in the fasted rat. The ratio of GSH/GSSG decreased from 16 in control animals to 7 in the fasted ischaemic rat, then to 1 in the fasted, ischaemic rat reperfused for 90 min. RTA of liver closely paralleled liver total GSH levels. TBAR was increased by ischaemia alone (50-100%), but more (400%) by 90 min reperfusion. 4. A complex series of molecular mechanisms including: (1) GSH depletion; (2) induction of CYP2E1 activity; (3) generation of reactive oxygen species; (4) lipid peroxidation; (5) cytokine release; and (6) leucocyte activation, are advanced to account for the toxic phenomena of surgical trauma and multiple system organ failure.

The effects of ether anaesthesia on oxidative stress in rats--dose response

Rats, with and without overnight fasting, were anaesthetised for 5, 15 and 30 min with diethyl ether, killed immediately and total glutathione (total GS), thiobarbituric acid-reacting substances (TBAR), radical-trapping activity (RTA), total cytochrome P450 (CYP), and 7-ethoxyresorufin O-deethylase (CYP1), 7-pentoxyresorufin O-dealkylase (CYP2B) and 4-nitrophenol hydroxylase (CYP2E1) activities of liver and kidney determined. Liver, after ether anaesthesia, but no fasting, showed 30-60% losses of total GS, RTA, and total CYP, after 5, 15 and 30 min of anaesthesia, while TBAR increased 10-, 20- and 35-fold for the same periods. Liver after ether anaesthesia and overnight fasting showed 50-85% losses of total GS, RTA and total CYP, for 0, 5, 15 and 30 min of anaesthesia, while TBAR increased 4-, 30-, 40- and 60-fold for the same periods of anaesthesia. Kidney changes were similar to those in liver. Liver CYP1 and CYP2B were decreased by 45% and 35%, respectively for 30 min of anaesthesia in fed rats, and by 80% and 30% respectively for 30 min of anaesthesia in fasted rats; in contrast, liver CYP2E1 was increased 30% by fasting alone and 70% by fasting plus 5 min of ether anaesthesia. Kidney CYP1 and CYP2B were similarly decreased by ether anaesthesia (70% and 50% respectively) in both fed and fasted rats, and CYP2E1 was similarly increased (by 40-90% in fed and 30-110% in fasted rats). The decrease in tissue total GS, RTA, total CYP, CYP1 and CYP2B, and the increase in lipid peroxidation products (TBAR), are all considered to be due to generation of reactive oxygen species and oxidative stress, associated with the increase in CYP2E1 activity that results from both fasting and exposure to diethyl ether.

Computer modelling in predicting carcinogenicity

The cytochrome P450-dependent mixed-function oxidases are the most important enzyme system in the oxidation of chemicals to their reactive intermediates which then interact with cellular components to provoke toxicity and carcinogenicity. These enzymes comprise a multifamily of proteins, two families of which, namely CYP1A and CYP2E, activate planar and small molecular weight compounds, respectively. A computer graphic procedure (COMPACT) has been developed which, based on the molecular shape and electronic structure of the chemical, determines whether the chemical will interact with these two particular cytochrome P450 families and thus be metabolized to toxic and carcinogenic intermediates. As the basal levels of these enzyme families are low, the ability of the chemical to induce them selectively, on repeated administration, is an important determinant of its carcinogenic potential. Inductive capability may be determined in short-term experiments (ENACT) utilizing a small number of animals. Thus the combination of COMPACT and ENACT provides a rapid and inexpensive means for the preliminary screening of chemicals, before the long term and expensive rodent lifetime bioassays are undertaken.

Effects of ether anaesthesia and fasting on various cytochromes P450 of rat liver and kidney

Fed and fasted, male, Wistar albino rats exposed to light ether anaesthesia and killed immediately or after 30 or 120 min recovery were compared with non-anaesthetized rats for changes in liver and kidney cytochrome P450 (CYP) activities. In fed rats, liver total CYP (nmol/mg protein) decreased by 30% immediately after ether, but was restored to normal levels after 30 min recovery; in fasted rats, liver total CYP increased by 20% by fasting alone, then decreased by 65% immediately after ether, and recovered to only 70% of control at 2 hr after ether. Rat liver cytochrome P4501A (CYP1A; 7-ethoxyresorufin O-deethylase or EROD activity) and cytochrome P4502B (CYP2B; 7-pentoxyresorufin O-dealkylase or PROD activity) were decreased after ether anaesthesia, similar to those for total CYP. In contrast, rat liver cytochrome P4502E1 (CYP2E1), determined by p-nitrophenol hydroxylation, increased by 40% by ether anaesthesia alone, 70% by fasting alone and 140% by ether plus fasting; these increases were confirmed by the CYP2E1-mediated activation of nitrosopyrrolidine and by immunoblot analysis using antibody to CYP2E1. In rat kidney, losses of total CYP, CYP1A and CYP2B, and increases of CYP2E1, induced by ether anaesthesia, were much more marked in fasted (90% loss in total CYP, 30% increase in CYP2E1) than in fed rats (slight loss in total cytochrome P450, 30% increase in CYP2E1). As maximum losses of total CYP in liver of fasted rats exposed to ether occurred at the time of maximum increase of CYP2E1 and maximum rate of generation of reactive oxygen species (ROS), it is suggested that the increase of CYP2E1, resulting from its stabilization by fasting and ether, leads to generation of ROS, increase in lipid peroxidation and consequent loss of total CYP, associated with the hepatic and renal necrosis seen in ether intoxication and surgical trauma.

Validation of a novel molecular orbital approach (COMPACT) for the prospective safety evaluation of chemicals, by comparison with rodent carcinogenicity and Salmonella mutagenicity data evaluated by the U.S. NCI/NTP

The molecular dimensions and electronic structures of 100 chemicals of structural diversity have been determined from molecular orbital calculations and molecular mechanics. From these parameters of molecular structure, those chemicals that are likely substrates of cytochromes P4501 and P4502E have been identified by the computer-optimized molecular parametric analysis of chemical toxicity (COMPACT) programme, and their potential toxicity, mutagenicity and carcinogenicity evaluated. The degree of correlation between COMPACT prediction of toxicity and rodent two species life-span carcinogenicity data is estimated to be 92%, and between COMPACT and Salmonella mutagenicity (Ames test) data is 64%. Anomalous rodent carcinogens are rationalized on the basis of biochemical mechanisms of metabolism, genotoxicity and carcinogenicity. Correlation of the Ames test data with rodent carcinogenicity data was 64%, but correlation of COMPACT plus Ames data versus rodent carcinogenicity data provided the highest correlation of 94%.