Covalent binding of benzo[a]pyrene to rat liver cytosolic proteins and its effect on the binding to microsomal proteins

Biomarker-based assessment of the toxicity of the antifungal clotrimazol to the microcrustacean Daphnia magna

Among the vast list of xenobiotics that may promote harmful effects in aquatic ecosystems, pharmaceuticals are currently a prominent class due to their ability to persist in these environments and also due to the lack of information regarding their effects on the different components of the aquatic biota. Antifungals in particular, despite their massive use, are not extensively studied in environbmental terms. The main objective of this study was to characterize the toxicity of the antifungal clotrimazole to the aquatic organism Daphnia magna. To attain this purpose, the effects of this compound were measured, focusing on the determination of acute lethality, and quantification of biomarkers, such as neurotoxicity (soluble cholinesterases, ChEs); and oxidative stress and metabolism (such as catalase, CAT; and glutathione-S-transferases, GSTs). The toxicity assessment with biomarkers was based on animals exposed to concentrations similar to those already found in surface waters in order to increase the ecological relevance of the obtained data. The results showed that exposure to clotrimazole was able to induce significant increases in both CAT amd GSTs activities. ChE activity was not significantly altered after clotrimazol exposure. In view of the above, it is concluded that the drug studied caused adverse effects in terms of oxidative stress, at an ecological relevant levels, showing that the presence of clotrimazol in the wild is not innocuous.

Effect of dietary caffeic and chlorogenic acids on in vivo xenobiotic enzyme systems

The plant phenols chlorogenic and caffeic acids were tested for their affinity to alter hepatic and intestinal xenobiotic Phase I and Phase II enzyme activities in mice. Mice were fed isocaloric and isonitrogenous powdered diets containing 0 and 0.2% caffeic and chlorogenic acids, respectively. Animals pre-treated with benzo(a)pyrene (B(a)P) were sacrificed 18 h after an oral dose (50 mg/kg Bwt). B(a)P induced animals exhibited higher (p < 0.05) microsomal AHH, UDPGT, P-450 and cytosolic GST liver enzyme activities. The presence of these phenolics in the diet was shown to have little effect in modulating hepatic xenobiotic activating-detoxification enzymes. The dietary intake of caffeic and chlorogenic acids was particularly evident with intestinal xenobiotic AHH and GST enzyme activity. These data indicate that the presence of caffeic and chlorogenic acids in the diet may have an integral role in modulating the carcinogenic potential of reactive xenobiotics such as B(a)P.

Glutathione-S-transferase is a target for covalent modification by a halothane reactive intermediate in the guinea pig liver

The anesthetic halothane is bioactivated by the liver cytochrome P450 system to the reactive intermediate, trifluoroacetyl chloride, which can acylate liver protein. Cytosolic glutathione-S-transferase (GST) was identified as a major target for protein adduct formation in guinea pig liver slices exposed to halothane. To determine if GST is also a target in vivo, male Hartley guinea pigs were exposed to 1% halothane in 40% O2 for 4 h. At 10 h post exposure, livers were removed and microsomal and cytosolic fractions prepared. Past studies have shown these conditions resulted in maximal covalent binding of halothane intermediates to hepatic protein. Protein was isolated by ethanol precipitation and washed with trichloroacetic acid to remove unbound metabolites. Cytosolic GST was isolated by gel filtration and S-hexyl-glutathione affinity chromatography to electrophoretic purity. Protein adducts were quantified using a covalently bound fluorine assay. Covalent binding of a halothane intermediate to cytosolic and microsomal protein was determined as 2.0 +/- 0.4 and 13.2 +/- 2.3 nmol F/mg protein, respectively. Liver glutathione depletion by buthionine sulfoximine pretreatment produced an increase in covalent binding only to cytosolic proteins (3.3 +/- 0.4 nmol F/mg protein). Adduct formation to cytosolic GST was determined to be 4.7 +/- 1.6 nmol F/mg protein. Glutathione-S-transferase is a target for covalent modification in the liver following an inhalation exposure to halothane.

Aflatoxin B1-binding proteins in primary cultured hepatocytes of chicken embryo: studies in vivo and in vitro

Among several aflatoxin B1 (AFB1)-binding proteins in primary cultured hepatocytes of chicken embryo, a cytosolic 25 kDa protein proved to be an AFB1-specific binding protein. Binding of AFB1 to a cytosolic protein of the same molecular mass as that seen in vivo was also detectable in a cell-free system and was dependent on the microsomal components. The binding was observed in the in vitro systems derived from AFB1-sensitive animals such as chicken, rabbit, and rat. These findings suggest that the 25 kDa AFB1-binding protein(s) is involved in the intracellular process of AFB1-mediated toxicity.

Formation and identification of protein adducts to cytosolic proteins in guinea pig liver slices exposed to halothane

The anesthetic halothane can be bioactivated to the reactive intermediate, trifluoroacetyl chloride, which can covalently bind to liver protein. The product of this reaction is trifluoroacetyl-N-epsilon-lysine which can act as a foreign epitope in altering both protein immunogenicity and antigenicity. An in vitro liver slice system was used to study the formation of protein adducts following exposure to halothane. Liver slices (30-35 mg wet weight, 250-300 microns thick) from adult male Hartley guinea pigs (600-800 g) were exposed to [14C]halothane (0.6-0.9 microCi, 1.0-1.7 mM) in 95% O2/5% CO2 for 1, 6 and 12 h. The slices were homogenized and subcellular fractions prepared. Proteins were resolved by electrophoresis and bound radioactivity was detected by scintillation counting and autoradiography. Greater than 80% of detectable radioactivity to whole liver cell protein was localized in the 20-30-kDa range and increased in a linear fashion over the 12-h incubation period. Covalent binding was localized to two proteins of 27 kDa and 26 kDa present in the cytosolic compartment. Purification followed by N-terminal amino acid sequence analysis of the 27-kDa protein has identified it to be homologous with glutathione-S-transferase b. This cytosolic protein appears to be the major target for trifluoroacetylation in liver slices exposed to halothane.

Glutathione S-transferase in intestine, liver and hepatic lesions of mummichog (Fundulus heteroclitus) from a creosote-contaminated environment

Cytosolic glutathione S-transferase (GSH transferase) activity towards 1-chloro-2,4-dinitrobenzene (CDNB) was elevated approximately three to four-fold in intestine and liver of mummichog (Fundulus heteroclitus) collected from a creosote-contaminated site in the Elizabeth River, Virginia. Intestinal GSH transferase activity at the most heavily contaminated site, at a moderately contaminated site and at a relatively clean site averaged 3.64, 2.83 and 1.11µmoles/min/mg respectively, while values for liver at these sites averaged 2.84, 1.75 and 0.93µmoles/min/mg. In addition, densitometric tracings of sodium dodecylsulfate-polyacrylamide gels of intestine and liver cytosol revealed a similar trend in the staining intensity of a 25.8 kD protein band, which lies within the molecular weight range of GSH transferase subunits. Activity in putative preneoplastic and neoplastic hepatic lesions of fish collected from the creosote-contaminated site was not significantly different from that of adjacent normal tissue. In the laboratory, dietary betanaphthoflavone (ßNF) treatment resulted in a three-fold increase in intestinal GSH transferase. Hepatic GSH transferase activity in the same fish was not affected by dietary ßNF although hepatic monooxygenase activity, measured as ethoxyresorufin O-deethylase (EROD), was. The results of this study indicate a response of the intestinal detoxification system to environmental contaminants and supports previous studies on the importance of intestinal metabolism of foreign compounds. Further, our results indicate the trend towards elevated GSH transferase in liver of feral fish could not be attributed to a cancerous disease state in these fish but indicates chemical induction in this organ as well.

Activation of microsomal glutathione transferase activity by reactive intermediates formed during the metabolism of phenol

The activity of microsomal glutathione transferase was increased 1.7-fold in rat liver microsomes which carried out NADPH dependent metabolism of phenol. Known phenol metabolites were therefore tested for their ability to activate the microsomal glutathione transferase. The phenol metabolites benzoquinone and 1,2,4-benzenetriol both activated the glutathione transferase in microsomes 2-fold independently of added NADPH. However, NADPH was required to activate the enzyme in the presence of hydroquinone. Catechol did not activate the enzyme in microsomes. The purified enzyme was activated 6-fold and 8-fold by 5 mM benzenetriol and benzoquinone respectively. Phenol, catechol or hydroquinone had no effect on the purified enzyme. When microsomal proteins that had metabolized [14C]phenol were examined by SDS polyacrylamide gel electrophoresis and fluorography it was found that metabolites had bound covalently to a protein which comigrated with the microsomal glutathione transferase enzyme. We therefore suggest that reactive metabolites of phenol activate the enzyme by covalent modification. It is discussed whether the binding and activation has general implications in the regulation of microsomal glutathione transferase and, since some reactive metabolites might be substrates for the enzyme, their elimination through conjugation.

Evidence that covalent binding of metabolically activated phenol to microsomal proteins is caused by oxidised products of hydroquinone and catechol

The metabolic activation of [14C]phenol resulting in covalent binding to proteins has been studied in rat liver microsomes. The covalent binding was dependent on microsomal enzymes and NADPH and showed saturation kinetics for phenol with a Km-value of 0.04 mM. The metabolites hydroquinone and catechol were formed at rates which were 10 or 0.7 times that of the binding rate of metabolically activated phenol. The effects of cytochrome P-450 inhibitors and cytochrome P-450 inducers on the metabolism and binding of phenol to microsomal proteins, suggest that cytochrome P-450 isoenzyme(s) other than P-450 PB-B or P-450 beta NF-B catalyses the metabolic activation of phenol. Furthermore, reconstituted mixed-function oxidase systems containing cytochrome P-450 PB-B and P-450 beta NF-B were (on basis of cytochrome P-450 content) 6 and 11 times less active in catalysing the formation of hydroquinone than microsomes. The isolated metabolites hydroquinone and catechol bound more extensively to microsomal proteins than phenol and the binding of these was not stimulated by NADPH. The binding occurring during the metabolism of phenol could be predicted by the rates of formation of hydroquinone and catechol and the rates by which the isolated metabolites were bound to proteins.

Covalent binding of benzo[a]pyrene to cytochrome P-450 beta NF-B2 and other proteins in reconstituted mixed-function oxidase systems

A reconstituted mixed-function oxidase system, containing the major beta-naphthoflavone-induced isozyme of rat liver cytochrome P-450 bound benzo[a]pyrene covalently in the presence of NADPH. NADPH-cytochrome P-450 reductase was required for binding and a maximum rate of adduct formation was obtained at 8 units of reductase per nmol cytochrome P-450. Phosphatidylcholine inhibited this reaction. Benzo[a]pyrene was bound to the cytochrome, but not to the reductase, as shown by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Approximately 6 molecules of benzo[a]pyrene bound to each molecule cytochrome P-450 during prolonged incubations. No binding occurred when the beta-naphthoflavone-induced isozyme of cytochrome P-450 was replaced by the major isozyme induced by phenobarbital, but both cytochromes incorporated benzo[a]pyrene to approximately the same extent when they were incubated together in the presence of the reductase and NADPH. Metabolically activated benzo[a]pyrene also bound covalently to purified epoxide hydrolase, when this enzyme was added to the reconstituted mixed-function oxidase system.