Formation and disposition of N-hydroxylated metabolites of aniline and nitrobenzene by isolated rat hepatocytes

Development of a New Threshold of Toxicological Concern Database of Non-cancer Toxicity Endpoints for Industrial Chemicals

In cases where chemical-specific toxicity data are absent or limited, the threshold of toxicological concern (TTC) offers an alternative to assess human exposure below which “there would be no appreciable risk to human health.” The application of TTC to non-cancer systemic endpoints has been pursued for decades using a chemical classification and Point of Departure (POD). This study presents a new POD dataset of oral subacute/subchronic toxicity studies in rats for 656 industrial chemicals retrieved from the Hazard Evaluation Support System (HESS) Integrated Platform, which contains hundreds of reliable repeated-dose toxicity test data of industrial chemicals under the Chemical Substances of Control Law in Japan. The HESS TTC dataset was found to have less duplication with substances in other reported TTC datasets. Each chemical was classified into a Cramer Class, with 68, 3, and 29% of these 656 chemicals distributed in Classes III, II, and I, respectively. For each Cramer Class, a provisional Tolerable Daily Intake (TDI) was derived from the 5th percentile of the lognormal distribution of PODs. The TDIs were 1.9 and 30 μg/kg bw/day for Classes III and I, respectively. The TDI for Cramer Class II could not be determined due to insufficient sample size. This work complements previous studies of the TTC approach and increases the confidence of the thresholds for non-cancer endpoints by including unique chemical structures. This new TTC dataset is publicly available and can be merged with existing databases to improve the TTC approach.

Structural features of cytochromes P450 and ligands that affect drug metabolism as revealed by X-ray crystallography and NMR

Cytochromes P450 (P450s) play a major role in the clearance of drugs, toxins, and environmental pollutants. Additionally, metabolism by P450s can result in toxic or carcinogenic products. The metabolism of pharmaceuticals by P450s is a major concern during the design of new drug candidates. Determining the interactions between P450s and compounds of very diverse structures is complicated by the variability in P450-ligand interactions. Understanding the protein structural elements and the chemical attributes of ligands that dictate their orientation in the P450 active site will aid in the development of effective and safe therapeutic agents. The goal of this review is to describe P450-ligand interactions from two perspectives. The first is the various structural elements that microsomal P450s have at their disposal to assume the different conformations observed in X-ray crystal structures. The second is P450-ligand dynamics analyzed by NMR relaxation studies.

Binding of nitrobenzene to hepatic DNA and hemoglobin at low doses in mice

Nitrobenzene (NB) is a widely used industrial chemical, and is considered a hazardous air pollutant. Evidence has recently showed that nitrobenzene is an animal carcinogen. We investigated the binding of 14C-NB to hepatic DNA and Hb in mice at low doses using an ultrasensitive method of accelerator mass spectrometry (AMS). In a dose-response profile, NB-DNA and NB-Hb adduct levels increased with increasing administered doses from 0.1 microg/kg b.w. to 10 mg/kg b.w. with a good linearity in a log/log presentation. At 2 h after NB administration, NB-DNA adduct levels were about twofold greater than that of NB-Hb at all doses. In the time course study NB-DNA adduct levels reduced rapidly through an exponential decay profile, whereas NB-Hb adducts showed a different decay mode, declining rather slowly to low levels. Our findings on the genotoxicity of NB do furnish a significant evidence in support of the probable carcinogenic property of NB previously reported.

Distinct mechanisms of oxidative DNA damage by two metabolites of carcinogenic o-toluidine

Mechanisms of DNA damage by metabolites of carcinogenic o-toluidine in the presence of metals were investigated by the DNA sequencing technique using (32)P-labeled human DNA fragments. 4-Amino-3-methylphenol, a major metabolite, caused DNA damage in the presence of Cu(II). Predominant cleavage sites were thymine and cytosine residues. o-Nitrosotoluene, a minor metabolite, did not induce DNA damage even in the presence of Cu(II), but addition of NADH induced DNA damage very efficiently. The DNA cleavage pattern was similar to that in the case of 4-amino-3-methylphenol. Bathocuproine and catalase inhibited DNA damage by these o-toluidine metabolites, indicating the participation of Cu(I) and H(2)O(2) in the DNA damage. Typical free hydroxyl radical scavengers showed no inhibitory effects on the DNA damage. o-Toluidine metabolites increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine in calf thymus DNA in the presence of Cu(II). UV-visible and ESR spectroscopic studies have demonstrated that 4-amino-3-methylphenol is autoxidized to form the aminomethylphenoxyl radical and o-nitrosotoluene is reduced by NADH to the o-toluolhydronitroxide radical in the presence and absence of Cu(II). Consequently, it is considered that these radicals react with O(2) to form O(-)(2) and subsequently H(2)O(2), and that the reactive species generated by the reaction of H(2)O(2) with Cu(I) participate in the DNA damage. Metal-mediated DNA damage by o-toluidine metabolites through H(2)O(2) seems to be relevant for the expression of the carcinogenicity of o-toluidine.

Oxidative DNA damage by a metabolite of carcinogenic and reproductive toxic nitrobenzene in the presence of NADH and Cu(II)

The mechanism of DNA damage induced by metabolites of nitrobenzene was investigated in relation to the carcinogenicity and reproductive toxicity of nitrobenzene. Nitrosobenzene, a nitrobenzene metabolite, induced NADH plus Cu(II)-mediated DNA cleavage frequently at thymine and cytosine residues. Catalase and bathocuproine inhibited the DNA damage, suggesting the involvement of H2O2 and Cu(I). Typical free hydroxyl radical scavengers showed no inhibitory effects on DNA damage. Nitrosobenzene caused the formation of 8-oxo-7, 8-dihydro-2'-deoxyguanosine in calf thymus DNA in the presence of NADH and Cu(II). ESR spectroscopic study has confirmed that nitrosobenzene is reduced by NADH to the phenylhydronitroxide radical even in the absence of Cu(II). These results suggest that nitrosobenzene can be reduced non-enzymatically by NADH, and the redox cycle reaction resulted in oxidative DNA damage due to the copper-oxygen complex, derived from the reaction of Cu(I) with H2O2.

Biotransformation of monochloroanilines in guppy, Poecilia reticulata

1. Guppies (Poecilia reticulata) were exposed for 96 h to 4-chloroaniline (4CA), 3-chloroaniline (3CA) and 2-chloroaniline (2CA) in a static exposure system. 4-Chloroacetanilide (N4CA), 3-chloroacetanilide (N3CA), and 2-chloroacetanilide (N2CA) were identified in test water as the respective biotransformation products. 2. Dependence of biotransformation on aqueous concentration was investigated with 4CA. The measured biotransformation product in water (N4CA) increased with concentration increase of parent compound, 4CA, and with exposure time. 3. Quantitative differences in net acetylation of the three monochlorinated anilines were examined. The amount of acetanilide found in water increased in the following order: N2CA < N3CA < N4CA. Mass balance calculations indicated the highest loss of aniline occurred for 2CA. This compound, which is highest in toxicity, had the smallest amount of acetylated product formed. 4. This study proves, for the first time, that the acetylation reaction in fish is reversible. Quantitative differences in the net deacetylation of monochloroacetanilides were observed. The amount of deacetylated product found in water increased in the order 4CA < 3CA < 2CA.

Hepatic microsomal N-hydroxylation of aniline and 4-chloroaniline by rainbow trout (Onchorhyncus mykiss)

1. N-Hydroxylation of aniline and 4-chloroaniline was quantified in rainbow trout microsomal preparations using h.p.l.c.-liquid scintillation methods. Radioactive phenylhydroxylamine and 4-chlorophenylhydroxylamine metabolites were identified by co-elution with non-labelled standards. The method provided resolution of metabolite standards, and quantification of both N-hydroxylated metabolites was achieved without derivatization. 2. The maximum velocities at 25 degrees C were 33.8 +/- 1.40 and 22.0 +/- 0.98 pmol/min per mg for aniline and 4-chloroaniline N-hydroxylation, respectively. The Km values were 1.0 +/- 0.11 and 0.8 +/- 0.11 mM for aniline and 4-chloroaniline N-hydroxylation, respectively. These activities were not induced by treatment of the trout with Aroclor 1254 under the conditions of this study. 3. When incubations were performed at 11 degrees C, the physiological temperature of rainbow trout in this study, the Vmax for 4-chloroaniline N-hydroxylation decreased from 22.0 to 6.4 pmol/min per mg and the Km decreased from 0.8 to 0.5 mM. 4. The pH optimum for 4-chloroaniline N-hydroxylation was 8.0 while the pH optimum for aniline N-hydroxylation ranged from 7.4 to 8.0, suggesting the possible contribution of different isoenzymes. 5. The demonstration of aniline and 4-chloroaniline N-hydroxylation by rainbow trout microsomes provides further insight into the high acute:subchronic toxicity ratios observed in fish exposed to these compounds.

Formation and disposition of nitrosochloramphenicol in rat liver

It has been suggested that in the chloramphenicol-induced aplastic anemia nitrosochloramphenicol may be involved as a toxic intermediate. We found that aminochloramphenicol, which reportedly is formed from chloramphenicol by intestinal bacteria, is N-oxygenated by liver microsomes of untreated rats with apparent Km = 0.4 mM and Vmax = 0.28 nmole/min/mg protein. These values are in close agreement with those reported for aniline N-oxygenation. Reductive reactions, however, eliminate the N-oxygenation products at markedly higher rates. As judged from hemoglobin-free single-pass liver perfusion experiments, N-hydroxy-chloramphenicol is reduced at rates faster than 300 nmole/min/g liver wet, and nitrosochloramphenicol is eliminated at rates faster than 1.5 mumole/min/g liver. At least two NADPH- and two NADH-dependent cytosolic enzymes are responsible for nitrosochloramphenicol reduction. Determination of the kinetic parameters of these enzymes by stop-flow analysis revealed the contribution of enzymes, one of it being alcohol dehydrogenase, with Michaelis constants in the micromolar range. Despite this high reducing capacity, about 10% of nitrosochloramphenicol reacted with GSH under formation of glutathionesulfinamidochloramphenicol and GSSG released from the liver into bile and venous effluent. At high nitrosochloramphenicol load these reactions led to glutathione depletion of the liver, caused membrane damage, and impaired bile production. At low nitrosochloramphenicol load, i.e. below 0.5 mumole/min/g, no relevant nitrosochloramphenicol passed the liver. These data together with the previously reported reactions of nitrosochloramphenicol within human blood suggest that nitrosochloramphenicol, if formed at all in the intestine or liver, is rather unlikely to be transferred to the critical target.

Kinetics of the formation and secretion of the aniline metabolite 4-aminophenol and its conjugates by isolated rat hepatocytes

The rates of secretion of 4-aminophenol and its sulphate and glucuronide conjugate were determined in cultures of rat hepatocytes with aniline and 4-aminophenol as substrates. When 4-aminophenol (300 microM) was used as substrate, 4-aminophenol disappeared from the medium within 30 min. Secretion of conjugates continued for more than 60 min, when 70% of the 4-aminophenol had been secreted as conjugates. At a low concn. of 4-aminophenol, the sulphate ester was the main metabolite, while secretion of the glucuronide showed a more than proportional rise ('lag phase') with increasing substrate concn. At higher concn. (greater than 300 microM) about equal amounts of both conjugates were formed. Without inorganic sulphate, sulphation of 4-aminophenol was greatly diminished and the lag phase in glucuronide secretion was not found. With 1 mM aniline as substrate up to 300 microM of conjugated 4-aminophenol was secreted with a linear time-dependence for at least two hours. With aniline as substrate the sulphate ester was the most important conjugate and lag phases in the secretion of both conjugates were minimal. Phenobarbitone pretreatment in vivo stimulated the secretion of conjugated products after incubation with aniline. No dramatic changes in the profile of the lag phases were seen. The differences in the conjugation profiles of both substrates can be explained by taking into consideration the differences in the expected intracellular concentrations.

The effect of erythrocytes and hemoglobin on sister chromatid exchange induction in cultured human lymphocytes exposed to aniline HCI

Erythrocytes [red blood cells (RBCs)] possess aniline hydroxylase activity. When aniline interacts with ferrohemoglobin in the presence of molecular oxygen, oxidation of nitrogen and ring carbons occurs. Thus, apart from the liver, RBCs may represent an important site of aniline metabolism. Because 2 metabolites of aniline, o-aminophenol and phenylhydroxylamine, can induce sister chromatid exchanges (SCEs), we examined the ability of RBCs and hemoglobin to activate aniline to genotoxic intermediates as evidenced by SCE induction in human lymphocytes. Aniline HCI (0.05-1.0 mM) induced significant concentration-related increases in the SCE frequency only in the whole blood cultures. Similarly, inhibition of cell cycle kinetics by aniline was observed only in the whole blood cultures, as shown by a concentration-dependent decrease in the percentage of third- (and later) division metaphases. Mitotic indices were not affected significantly at any concentration of aniline or hemoglobin. Hemoglobin (500 or 1,000 micrograms/ml) alone induced significant concentration-related increases in SCEs in the mononuclear leukocyte cultures. Therefore, human mononuclear leukocytes do not activate aniline to genotoxic intermediates capable of inducing SCEs during a 48-hr exposure. However, the inclusion of RBCs and granulocytes provides an activation system as demonstrated by a small, but statistically significant increase in the SCE frequency in the whole blood cultures. The weak genotoxicity of hemoglobin may be related to production of oxygen radicals during autoxidation to methemoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)