Microsomal and nuclear metabolism of 3-methylcholanthrene

Application of fused-grid-based CYP-Template systems for genotoxic substances to understand the metabolisms

Understanding of metabolic processes is a key factor to evaluate biological effects of carcinogen and mutagens. Applicability of fused-grid Template* systems of CYP enzymes (Drug Metab Pharmacokinet 2019, 2020, 2021, and 2022) was tested for three phenomena. (1) Possible causal relationships between CYP-mediated metabolisms of β-naphthoflavone and 3-methylcholanthrene and the high inducibility of CYP enzymes were examined. Selective involvement of non-constitutive CYP1A1, but not constitutive CYP1A2, was suggested on the oxidative metabolisms of efficient inducers, β-naphthoflavone and 3-methylcholanthrene. These results supported the view of the causal link of their high inducibility with their inefficient metabolisms due to the lack of CYP1A1 in livers at early periods after the administration of both inducers. (2) Clear differences exist between human and rodent CYP1A1 enzymes on their catalyses with heterocyclic amines, dioxins and polyaromatic hydrocarbons (PAHs). Reciprocal comparison of simulation results with experimental data suggested the rodent specific site and distinct sitting-preferences of ligands on Template for human and rodent CYP1A1 enzymes. (3) Enhancement of metabolic activation and co-mutagenicity have been known as phenomena associated with Salmonella mutagenesis assay. Both the phenomena were examined on CYP-Templates in ways of simultaneous bi-molecule bindings of distinct ligands as trigger and pro-metabolized molecules. α-Naphthoflavone and norharman served consistently as trigger-molecules to support the oxidations of PAHs and arylamines sitting simultaneously as pro-metabolized molecules on Templates of CYP1A1, CYP1A2 and CYP3A4. These CYP-Template simulation systems with deciphering capabilities are promising tools to understand the mechanism basis of metabolic activations and to support confident judgements in safety assessments.

Enantioselective aliphatic hydroxylations of racemic 1-hydroxy-3-methylcholanthrene by rat liver microsomes

Enantiomeric pairs of 1-hydroxy-3-hydroxymethylcholanthrene (1-OH-3-OHMC), 3-methylcholanthrene (3MC) trans- and cis-1,2-diols, and 1-hydroxy-3-methylcholanthrene (1-OH-3MC) were resolved by HPLC using a covalently bonded (R)-N-(3,5-dinitrobenzoyl)phenylglycine chiral stationary phase (Pirkle type 1A) column. The absolute configuration of an enantiomeric 3MC trans-1,2-diol was established by the exciton chirality CD method following conversion to a bis-p-N,N-dimethylaminobenzoate. Incubation of an enantiomeric 1-OH-3MC with rat liver microsomes resulted in the formation of enantiomeric 3MC trans- and cis-1,2-diols; the absolute configurations of the enantiomeric 1-OH-3MC and 3MC cis-1,2-diol were established on the basis of the absolute configuration of an enantiomeric 3MC trans-1,2-diol. Absolute configurations of enantiomeric 1-OH-3-OHMC were determined by comparing their CD spectra with those of enantiomeric 1-OH-3MC. The relative amount of three aliphatic hydroxylation products formed by rat liver microsomal metabolism of racemic 1-OH-3MC was 1-OH-3-OHMC greater than 3MC cis-1,2-diol greater than 3MC trans-1,2-diol. Enzymatic hydroxylation at C2 of racemic 1-OH-3MC was enantioselective toward the 1S-enantiomer over the 1R-enantiomer (approximately 3/1); hydroxylation at the C3-methyl group was enantioselective toward the 1R-enantiomer over the 1S-enantiomer (approximately 58/42). Rat liver microsomal C2-hydroxylation of racemic 1-OH-3MC resulted in a 3MC trans-1,2-diol with a (1S,2S)/(1R,2R) ratio of 63/37 and a 3MC cis-1,2-diol with a (1S,2R)/(1R,2S) ratio of 12/88, respectively.

Polycyclic Aromatic Hydrocarbon Binding Macromolecules. Identification, Characterization and Temperature Activation of a 4.5 S Binding Nucleoprotein

A macromolecule binding3H-methyIcholanthrene (3H-MCA) and3H-benzo(a)pyrene (3H-BaP) and sedimenting in the 4-5 S region of sucrose gradient (4.5 S) was identified in rat liver cytosol. The binding was displaced by 100-fold molar excess unlabeled ligands whereas 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) was ineffective. The dissociation constant for both polycyclic aromatic hydrocarbons (PAHs) was of the order of 10−3M or lower. Both3H-MCA and3H-BaP bound to 4.5 S in a non covalent manner, since 92 % of the bound radioactivity was extractable with ethyl ether. Furthermore the binding was stronghly reduced by urea 8 M and by guanidine. HCl 4 M (99 and 70 % respectively). Thin layer chromatography of the ehtyl ether-solubilized radioactivity showed a peak comigrating with PAHs used as standards. When chromatographed on Sephadex G-200, 4.5 S was eluted as a sharp peak with an apparent molecular weight of 50-60,000 daltons. Enzyme treatment of liver cytosol showed that the 4.5 S binding sites were destroyed by micrococcal nuclease (92 % of inhibition). Papain and phosphodiesterase I and II reduced the binding to 50 %, whereas DNase I, DNase II, RNase, phospholipase A2andC and trypsin were ineffective. These data suggest that the PAHs binding macromolecule of rat liver cytosol is a protein associated with a polynucleotide. The binding of both PAHs was enhanced by increasing the incubation temperature, the maximum being reached after 20-30 min at 37 °C. After 2.5 min at 65 °C, binding sites were completely destroyed. The same temperature-induced « activation » was obtained also by prewarming the cytosol at 37 °C in the absence of ligands.

The metabolism of 3-methylcholanthrene by liver and lung microsomes: effect of enzyme inducing agents

The metabolism of 3-methylcholanthrene (3-MC) by rat liver and lung microsomes was investigated. The animals were pretreated with various inducing agents to determine the effect which microsomal induction has on metabolic profile. Metabolites were separated by high pressure liquid chromatography. Major metabolism was associated with three peaks: 2-hydroxy-3-MC, 1-hydroxy-3-MC and a peak that co-chromatographs with 3-MC 11,12-oxide and probably contains phenols. Pretreatment of animals with various inducing agents caused an increase in total metabolism as well as in specific metabolites. However, lung microsomes from phenobarbital treated animals were less efficient at metabolizing 3-MC than control microsomes. Liver and lung microsomes converted 3-MC to qualitatively similar products. No metabolites were formed by lung microsomes that were not also formed by those from liver.

Fungal oxidation of 3-methylcholanthrene: formation of proximate carcinogenic metabolites of 3-methylcholanthrene

The filamentous fungus, Cunninghamella elegans, was found to metabolize the potent carcinogen, 3-methylcholanthrene (3-MC) to 1-hydroxy-3-MC, 2-hydroxy-3-MC, 1-keto-3-MC, 2-keto-3-MC and trans-9,10-dihydrodiols of 1-hydroxy-3-MC. In addition several unidentified derivatives of 3-MC were found. The metabolites formed were separated by high pressure liquid chromatography (HPLC) and identified by comparison of retention times, absorbance, fluorescence and mass spectra with those of synthetic standards. Incubation of (+/-)-1-hydroxy-3-MC and (+/-)-2-hydroxy-3-MC with cells of C. elegans indicated that 1-hydroxy-3-MC is metabolized to form diasteromerically related trans-9,10-dihydrodiols of 1-hydroxy-3-MC. Experiments with 3-[14C]MC showed that over a 48-h period, 8.7% of the hydrocarbon was oxidized to organic solvent-soluble metabolic products. Most of the metabolites were polar products, some of which co-chromatographed with trans-9,10-dihydrodiols of 1-hydroxy-3-MC. The results show that C. elegans has the ability to oxidize 3-MC to metabolites that have been implicated as proximate carcinogenic forms of 3-MC in higher organisms.