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Binding characteristics of 4-S proteins from rat and mouse liver. High affinity of ellipticines
1Laboratoire de Pharmacologie et de Toxicologie Fondamentales, Centre National de la Recherche Scientifique, Toulouse, France.
Abstract:
The binding characteristics of 4-S components (carcinogen-binding protein) from livers of Sprague-Dawley rats, C57BL/6 and DBA/2 mice have been examined before and after separation from other binding components presents in the cytosol. Competitive potency of 3-methylcholanthrene, benzo[alpha]pyrene, beta-naphthoflavone and 20 ellipticines, a series of compounds differently substituted on the dimethyl-pyrido-carbazole nucleus and deprived of carcinogenic activity, has been determined with [3H]3-methylcholanthrene and/or [3H]benzo[a]pyrene as radioligands. The inducing ability of the same compounds for aryl hydrocarbon hydroxylase and for ethoxyresorufin-O-deethylase has been compared to their affinity for the 4-S protein and the Ah receptor respectively. The main results of this study are as follows. 1. The intrinsic binding characteristics of 4-S proteins were dependent on both the nature of the radioligand used and the presence or absence of other cytosolic binding components. 2. The heterocyclic ellipticines were revealed as strong ligands for the carcinogen-binding protein (stronger than benzo[alpha]pyrene for five derivatives substituted in the A ring of ellipticine), with IC50 values ranging from 0.047 microM (8-hydroxyellipticine) to 5.8 microM (N2-ethyl-9-hydroxyellipticinium). 3. When the affinity of ellipticines was plotted versus their inducing ability of aryl hydrocarbon hydroxylase and ethoxyresorufin-O-deethylase, it appears that a good correlation exists for the Ah receptor but not for the 4-S protein. It is concluded that these data, as well as the lack of enzymatic induction after benzo[alpha]pyrene treatment of DBA/2 mice, which display a high level of 4-S protein, do not support the implication of this binding component in the positive control of cytochrome P-450 induction.
Insights
This study investigated the carcinogen-binding protein (4-S protein) in rat and mouse livers. Ellipticines showed strong binding to the 4-S protein, but this binding did not correlate with enzyme induction, suggesting it
Area of Science:
- Biochemistry
- Pharmacology
- Toxicology
Background:
- The 4-S protein, a carcinogen-binding component in liver cytosol, plays a role in xenobiotic metabolism.
- Understanding its binding characteristics and relationship with enzyme induction is crucial for deciphering toxicological pathways.
Purpose of the Study:
- To characterize the binding properties of the 4-S protein from rat and mouse livers.
- To compare the binding affinity of various compounds, including ellipticines, to the 4-S protein and the aryl hydrocarbon receptor (Ah receptor).
- To investigate the correlation between compound affinity for the 4-S protein/Ah receptor and their ability to induce specific enzymes.
Main Methods:
- Competitive binding assays using radioligands ([3H]3-methylcholanthrene and [3H]benzo[a]pyrene).
- Determination of IC50 values for various ligands, including ellipticines.
- Comparison of enzyme-inducing abilities (aryl hydrocarbon hydroxylase, ethoxyresorufin-O-deethylase) with binding affinities.
Main Results:
- The binding characteristics of 4-S proteins were influenced by the radioligand and the presence of other cytosolic components.
- Ellipticines demonstrated potent ligand activity for the 4-S protein, with some exhibiting stronger binding than benzo[a]pyrene.
- A correlation between ligand affinity and enzyme induction was observed for the Ah receptor, but not for the 4-S protein.
Conclusions:
- The 4-S protein binds heterocyclic ellipticines strongly, but this interaction does not appear to mediate cytochrome P-450 induction.
- Data suggest that the 4-S protein is not directly involved in the positive control of cytochrome P-450 induction, despite its high levels in certain mouse strains.