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The Ah receptor, a soluble protein implicated in the mechanism of action of the toxic halogenated aryl hydrocarbons has been examined in rodent livers. Due to the difficulty of making reliable quantitative determinations on binding parameters for hydrophobic compounds in cytosols that contain several components, Ah receptors from livers of Sprague-Dawley rats and C57BL/6 mice have been separated, in a preparative manner, using sucrose density gradient centrifugation in a vertical rotor. The rationale of this approach is supported by the results obtained and the major conclusions are as follows.1. The intrinsic binding characteristics of Ah receptors were dependent on the presence or absence of other cytosolic binding components (light-density component and 4-S carcinogen-binding protein).2. In contrast with many previous unsuccessful attempts, the separation of the C57BL/6 Ah receptor allowed the unambiguous detection of a 9-S binding peak with [3H]benzo[a]pyene as a radioligand.3. The intrinsic binding characteristics of the separated Ah receptors of Sprague-Dawley rats and C57BL/6 mice were similar if not identical.4. A good correlation exists between the competitive potency (IC,o) of chemicals and their ability to induce aryl hydrocarbon hydroxylase activity, except for 7-hydroxyellipticine which binds to the Ah receptors of rat and mouse liver (IC50 z 5 -10 pM) without inducing aryl hydrocarbon hydroxylase.5. When coadministered with various inducers, 7-hydroxyellipticine antagonizes (from about 20% to 65%) the inducing ability of chemicals displaying similar (ellipticines) or weaker (chlorpromazine, phenothiazine) binding affinities for the Ah receptor.The cytosol from liver or other organs of rodents contains several components that bind halogenated aromatic compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) [l -31 and polycyclic aromatic hydrocarbons such as 3-methylcholanthrene and benzo[a]pyrene [4 -61. For some ten years many laboratories have focused attention on the Ah receptor (aromatic hydrocarbon), one function of which is the regulation of induction of aryl hydrocarbon hydroxylase and other drug-metabolizing enzymes associated with the Ah gene locus [7,8]. Most investigations concern the physico-chemical properties of this protein [2, 9 -111, its mechanism of action [7] (which is probably similar to those of receptors for steroid hormones [12]), the possible existence of an endogenous ligand and the role of the receptor in the toxicity of dioxin, dibenzofuran and biphenyl derivatives [13,14]. More recently, several laboratories have investigated the properties and function(s) of another soluble protein, the 4-S carcinogen-binding

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[a]pyrene, P-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-0-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[a]pyrene for five derivatives substit'uted in the A ring of ellipticine), with IC50 values ranging from 0.047 pM (8-hydroxyellipticine) to 5.8 pM (Nz-ethyl-9-hydroxyellipticinium).3. When the affinity of ellipticines was plotted versus their inducing ability of aryl hydrocarbon hydroxylase and ethoxyresorufin-0-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[a]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.

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