Binding characteristics of 4‐S proteins from rat and mouse liver High affinity of ellipticines

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[a]pyrene, .beta.-napthoflavone and 20 ellipticines, a series of compounds differentially substituted on the dimethyl-pyrido-carbazole nucleus and deprived of carcinogen 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 (stronger than benzo[a]pyrene for five derivatives in the A ring of ellipticine), with IC50 values ranging from 0.047 .mu.M (8-hydroxyellipticine) to 58.8 .mu.M (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[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.