INTERACTIONS OF INDOLO[3,2-B]CARBAZOLES AND RELATED POLYCYCLIC AROMATIC-HYDROCARBONS WITH SPECIFIC BINDING-SITES FOR 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN IN RAT-LIVER

In the present study we have investigated the capacity of various comp ounds sterically related to indolo[3,2-b]carbazole to inhibit specific 2,3,7,8-tetrachloro[1,6-H-3]dibenzo-p-dioxin binding in rat liver cyt osol, as analyzed by electrofocusing in polyacrylamide gels. When the two nitrogen atoms of indolo[3,2-b]carbazole (IC50 = 3.6 nm) were repl aced with sulfur atoms, the affinity for the specific binding sites (I C50 = 3.3 nm) was similar to that of the parent compound, whereas the affinity decreased when the two nitrogen atoms were replaced with oxyg en atoms (IC50 = 29 nm). Substitution with methyl groups at positions 5 and 11 (on the nitrogens) of indolo[3,2-b]carbazole resulted in incr eased affinity (IC50 = 1.2 nm), compared with that of the parent compo und, whereas dimethylation at the 4,10- or 2,8-positions decreased the affinity (IC50 = 19 nm and IC50 > 150 nm, respectively). Substitution at positions 5 and 11 of indolo[3,2-b]carbazole with substituents lar ger than methyl, as in 5,11-diethylindolo[3,2-b]carbazole (IC50 = 8.9 nm), diacetylindolo[3,2-b]carbazole (IC50 = 11.2 nm), 5,11-dibutylindo lo[3,2-b]carbazole (IC50 > 150 nm), and di(NS,NS-dimethylaminoethyl)in dolo[3,2-b]carbazole (IC50 > 1500 nm), also decreased the affinity. In troduction of oxygen in, or hydroxylation of, the middle ring of indol o(3,2-b]carbazole, giving indolo[3,2-b]carbazole-6,12-quinone (IC50 > 150 nm) or 6,12-dihydroxyindolo[3,2-b]carbazole (IC50 > 1500 nm), resp ectively, also lowered the affinity. We calculated the Gibbs free ener gy of solvation of the analogue isoquino[3,4-b]phenanthridine (IC50 = 137 nm), relative to that of dibenz[a,h]anthracene (IC50) = 2.5 nM), i n water to be -6 kcal/mol by free energy perturbation, which indicates that the most important explanation for the observed difference in bi nding affinity is the smaller difference in relative free energy of bi nding at the binding sites, compared with the Gibbs free energy of sol vation of the two compounds.