Absence of positive co-operativity in the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to its cytosolic receptor protein

The role of positive co-operativity in stabilizing the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the rat hepatic cytosolic TCDD receptor protein (Ah receptor) was investigated. The binding mechanism of TCDD was determined by kinetic means through equilibrium and saturation binding studies, and Scatchard and Hill plot analysis. In all studies, the slope of the Hill plot was close to 1.0, indicating the absence of positive co-operativity. Interpretation of the Scatchard plot was however complicated by the fact that both linear and nonlinear plots were experimentally obtained. The nonlinearity was shown to be an experimental artifact and a consequence not of co-operativity, but of high levels of nonspecific binding. The high level of nonspecific binding could be attributed to: (1) lipophilicity of the TCDD ligand, and (2) inefficient competition of receptor-bound [3H]TCDD. When nonspecific binding was minimized, the Scatchard slope was linear and in agreement with the Hill coefficient, thus indicating the lack of positive co-operativity in the binding of TCDD to the Ah receptor.