Figure 12
Binding to a low affinity second binding site with hypothetically large R2T and K d2 or K i2 values mimic NSB in homologous and heterologous competition data. A. The one site modelfree (α = 0.2) and two sites modelfree (α = 0) generated homologous competition data with Kd2 = R2T/0.2. With increasing R2T and Kd2, the two sites modelfree produces increasingly long plateaus of total bound [3H]EB, similar to NSB. Low-affinity specific binding is distinguished from NSB only when cold EB concentration exceeds Kd2. B. Heterologous competition of [3H]EB with a nicotine-like competitor at a low affinity site can mimic NSB. The one site modelfree with α = 0.2 generated heterologous competition data with Ki1 = 0.84 nM for the competitor (value for nicotine [18]). With these values and R2T = 2.4 nM and α = 0, the two sites modeltotal fits these data well up to a competitor concentration of Ki2. Low-affinity specific binding is distinguished from NSB only when the competitor concentration exceeds Ki2. C. With a superhigh affinity competitor, increasing R2T and varying Ki2 with heterologous competition of [3H]EB at a low affinity site can mimic NSB. The one site modelfree with α = 0.2 generated heterologous competition data with Ki1 = 1.3 × 10-4 nM. With α = 0, the two sites modeltotal fits well to these data up to a competitor concentration of Ki2. Low-affinity specific binding is distinguished from NSB only when competitor concentration exceeds Ki2.