Saturation binding isotherm and Scatchard plot analysis of ¹²⁵I-Fg binding to D3-activated transfected CHO cells. Transfected CHO cells expressing (A) Pen^a or (B) Pen^b forms of GPIIb-IIIa were treated with D3 at a concentration of 200 μg/mL and then incubated with increasing concentrations of ¹²⁵I-Fg at room temperature for 45 minutes. Data are plotted as molecules of ¹²⁵I-Fg specifically bound versus fibrinogen concentration. Scatchard plot analyses of the specific binding data are shown in the insets. Data are replotted according to the Scatchard method using the GraphPad linear regression program. kd Values for both Pen^a and Pen^b forms of GPIIb-IIIa are 400 and 455 nmol/L, respectively. (C) Summary of kd values from five different experiments. kd Values derived from the Scatchard plots are summarized. Both forms of GPIIb-IIIa bound to Fg with similar affinity, 428 nmol/L for Pen^a- and 457 nmol/L for Pen^b-transfected CHO cells, respectively (P = .78). Note that there is no significant difference in affinity for Fg between the Arg₁₄₃ (Pen^a) and Gln₁₄₃(Pen^b) forms of the GPIIb-IIIa complex.