You isolate a crude fraction from the alien cells containing receptors for XFF. You perform a binding assay and plot the XFF receptors occupied (y-axis) at several XFF concentrations. You obtain a Michaelis–Menten-like simple binding curve that indicates a 1 XFF:1 receptor interaction and approaches a maximal level of binding you define as Bmax. From the curve you extrapolate the Kd for XFF binding as the concentration at which 50% of the receptors are bound. What value reflects the affinity of the receptor for the ligand?
You have isolated a primary signal factor from the blood of a space alien, which you name XFF (for X-files factor). You also have isolated various types of alien cells which appear to interact with XFF. Based on preliminary data, you think that XFF could be similar to one of three compounds normally found in human blood: epidermal growth factor (EGF), epinephrine, or estrogen.
A) Kd
B) Bmax
C) Kd − Bmax
D) 1/Bmax
E) The affinity cannot be determined from the information available.