Experimental Forms of Data from Saturation Radioligand Assays

2c Experimental Forms of Data from Saturation Radioligand Assays. 

As we did with fractional occupancy, we can predict the data that would result from a toxicant-receptor interaction that was modeled by Eq. (19.14). Let s see what theoretical data would look like if we assume (for the sake of easy calculations) that the Bmax = 100 and the KD = 1 nM. (This is a similar exercise to what we did with fractional occupancy.) Picking arbitrary [F] concentrations, Table 19.3 shows the derived values [try this yourself using Eq. (19.14)]. 

One of the reasons, however, that Scatchard plots still are used (or at least often expected for publication) is that they permit quick visual estimates of the 5max (i.e., the Fmax is the X-intercept of the extrapolated line) and of relative toxicant affinities (reflected as the negative reciprocal of the slope). The steeper the slope, the smaller the Kd, and the greater the affinity of the toxicant for the receptor. 

TABLE 19.3. Derived Values for Theoretical Saturation Plot 

A potential problem associated with Scatchard analysis occurs when the range of radiolabeled toxicant concentrations used is too narrow, such that the highest concentration does not equal or exceed the KD (that concentration of toxicant which results in the occupation of one half of the total binding sites). Although not apparent from a Scatchard plot of such data, estimates of 5max under such conditions are prone to significant error. 

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