Competition:Analysis of Data:Plotting Data

Plotting Competition Studies on Semilog Graph

This page explains the different methods that can be used for plotting the data from competition experiments. It is helpful to go through this page even if you are not doing competition studies.

1. How are the data from a competition experiment plotted?
2. Why do you need to use the log of the concentration of the unlabeled drug in a competition experiment?
3. The bottom of the curve is not zero in the graphs shown here. Why isn't the bottom zero?

Return to # 2 under Overview of Data

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X-axis is the log of the concentration of the unlabeled drug
Y-axis is either
- the amount of radioligand bound, or
- the % of Control where the control is the cpm bound in the absence of unlabeled drug.

Data points form a sigmoid curve and can be analyzed using the equation for a sigmoid curve as will be shown in the Analysis of Sample Data.

The concentration of radioligand used spans three to five log units. If a linear scale was used, the plot would be quite wide. Most of the data points would be close to the Y-axis.

Even if you remove the last two data points from the previous graph, most of the data points will still be close to the Y-axis.

Compare the plot with the Log [Drug] versus the linear plot with [Drug]. Notice that with the sigmoid plot

Data points are evenly spread out along the X-axis
The data take on the shape of a sigmoid curve.
The shape of the curve is linear between 80 and 20% of bound radioligand.

There is always some non-saturable, nonspecific binding which is not displaced by the unlabeled ligand. Since these sites can't be saturated, the unlabeled ligand doesn't compete with the labeled ligand. You may recall that this is the same type of experiment used to determine the amount of unlabeled ligand to use to block specific sites.