Competition:Analysis of Data:Sample Experiment:Ki Determination
Conversion of the IC50 value to a Ki value
On this page you will convert the IC50 value from the sample experiment to a Ki value.
As explained under Analysis of Data (Overview) the Ki value can be obtained from the IC50 value using the Cheng-Prusoff equation (Biochem. Pharmacol. 22:3099-3108, 1973):
Where D is the concentration of the radioligand and Kd is the Kd for the radioligand binding to the receptor under the same experimental conditions used in the competition experiment.
In order to use this equation one needs to know:
- The concentration of [3H]RX821002 used in the competition assay. This concentration is calculated from the average cpm detected with the 20 µl aliquots which were added to scintillation vials during the set up of the sample experiment. These counts are usually referred to as the Total Added counts.
- The Kd for [3H]RX821002 for the Alpha-2B adrenergic receptor. The Kd for [3H]RX821002 binding to the Alpha-2B adrenergic receptor is 890 pM. This was previously determined using a saturation experiment using the same experimental conditions as the sample competition experiment.
Calculation of the concentration of [3H]RX821002 present in each assay tube
The table below shows the Total Added values for [3H]RX821002.
- Calculate the average Total Added and put the results in column four in the table below.
- Convert the average cpm to pM units. The equation used to convert cpm to pM units is
given below. Use this equation to determine a factor for converting cpm to pM units given
a counting efficiency of 0.36 and a specific activity of [3H]RX821002 of 59
Ci/mmol using the equation below.
Round your answers to the nearest whole number
Unlabeled ligands can bind to more than one site. Continue
to learn about two-site competition curves. 
