Q.41
Receptor R is over expressed in CHO cells and analysed for expression.
6 × 107 cells were incubated with its radioactive ligand
(specific activity 100 counts per picomole).
If the total counts present in the cell pellet was 1000 cpm,
the average number of receptors R per cell is
(assume complete saturation of receptors with ligand and
one ligand binds to one receptor)
Options:
(A) 104
(B) 103
(C) 105
(D) 107
Calculation of Receptor Number per Cell Using Radioligand Binding Assay
Radioligand binding assays are commonly used to estimate receptor expression levels
on cells. This numerical problem demonstrates how to calculate the average number
of receptors per cell when complete saturation of receptors is assumed.
Question Overview
- Number of CHO cells = 6 × 107
- Specific activity of ligand = 100 counts per picomole
- Total counts in cell pellet = 1000 cpm
- Assumption: One ligand binds to one receptor (complete saturation)
Step 1: Amount of Ligand Bound
Ligand bound (in picomoles) is calculated as:
Ligand (pmol) = Total counts / Specific activity
= 1000 / 100 = 10 pmol
Step 2: Convert Picomoles to Number of Molecules
1 pmol = 6.022 × 1011 molecules
Therefore,
10 pmol = 6.022 × 1012 molecules
Since one ligand binds to one receptor, this equals the total number of receptors.
Step 3: Calculate Receptors per Cell
Receptors per cell = Total receptors / Number of cells
= (6.022 × 1012) / (6 × 107)
≈ 1 × 105 receptors per cell
Correct Answer
Option (C): 105 receptors per cell
Important Exam Tips
- Complete saturation means every receptor is occupied.
- Always convert pmol to molecules using Avogadro’s number.
- Divide total bound ligand molecules by total number of cells.
Conclusion
Based on radioligand binding calculations, each CHO cell expresses approximately
105 receptors. Hence, the correct answer is
Option (C).
