Usually receptor expression unit is pmol/mg of protein. What does it mean? I found some article publish that their receptor expression level is 1.5-2 pmol/mg protein. What does it mean? When we count the receptor number by radioligand method we found unit CPM. what is the relationship between CPM & pmol/mg?
picomole: A unit of substance equal to one trillionth (10-12) of a mole.
CPM: counts per minute
In Radiometric Assays, after preparation a very low concentration solution of substrate (millimolar concentration of substrate) the radioactive compound is used. It is possible to calculate the amount of it according the specific radioactivity indicated by the producer. Then, the specified amount of nano-moles of the mixture is added in the counter to obtain the CPM (count per minutes). For obtaining specific radioactivity in terms of (CPM/ nmol), the CPM will be divided by the number of nano-moles used in the solution.
According the protocol, and measuring the radioactivity at the beginning and at the end of the reaction, it is possible to calculate the amount of radioactive that was incorporated in the product. Dividing the CPM obtained from the counter by the specific activity calculated as described above, the nano-mole of the product produced during the reaction period can be calculated. Then, the enzyme activity can be obtained by dividing the amount of product (nano-mole) by reaction time and volume of reaction solution (nmol/ (ml min) or U/min).
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