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Abstract
In radioligand binding studies, the accuracy of the specific activity of the radiolabeled ligand has a large impact on the calculated receptor concentration and on the calculated affinities of both labeled and competitive ligands.
A radioreceptor assay for anticholinergic drugs was employed to determine the specific activity of 3H-N-methylscopolamine in which the assumption was made that the labeling does not affect the interaction between the ligand and the muscarinic receptor. With this method a specific activity of 48 Ci/mmole was determined which was about 60% of the manufacturer's specification. However, the specific activity as determined by mass-spectrometry with field desorption showed 2.8 tritium-atoms per molecule NMS corresponding with an specific activity of 84.5 Ci/mmole. The only explanation for this discrepancy was a racemisation of the label, which could be proven by titration of the label with an increasing receptor concentration showing that 40% was not bound at all. This was attributed to the formation of the (+)-isomer.