Discrepancies in the specific activity of 3H-N-methylscopolamine as determined by mass spectrometry and radioreceptor assay due to racemisation.
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.
Ensing
K
K
Department of Toxicology, State University, AW Groningen, The Netherlands.
Bloemhof
D A
DA
in 't Hout
W G
WG
de Zeeuw
R A
RA
D000818:Animals; D002417:Cattle; D013058:Mass Spectrometry; D019832:N-Methylscopolamine; D011869:Radioligand Assay; D012602:Scopolamine Derivatives / Q000378:metabolism*; D013237:Stereoisomerism; D014316:Tritium
D012602:Scopolamine Derivatives; D014316:Tritium; D019832:N-Methylscopolamine