Cholinergic elements in a human choriocarcinoma cell line

Components of the cholinergic system have been identified in the JEG choriocarcinoma cell line. [³H]Quinuclidinyl benzilate (QNB) was used to identify high affinity muscarinic binding sites in a whole cell preparation. Specific binding was saturable with respect to QNB concentration and revealed a binding site density of 27 fmoles/mg protein. The bimolecular rates of association, 2.24 × 10⁷ M^-1 min^-1, and dissociation, 4.2 × 10^-3min^-1, revealed a dissociation constant (K_d) of 180 pM which agreed closely with that derived from saturation isotherms, 245 pM. Muscarinic antagonists and agonists were able to compete effectively for these binding sites, whereas non-muscarinic compounds were not. Cholinesterase activity was also demonstrated with substrate preference consistent with that of acetyl-cholinesterase (acetylcholine > acetyl-β-methylcholine > butyrylcholine) hydrolyzing 2.42 ± 0.19 × 10^-3 μmoles acetylcholine·min^-1 ·(mg protein)^-1. No choline acetyltransferase activity was detected in these cells, however.