PT  - JOURNAL ARTICLE
AU  - Vimesh Avlani
AU  - Lauren T. May
AU  - Patrick M. Sexton
AU  - Arthur Christopoulos
TI  - Application of a Kinetic Model to the Apparently Complex Behavior of Negative and Positive Allosteric Modulators of Muscarinic Acetylcholine Receptors
AID  - 10.1124/jpet.103.059840
DP  - 2004 Mar 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 1062--1072
VI  - 308
IP  - 3
4099  - http://jpet.aspetjournals.org/content/308/3/1062.short
4100  - http://jpet.aspetjournals.org/content/308/3/1062.full
SO  - J Pharmacol Exp Ther2004 Mar 01; 308
AB  - The binding of allosteric modulators to G protein-coupled receptors (GPCRs) is often described by an equilibrium allosteric ternary complex model (ATCM). This study evaluated the effects of three modulators on the binding of [3H]N-methylscopolamine ([3H]NMS) to the human M2 muscarinic acetylcholine receptor (mAChR). The binding of each modulator was more complex than predicted by the ATCM; the inhibitors heptane-1,7-bis-(dimethyl-3-phthalimidopropyl)-ammonium bromide and gallamine yielded biphasic curves that were described empirically by a two-site binding model, whereas the enhancer alcuronium yielded a bell-shaped curve. Radioligand dissociation assays revealed that the modulators retarded [3H]NMS kinetics such that the system never attained equilibrium. Subsequent application of a kinetic ATCM accommodated and quantified all experimental observations. Our findings confirm and extend previous studies on the use of a kinetic ATCM for mAChR allosteric enhancers, but also highlight how complex curves displayed by allosteric inhibitors can be misinterpreted in terms of multisite orthosteric binding. It is possible that similar behavior of other allosteric modulators at GPCRs may reflect nonequilibrium binding artifacts rather than deviation from an ATCM. The American Society for Pharmacology and Experimental Therapeutics