Ligand-induced changes in membrane-bound acetylcholine receptor observed by ethidium fluorescence. 3. Stopped-flow studies with histrionicotoxin.
Rapid kinetic studies of histrionicotoxin interactions with membrane-bound acetylcholine-receptor showed a conformational change in the receptor-histironicotoxin complex as reflected by a decrease in fluorescence intensity of the extrinsic probe ethidium. The simplest kinetic mechanism consistent with the observed data is one in which a rapid preequiliibrium exists between receptor and toxin (K = 3.33 micrometers), followed by a slow conformational change (k1 congruent to 2 X 10(-2) s-1 and k-1 congruent to 1.5 X 10(-3) s-1). The overall equilibrium constant (Kov) determined from a fit of the amplitude dependence on toxin concentration had a value of 0.25 micrometer. The data preclude kinetic mechanisms where histrionicotoxin acts as an effector, shifting equilibria between preexisting, discrete, and slowly interconverting receptor forms.
D000109:Acetylcholine; D000664:Amphibian Venoms / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D004557:Electric Organ / Q000378:metabolism*; D004996:Ethidium; D005399:Fishes; D007700:Kinetics; D008024:Ligands; D008433:Mathematics; D010276:Parasympatholytics / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D013997:Time Factors
D000664:Amphibian Venoms; D008024:Ligands; D010276:Parasympatholytics; D011950:Receptors, Cholinergic; D004996:Ethidium; D000109:Acetylcholine