The hydrophobic photoreagent 3-(trifluoromethyl)-3-m-([125I] iodophenyl) diazirine is a novel noncompetitive antagonist of the nicotinic acetylcholine receptor

We have shown previously that the lipophilic photoreagent 3-(trifluoromethyl)3-m-(125I]iodophenyl)-diazirine (125I]TID) photolabels all four subunits of the Torpedo nicotinic acetylcholine receptor (AChR) and that greater than 70% of this photoincorporation is inhibited by cholinergic agonists and some noncompetitive antagonists, including histrionicotoxin (HTX), but not phencyclidine (PCP; White, B.H., and Cohen, J.B. (1988) Biochemistry 27, 8741-8751). We have now examined the effects of nonradioactive TID on (a) AChR photoincorporation of 125I]TID, (b) AChR-mediated ion transport, and (c) AChR binding of several cholinergic ligands. We find that TID inhibits 125I]TID photoincorporation into the AChR to the same extent as carbamylcholine. The saturable component of 125I]TID photolabeling is half-maximal at 4 microM 125I]TID with 0.5 mol specifically incorporated per mol of AChR after 30 min photolysis with 60 microM 125I]TID. Repeated labeling of membranes at a fixed 125I]TID concentration gave results consistent with a maximal incorporation of one 125I]TID molecule per AChR. Nonradioactive TID also noncompetitively inhibits agonist-stimulated 22Na+ efflux from Torpedo vesicles with an IC50 of 1 microM. Furthermore, TID inhibits allosterically the binding of 3H]HTX, decreasing its affinity for the AChR 5-fold both in the presence and absence of agonist. In contrast, TID has little effect on 3H]PCP binding in the absence of agonist but completely inhibits it in the presence of agonist. TID enhances the cooperativity of 3H]nicotine binding. 125I]TID is thus a photoaffinity label for a novel noncompetitive antagonist binding site on the AChR that is linked allosterically to the binding sites of both agonists and other noncompetitive antagonists. The 125I]TID site is presumably located within the central pore of the AChR.