Pyrenesulfonyl azide: a covalent probe permitting in vitro desensitization of labeled acetylcholine-rich membrane fragments from Torpedo californica.

Acetylcholine receptor-rich membrane fragments from $\text{Torpedo}\text{californica}$ electroplax after covalent labelling at the protein-lipid boundary by nitrenes generated $\text{in}\text{situ}$ from pyrenesulfonyl azide can bind ${}^{125}\text{I]-α-bungarotoxin}$. The covalent attachment of 6–8 molecules of the fluorescent probe/receptor molecule also does not perturb the marked effect on the rate of α-bungarotoxin binding to electroplax membranes exerted by their preincubation with carbamylcholine. This phenomenon, which is analogous to pharmacological desensitization of receptors in synaptic junctions, is fully reversible upon removal of carbamylcholine (Quast, V., Schmerlik, M., Lee, T., Witzemann, V., Blanchard, V. and Raftery, M.A. (1978) Biochemistry $\text{17}$, 2405–2414). Torpedo electroplax membranes, whether tagged with the covalent probe or freshly isolated, regain the original fast rate of α-bungarotoxin binding upon dilution of carbamylcholine.