A new class of photoactivatable and cleavable derivatives of neurotoxin II from Naja naja oxiana. Synthesis, characterisation, and application for affinity labelling of the nicotinic acetylcholine receptor from Torpedo californica.

A new series of photoactivatable and cleavable derivatives of neurotoxin II from the cobra Naja naja oxiana is investigated which can be used for mapping the surface topology of the nicotinic acetylcholine receptor from Torpedo electric tissue. The preparation and characterisation of five toxin derivatives, each with a radioactive 125I-azidosalicylamidoethyl-1,3'-dithiopropyl group in a defined position within the primary structure, are described. The photoinduced cross-linking reaction of the toxin derivatives with membrane-bound receptor is investigated. The photoactivatable group located at position K25 reacts almost exclusively with the delta subunit of the receptor, whereas the K15 derivative reacts with the alpha and beta subunits. The other derivatives did not react with the receptor to any significant extent. It is shown that, with respect to the receptor subunits, the cross-linking pattern depends on the length and chemical nature of the cross-linking group.