Radiolabeled .alpha.-bungarotoxin derivatives: kinetic interaction with nicotinic acetylcholine receptors

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Abstract
The binding interactions of purified [3H]-.alpha.-Bgt [.alpha.-bungarotoxin] and monoiodinated and diiodinated derivatives of .alpha.-Bgt with membrane-bound nicotinic acetylcholine receptors (nAChR) from Torpedo californica electroplax and rat brain were characterized by several kinetic and equilibrium techniques. [3H]-.alpha.-Bgt and 125I-labeled monoiodinated .alpha.-Bgt ([125I]-.alpha.-Bgt) exhibited comparable specificities and affinities for nAChR. Affinity of nAChR for 125I-labeled diiodinated .alpha.-Bgt ([125I2]-.alpha.-Bgt) was reduced, and [125I2]-.alpha.-Bgt=nAChR complexes showed anomalous biphasic dissociation kinetics. [125I]-.alpha.-Bgt and [125]-.alpha.-Bgt binding was inhibited most potently by native .alpha.-Bgt as opposed to iodinated toxins. [3H]-.alpha.-Bgt was the radiotoxin most resistant to inhibitory influences. The use of well-characterized, chemically modified .alpha.-Bgt derivatives may identify ligand binding microheterogeneities and tissue-specific receptor subclasses.