Primary structure and binding properties of iodinated derivatives of alpha-bungarotoxin

Iodination of alpha-bungarotoxin (alpha BuTx) gives rise to two products, mono- and diiodo-alpha BuTx. A combination of enzymatic digestion and Edman degradation revealed that the iodinated derivatives contain label in residue Tyr 54 exclusively. This labeling pattern is reminiscent of the reactivity toward iodination of "short" neurotoxins. The binding properties of the two derivatives were compared. At 20 degrees C, in 10 mM sodium phosphate, pH 7.4, the association of mono-125I-alpha BuTx to detergent-solubilized Torpedo californica electroplax acetylcholine receptor is characterized by a rate constant of 5.2 X 10(6) M-1 s-1, which is indistinguishable from the on-rate constant of the native toxin. Introduction of the second iodine atom reduces the association rate constant to 1.7 X 10(6) M-1 s-1. The diiodo derivative was also found to bind more slowly than the monoiodinated toxin by factors of 2.6 and 2.5 to partially purified preparations of acetylcholine receptor from denervated chick muscle and of toxin receptor from chick brain, respectively. No difference in dissociation kinetics was observed; both derivatives bind irreversibly to electric tissue and muscle acetylcholine receptor and dissociate from the neuronal receptor with an off-rate constant of 5.9 X 10(-5) s-1. In muscle, two sets of binding sites are kinetically distinguishable as originally observed by Brockes and Hall (Brockes, J. P., and Hall, Z. W. (1975) Biochemistry 14, 2092-2099). Di-125I-alpha BuTx binds more slowly to each site than mono-125I-alpha BuTx by factors of 2.0 and 3.7, respectively.