The equilibrium binding of [125I]-α-bungarotoxin to muscle fractions derived from normal and denervated, slow and fast muscle was examined. The toxin bound to normal slow and fast muscle with an affinity constant (K) in the order of 1081./mol. The number of receptor sites (N) was approximately 8.5 pmol/g of muscle. The toxin bound to both types of denervated muscle with an affinity constant of 1091./mol. The number of receptor sites (N) was more than tenfold higher than the number of sites in the corresponding normal muscle. Binding of the toxin to the receptor derived from both normal and denervated muscle closely approximated the isotherm predicted from the mass law equation for the interaction of 1 mol of homogeneous ligand with 1 mol of identical and independent sites. Further experiments suggested that the observed difference between normal and denervated muscle in the free energy of binding may be attributed to factors other than the primary structure of the receptor. The long term exposure of both types of normal muscle to the nonionic detergent Triton X-100 converted all binding to a high affinity set of sites (K, 1091./mol) with no change in the total number of sites (N). This conversion went through a complex intermediate state, in which the binding did not fit the model of a ligand interacting with a single set of noninteracting sites. In contrast, the long term exposure of either slow or fast denervated muscle to detergent had no effect on either the total number of binding sites (N) or the binding affinity (K). These data suggest that extra-junctional receptors in denervated muscle may be different than normal junctional receptors and that the difference may reflect different molecular interactions of the receptor with its environment rather than a difference in primary structure of the receptor protein.
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