Binding of Clostridium botulinum Type C Neurotoxin to Rat Brain Synaptosomes1

The binding of Clostridium botulinum type C neurotoxin to rat brain synaptosomes was determined by the use of 125I-neurotoxin. The binding was independent of the incubation temperature (0 degrees C and 37 degrees C) and was equilibrated in 10 min. The dose dependent of 125I-toxin binding to synaptosomes at 0 degrees C showed that there were two kinds of toxin receptors on the synaptosomal membrane; the association constants and maximum binding values were 1.05 x 10(10 M-1, 5.25 x 10(-13) mol/mg of synaptosomal protein and 5.00 x 10(6) M-1, 5.00 x 10(-12) mol/mg of synaptosomal protein, respectively. When the incubation of toxin with synaptosomes was continued at 37 degrees C after 125I-toxin had been pre-incubated with synaptosomes at 0 degrees C for 10 min, the displacement of labeled toxin by the addition of excess amounts of unlabeled toxin decreased slightly with increasing incubation time, and finally 0.4% of the bound 125I-toxin was not displaced from synaptosomes. The binding of 125I-toxin to synaptosomes was inhibited by anti-heavy chain IgG and a monoclonal antibody which neutralized toxin and recognized heavy chain. These results suggest that the binding sites of toxin to synaptosomes are localized on heavy chain and a small amount of the bound toxin is incorporated into the synaptosomal membrane or synaptosomes.