Characterization of Clostridium botulinum type B neurotoxin associated with infant botulism in Japan

The neurotoxin of strain 111 (111/NT) associated with type B infant botulism showed antigenic and biological properties different from that (Okra/NT) produced by a food-borne botulism-related strain, Okra, The specific toxicity of 111/NT was found to be about 10 times lower than that of Okra/NT, The monoclonal antibodies recognizing the light chain cross-reacted with both neurotoxins, whereas most of the antibodies recognizing the carboxyl-terminal half of the heavy chain of Okra/NT did not react to 111/NT, Binding experiments with rat brain synaptosomes revealed that I-125-labeled 111/NT bound to a single binding site with a dissociation constant (K-d) of 2.5 nM; the value was rather lower than that (0.42 nM) of I-125-Okra/NT for the high-affinity binding site. In the lipid vesicles reconstituted with ganglioside GT1b, I-125-Okra/NT interacted with the amino-terminal domain of synaptotagmin 1 (Stg1N) or synaptotagmin 2 (Stg2N), fused with the maltose-binding protein, in the same manner as the respective full-length synaptotagmins, and the K-d values accorded with those of the low- and high-affinity binding sites in synaptosomes. However, I-125-111/NT only exhibited a low capacity for binding to the lipid vesicles containing Stg2N, but not Stg1N, in the presence of ganglioside GT1b, Moreover, synaptobrevin-2, an intracellular target protein, was digested to the same extent by the light chains of both neurotoxins in a concentration-dependent manner. These findings indicate that the 111/NT molecule possesses the receptor-recognition site structurally different from Okra/NT, probably causing a decreased specific toxicity.