The role of the interchain disulfide bond in governing the pharmacological actions of botulinum toxin

All serotypes of botulinum toxin possess a disulfide bond that links the heavy chain and light chain components of the holotoxin. Experiments were done to assess the functional significance of this covalent bond, and the work was facilitated by use of mercurial compounds that modify residues in the vicinity of the catalytic site. The data indicated that reduction of the interchain disulfide bond had two major effects: 1) changing conformation or orientation of the two chains, which diminished toxicity against intact cells, and 2) loosening or relocating a heavy chain belt segment that encircles the light chain and occludes the catalytic site. Interestingly, disulfide bond reduction of all serotypes produced conformational changes that diminished toxicity against intact cells, but it produced conformational changes that led to exposure of the catalytic site in only three serotypes. For the other serotypes, the catalytic site was accessible even before disulfide bond reduction. Neither of the major structural effects was dependent upon separation of the heavy chain and light chain components of the toxin, nor were they dependent on toxin substrate. Depending on the initial state of the toxin molecule, the combination of disulfide bond reduction and treatment with a mercurial compound could abolish toxicity. Therefore, this combination of treatments was used to convert active toxin into a parenteral vaccine. Administration of the modified toxin evoked a substantial IgG response, and it produced complete protection against a large dose of native toxin.