Molecular differences between type A botulinum neurotoxin and its toxoid

The neurotoxins (seven serotypes, Mr approximately 150,000) produced by Clostridium botulinum cause the neuroparalytic disease botulism. Prophylaxis, definitive diagnosis and the only effective therapy for botulism depend, at present, on chemically detoxified form(s) of the neurotoxins, i.e. toxoids (immunogens), and the antisera raised with the immunogens. And yet, the toxoids currently used for immunization of humans and animals and for raising antibody are very crude preparations (approximately 90% impure) of the neurotoxins. Hence, the highly heterogenous toxoids were not suitable for physicochemical studies. We have detoxified a pure (greater than 99%) neurotoxin (serotype A) with formaldehyde. The native neurotoxin is composed of two subunit chains (Mr 53,000 and 97,000). The physicochemical properties of the toxoid (immunogenic in rabbits) were analyzed. The chemical modification produced inter- and intrasubunit covalent links at multiple sites and thus extensive aggregation of the neurotoxin. The secondary structure parameters (alpha-helix, beta-sheet, beta-turn and random coil) of the native protein were not significantly altered. Tertiary structure, as measured by exposure of tyrosine residues and fluorescence quantum yield of tryptophan residues, was considerably altered. The data imply that conformational (topographical) antigenic determinants may not contribute significantly to the serological property of the neurotoxin.