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Tetanus toxin. The effect of chemical modifications on toxicity, immunogenicity, and conformation.
Tetanus toxin has been isolated from the extract of Clostridium tetani and analyzed for purity using various methods, e.g. sedimentation velocity, gel filtration, polyacrylamide gel electrophoresis, immunoelectrophoresis, and immunodiffusion. The homogeneous toxin, characterized by a minimum lethal dose of 10 pg (18- to 20-g mouse), was judged to be of high purity. The amino acid composition was determined and found to be in good agreement with reported values for both filtrate and extract toxin. The corrected sedimentation coefficient, so20,w, was found to be 7.5, and the molecular weight was estimated to be 150,000. These values agree closely with those reported by others. The toxin was modified using the conventional formaldehyde reaction to produce toxoid and both reductive methylation and carbamylation, which are highly specific for lysyl residues. Under certain reaction conditions, carbamylation of the toxin completely eliminated toxicity. Whereas reductive methylation yielded a high degree of conversion of lysine to dimethyllysine and monomethyllysine, the toxicity, albeit greatly reduced, was never completely eliminated. The circular dichroic spectrum of each chemically modified toxin was obtained, resolved into Gaussian components, and compared with that of native toxin, which is estimated to contain about 20% alpha helix and 23% beta structure. The far ultraviolet circular dichroic spectra of toxin and toxoid were characterized by negative extrema at 208 nm and 217 nm attributable to ordered secondary structure, and toxin also exhibited a distinct shoulder at 223 nm. Carbamylated toxin and methylated toxin were characterized by negative extrema at 210 nm and 206 nm, respectively, and both exhibited shoulders at 216 to 217 nm and 223 nm. The toxin and derivatives exhibited multiple negative extrema above 250 nm which were assigned to the various aromatic residues. There were differences in the spectra of the toxin and derivatives over the entire wavelength region, thus suggesting changes in the local environment of various chromophores. In particular, the rotational strengths of many of the bands assigned to tryptophan, tyrosine, and phenylalanine were altered in the derivatives. Also, in the far ultraviolet region of the circular dichroic spectrum, the data were suggestive of some reduction in the amount of both alpha helix and beta structure in the derivatives. However, there was no evidence of extensive conformational changes, e.g. unfolding, in the modified toxins. Presently, it is not known if the small conformational differences between toxin and toxoid are important in the loss of toxicity with the retention of immunogenicity in the derivative. The modification data are consistent with the hypothesis that separate amino acid residues are involved in toxicity and immunogenicity.
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