Conformational and molecular weight studies of tetanus toxin and its major peptides

Two forms of tetanus toxin have been purified from Clostridium tetani cultures. These forms, obtained from filtrate and cellular extracts, were characterized by analytical ultracentrifugation using both conventional and meniscus-depletion sedimentation equilibrium. The molecular weight of filtrate toxin was found to be 128,000 +/- 3,000, while the extract toxin, which tended to self-associate, appeared somewhat larger, 140,000 +/- 5,000. The heavy and light chains were prepared from filtrate toxin, and their molecular weights were estimated to be 87,000 and 48,000, respectively, using polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The circular dichroic spectra of the extract and filtrate toxins are quite similar between 200-300 nm indicating that no major conformational difference exists between the two. The toxins contain both alpha-helicity and beta-structure. Interestingly, the isolated chains contain appreciable helicity (e.g., the sum of the chain helicities is over 80% of that found in filtrate toxin), but they appear to have relatively low contents of beta-structure. The sum of the spectra of the chains in both the near- and far-ultraviolet does not yield that found for filtrate toxin, although the similarity is far more striking than the difference. The prominent 293.5 nm negative circular dichroic band of tetanus toxin can be assigned to tryptophanyl residues almost exclusively in the heavy chain. The similarity in the magnitude of this band in the separated chain and toxin suggests that the microenvironments of the contributing tryptophans change very little when toxin is dissociated into its constituent chains.