Abstract— The isolation and purification of an antigenic polypeptide fragment from tetanus toxin is described. The main physico-chemical, chemical, immunological, and pharmacological characteristics of this fragment, designated as B-IIb fraction, are reported. It is a polypeptide with a molecular weight close to 46,000. Its amino acid composition is on the whole comparable with that of the toxin. It contains one disulphide link and two free sulphhydryl groups which are not directly available for reaction. Tyrosine and lysine were found to be the two N-terminal groups. However, that B-IIb fraction has a subchain structure is still to be demonstrated. There is some evidence that B-IIb fraction may consist of ‘isofragments’. This toxin fragment shows a cross-reaction with intact toxin and a specific flocculating activity of about three times that of the latter. In contrast, however, B-IIb fraction exhibits a specific toxicity approximately one hundred thousand times lower than the intact toxin. Although practically atoxic, this toxin fragment is still able to bind to gangliosides with an affinity which is even greater than that of the toxin. It is also capable of migrating towards the central nervous system by a mechanism of retrograde axonal transport as shown in peripheral adrenergic, sensory and motoneurons.
These unique features of B-IIb fraction are discussed in relation to the use of such fragments both for competing in vivo with the attachment of the toxin in the central nervous system and for specifically carrying therapeutic and pharmacological drugs into the central nervous system by neural route.