Quantitative comparison between tetanus toxin, some fragments and toxoid for binding and axonal transport in the rat

The fragments BIIb and C of tetanus toxin, which contain its binding domain, were quantitatively compared with native toxin and a toxoid with respect to axonal transport from the gastrocnemius muscle to the spinal cord in rats. Against 125I-toxin, the dose-ascent curve of labelled toxoid was shifted by a factor of 3-5 to higher concentrations, whereas the ascent of the labelled binding fragments was at least 50-100 times less. The binding fragments also differed from tetanus toxin by their very low affinity to rat brain membranes buffered to pH 7.5 in saline, but were equivalent with the toxin in buffer of low molarity and low pH. We conclude that additional parts of the toxin molecule have to complement the binding domain for expression of the full binding and transport characteristics of the toxin.