Tetanus toxin-[2,3-3H]propionamid. Synthesis, specific activity, immunoreactivity, toxicity and ascent into the spinal cord.

Tetanus toxin was tritiated with various amounts of N-succinimidyl-[2,3-3H]propionate. A tenfold molar excess of reagent over toxin was used for the introduction of about 5 nmol of [2,3-3H]propionate into 1 nmol of toxin, corresponding to a specific activity of about 48 MBq/mg protein. The labelling procedure did not change significantly the immunoreactivity of tetanus toxin, but reduced the toxicity in mice from 12 ng/kg to 16 ng/kg. Tetanus toxin-[2,3-3H]propionamid was carried through the peripheral nerves into the spinal cord. It could be used instead of 125I-labelled toxin for the detection of anti-tetanus toxin antibodies with a detection limit of 0.25 mU/ml of serum.