Inactivation of Clostridium botulinum type A neurotoxin by trypsin and purification of two tryptic fragments

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Abstract
1 Limited treatment of Clostridium botulinum type A neurotoxin with trypsin resulted in the cleavage of the heavy (95 000 Da) subunit at approximately the mid-position and a loss of toxic activity. The rate of toxicity loss was considerably faster than that of mid-chain cleavage; thus a loss of toxicity in excess of 90% was accompanied by only 30–35% mid-chain cleavage of the heavy subunit. 2 A study of the binding of125I-labelled neurotoxin to rat brain synaptosomes suggesting the presence of at least two sites of tryptic action on the 95 000-Da binding subunit. 3 Prolonged treatment of the neurotoxin with trypsin resulted in the complete digestion of a 46 000-Da fragment of the heavy subunit, leaving intact a soluble fragment of approximately 105 000 Da containing the light subunit linked to the remaining (49 000-Da) protion of the heavy subunit. This fragment exhibited less than 0.01% of the original toxicity and gave immunoprecipitation reactions indistinguishable from the native toxin. 4 The 49 000-Da portion of the heavy chain was purified from the 105 000-Da fragment of the toxin and the sequence of the first 35 amino acids determined. The sequence of the first 10 residues was found to be identical to that previously reported for the heavy subunit showing that the 49 000-Da fragment represents the NH2-terminal portion of the heavy chain and that this region is resistant to tryptic action. 5 It is suggested that the primary site(s) of tryptic action on the heavy subunit of botulinum type A neurotoxin is close to the COOH terminus and that cleavage of the polypeptide chain in this region results in a closs of toxic activity mediated by the destruction of the neurotoxin-binding site.