BOTULINUM-A LIKE TYPE-B AND TETANUS TOXINS FULFILLS CRITERIA FOR BEING A ZINC-DEPENDENT PROTEASE

Although botulinum neurotoxin (BoNT) types A and B and tetanus toxin ( TeTx) are specific inhibitors of transmitter release whose light chain s contain a zinc-binding motif characteristic of metalloendoproteases, only the latter two proteolyse synaptobrevin. Chelation of zinc or it s readdition at high concentration hindered blockade of neuromuscular transmission by BoNT/A and B, indicating that type A also acts via a z inc-dependent mechanism. Such treatments prevented proteolysis of syna ptobrevin II in rat brain synaptic vesicles by BoNT/B and TeTx but onl y the activity of the latter was antagonised appreciably by ASQFETS, a peptide spanning their cleavage site. The toxins' neuroparalytic acti vities were attenuated by phosphoramidon or captopril, inhibitors of c ertain zinc requiring proteases. However, these agents were ineffectiv e in reducing the toxins' degradation of synaptobrevin except that a h igh concentration of captopril partially blocked the activity of TeTx but not BoNT/B, as also found for these drugs when tested on synaptoso mal noradrenaline release. These various criteria establish that a zin c-dependent protease activity underlies the neurotoxicity of BoNT/A, a finding confirmed at motor nerve endings for type B and TeTx. Moreove r, the low potencies of captopril and phosphoramidon in counteracting the toxins' effects necessitate the design of improved inhibitors for possible use in the clinical treatment of tetanus or botulism.