In vivo protection against Tityus serrulatus scorpion venom by antibodies raised against a discontinuous synthetic epitope.

Scorpion stings cause human fatalities in numerous countries. Serotherapy is the only specific means to try to circumvent the noxious effects of venom toxins. TsNTxP is a natural anatoxin from the venom of the scorpion Tityus serrulatus that may be useful to raise therapeutic anti-venom sera. Linear epitopes recognized by anti-TsNTxP antibodies have previously been mapped. Here, we attempted to identify discontinuous epitopes in TsNTxP since neutralizing epitopes are often associated with such complex entities. One hundred and fifty-three octadecapeptides with the general formula (P1)-(Gly-Gly)-(P2) were synthesized by the Spot method on cellulose membranes. P1 and P2 were octapeptides from the TsNTxP N-terminal and C-terminal sections, respectively. Each sequence of eight amino acids was frameshifted in turn by three residues, in order to cover TsNTxP entire sequence. Binding of neutralizing anti-TsNTxP rabbit antibodies to spotted peptides revealed GREGYPADGGGLPDSVKI as the more reactive peptide sequence. This epitope was made from the first eight residues of the protein (GREGYPAD) and from residues 47 to 54 (GLPDSVKI) of the C-terminal part of TsNTxP. BALB/c mice were immunized with synthetic GREGYPADGGGLPDSVKI peptide conjugated to ovalbumin. One week after the last immunization, in vivo protection assays showed that immunized mice could resist a challenge by an amount of T.serrulatus whole venom equivalent to 1.75 LD(100), a dose that killed all control non-immune mice. Based on molecular models of TsNTxP and related Tityus toxins, we found that the above peptide matches with a discontinuous epitope, well exposed at the toxin molecular surface which contains residues known to be important for the bioactivity of toxins.