An increasing number of analgesic peptides have been found in the tail toxicyst, but there has been little research into their analgesic domains. Where are the analgesic domains in a conservative beta alpha beta beta topology conformation of the analgesic peptides? We have carried out research to address this question. On account of the importance of disulfide bonds in the study of protein structure, the conformational stability, catalytic activity and folding, and site-directed mutagenesis in disulfide bridges have been used to look for the analgesic domain in a mature antitumor-analgesic peptide from the venom of the Chinese scorpion Buthus martensii Karsch (BmK AGAP). The mouse-twisting assay was used to examine the analgesic activity of 12 mutants, in which two mutants (C22S, C46S) and (C16S, C365), exhibited lower relative activity. Following the conformational analysis, one domain, called the "core domain", was found to be the key to the analgesic activity. (C) 2010 Elsevier Inc. All rights reserved.