Structure elucidation of prothoracicotropic hormone from the silkworm

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Two kinds of prothoracicotropic hormones, 22K-PTTH and bombyxin(4K-PTTH), have been isolated from adult heads of the silkmoth, Bombyx mori. 22K-PTTH, that activates Bombyx prothoracic glands to induce synthesis and release of ecdysone, consists of many heterogeneous molecular species, one of which has been isolated and its N-terminal sequence of 13 amino acid residues determined.Bombyxin, that can activate prothoracic glands of another silkmoth, Samia cynthia ricini, consists of two peptide chains, A and B chains with 20 and 28 amino acid residues, respectively, which are connected by disulfide bonds to each other. It has a considerable sequence homology (40%) with vertebrate insulin. Thermolysin digestion of bombyxin-II, one of various molecular form of bombyxins, afforded two cystine peptides. Sequencing of the peptides revealed that one contained one cystine and the other two. Hte former established the connection fo A(20)-B(22). As to the latter cystine peptide, there were three possible structures with different disulfide bonds, which were all synthesized. They were separated on reverse phase HPLC, one of them was co-eluted with the natural thermolysin peptide. Thus, the connection of A(6)-A(11) and A(7)-B(10) was established. This mode of disulfide bridges is identical with that of insulin. Such homologous structure of bombyxin-II with insulin permitted us to construct a three dimensional structural model of bombyxin-II, which showed a considerable difference in the surface of the molecules. This difference might account for the difference of biological activity.Monoclonal antibldy was made against an N-terminal synthetic decapeptide of A chain coupled with bovine serum albumin. Immunohistochemistry using this antibody showed that bombyxin is produced in median 4 pairs of neurosecretory cells, transferred to corpora allata and released into the hemolymph.