Solution structure of the toxic octapeptide, lophyrotomin.

Lophyrotomin is a toxic octapeptide, first isolated from larvae of the sawfly Lophyrotoma interrupta, which causes the death of cattle and sheep. It appears to act principally on the liver, however very little is known about the cellular site and mechanism of action. In the present study lophyrotomin was synthesized using solid-phase peptide synthesis, and the structure examined with two-dimensional nuclear magnetic resonance (NMR) spectroscopy. Two-dimensional correlation experiments (COSY and TOCSY) enabled the assignment of many of the resonances. Conventional NOESY experiments did not produce inter-residue information, however the alternative rotating frame NOE experiment (ROESY) resulted in intra-residue alpha N, and sequential alpha N and NN NOEs, permitting the sequence-specific assignment of all resonances. The presence of few additional shortage NOEs and the absence of any long-range NOEs in the ROESY spectra indicated a lack of persistent secondary structure. The results from circular dichroism (CD) spectroscopy experiments were consistent with the NOE data, as addition of high concentrations of the denaturant urea produced no changes in the lophyrotomin CD spectrum. This conclusion was further supported by 13C spin-lattice relaxation studies, which indicated that the peptide is a flexible molecule, by examination of the alpha-carbon chemical shifts, and by amide proton exchange rate measurements. Consequently it appears that if this peptide has to adopt a well defined structure to exert its biological activity, it must do so on interaction with other molecules, such as a receptor.