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 s heep. It appears to act principally on the fiver, however very little is known about the cellular site and mechanism of action. In the prese nt study lophyrotomin was synthesized using solid-phase peptide synthe sis, 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. Con ventional NOESY experiments did not produce inter-residue information, however the alternative rotating frame NOE experiment (ROESY) resulte d in intra-residue alphaN, and sequential alphaN and NN NOEs, permitti ng the sequence-specific assignment of all resonances. The presence of few additional short-range NOEs and the absence of any long-range NOE s in the ROESY spectra indicated a lack of persistent secondary struct ure. 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 spe ctrum. This conclusion was further supported by C-13 spin-lattice rela xation studies, which indicated that the peptide is a flexible molecul e, by examination of the alpha-carbon chemical shifts, and by amide pr oton 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. (C) Munksgaard 1993.