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Two peptides, SGCI and SGTI, that inhibited chymotrypsin and trypsin, respectively, were isolated from the haemolymph of Schistocerca gregaria. Their primary structures were found to be identical with SGP-2 and SGP-1, two of a series of peptides isolated from ovaries of the same species (A. Hamdaoui et al., FEES Lett, 422 (1998) 74-78). All these peptides are composed of 35-36 amino acid residues and contain three homologous disulfide bridges. The residues imparting specificity to SGCI and SGTI were identified as Leu-30 and Arg-29, respectively. The peptides were synthesised by solid-phase peptide synthesis, and the synthetic ones displayed the same inhibition as the natural forms: SGCI is a strong inhibitor of chymotrypsin (K-i = 6.2 x 10(-12) M), and SGTI is a rather weak inhibitor of trypsin (K-i = 2.1 x 10(-7) M). The replacement of P-1 then P-1’ residues of SGCI with trypsin-specific residues increased affinity towards trypsin 3600- and 1100-fold, respectively, thus SGCI was converted to a strong trypsin inhibitor (K-i = 5.0 x 10(-12) M) that retained some inhibitory affinity towards chymotrypsin (K-i = 3.5 x 10(-8) M). The documented role of both P-1 and P-1’ highlights the importance of S-1’P-1’ interactions in enzyme-inhibitor complexes. (C) 1999 Elsevier Science B.V. All rights reserved.