Identification of a Kunitz inhibitor from Albizzia kalkora and its inhibitory effect against pest midgut proteases

Abstract

A purification protocol, involving water extraction, ammonium sulfate precipitation, Sepharose 4B-trypsin affinity and FPLC Superdex G-75 chromatography, was employed to isolate a trypsin inhibitor from Albizzia kalkora seeds. The inhibitor, which had a molecular mass of 19,768.23 Da, consisted of two disulfide-linked polypeptide chains with approximate molecular mass of 15.5 and 4.5 kDa, respectively. It was stable from pH 2–12 for 24 h, whereas it was unstable either above 80°C for 10 min or under reduced condition over 60 min. The inhibitor, which inhibited trypsin activity with an apparent K _i of 2.5 × 10⁻⁷ M, had one reactive site involved with a lysine residue. Disulfide linkage and lysine residue were important in maintaining its active conformation. Partial amino acid sequence of the purified protein showed a high degree of homology with various members of the Kunitz inhibitor family. Moreover, trypsin-like proteases from larval Helicoverpa armigera, Spodoptera exigua, and Pieris rapae were inhibited for 85, 57, and 68% respectively, by the inhibitor at 45 μg ml⁻¹.