Purification, some properties and the complete primary structures of two protease inhibitors (DE-3 and DE-4) from Macrotyloma axillare seed.

The Macrotyloma axillare plant, belonging to the Leguminosae family, is a perennial climbing or trailing herb 0.2--3.5 m long. The plant is indigenous to South Africa and it occurs in the warm dry northern parts of the Transvaal. It has been introduced into Australia, where the seed are used as animal food. Two protease inhibitors, DE-3 and DE-4, were purified from Macrotyloma axillare seed by gel filtration on Sephadex G-50 followed by ion-exchange chromatography on DEAE-cellulose. They each comprise 76 amino acid residues including 14 half-cystine residues. The complete primary structures of the two protease innibitors have been elucidated and their sequences are 67% identical. The inhibitor specificities, the sequences, the invariant amino acid residues and the reactive inhibitor sites of protease inhibitors DE-3 and DE-4 resemble the corresponding properties of the Bowman-Birk double-headed protease inhibitor group. The cysteine residues are in similar locations to those in protease inhibitors of known structure so they are presumed to link similarly.