A cysteine protease from young stems of asparagus: isolation, properties, and substrate specificity

A protease was purified from the growing point of asparagus, Asparagus officinalis, using a cystatin-Sepharose column. The asparagus protease is the first protease isolated from the growing point of a plant tissue and from Liliaceae. Its molecular mass was estimated to be 28 kDa by SDS-PAGE. The optimum pH of the enzyme was 7 at 30 degrees C using casein as a substrate. The enzyme was strongly inhibited by monoiodoacetic acid, but not by diisopropylfluorophosphate, suggesting that it is a cysteine protease. Asparagus protease had broad specificity on the hydrolysis with oxidized B-chain of insulin as a substrate. However, for the P2 position of the cleavage site, the large hydrophobic side chains of amino acid residues such as Phe, Val, and Leu were considerably preferred. Asparagus protease was similar to papain about specificity at the P2 position. The N-terminal sequence of the first 12 residues was identified and 8 residues among them agreed with that of papain, accompanying the addition of one residue (Ala) to that of papain.