Isolation and characterization of a serine protease from the storage roots of sweet potato ( Ipomoea batatas [L.] Lam)

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A novel endopeptidase, IBSP82, has been purified from storage roots of sweet potato ( Ipomoea batatas [L.] Lam) to electrophoretic homogeneity by four purification steps: ammonium sulfate precipitation, Superdex ® 75 FPLC GFC, Superdex ® 200 FPLC GFC and Tricine–sodium dodecyl sulfate–polyacrylamide (Tricine–SDS–PAGE) gel cutting. Its M r was estimated to be 82 kDa by Tricine–SDS–PAGE. The enzyme activity of IBSP82 was strongly inhibited by 2 mM PMSF, but not at all by 2 mM iodoacetic acid, 5 mM EDTA or 20 uM pepstatin A. These results indicated that it was a serine type protease. The preferential cleavage sites for this protease were found to be hydrophobic and aromatic amino acid residues at the P 1 sites. It was also found that this protease was able to cleave denatured sweet potato β-amylase but not denatured sweet potato trypsin inhibitors. The native sweet potato trypsin inhibitors neither inhibit IBSP82’s activity nor serve as its substrate.