An aspartic type protease degrades trypsin inhibitors, the major root storage proteins of sweet potato (Ipomoea batatas (L.) Lam cv. Tainong 57)

Roots of sprouted sweet potato (Ipomoea batatas [L.] Lam) were used as materials to purify proteases which degraded trypsin inhibitors (TIs), the root storage proteins of sweet potato (SP). The commercial pepstatinagarose (crosslinked, 6%) was chosen as an affinity column for purification. The purified protease has a molecular mass of about 64 kDa on the gelatin-SDS-PAGE gel and was inhibited by pepstatin but not by E-64 on the gelatin-SDS-PAGE gel. Therefore, it might belong to the aspartic type. Using the trypsin inhibitor activity staining method as a criterion for TI degradations, we found that this aspartic type protease could degrade purified TIs in the presence or absence of 5 mM DTT and the hydrolysis was complete in the former condition. The physiological role of aspartic type protease in the degradation of SPTIs is discussed.