Dehydroascorbate reductase and monodehydroascorbate reductase activities of trypsin inhibitors, the major sweet potato (Ipomoea batatas [L.] Lam) root storage protein

Trypsin inhibitors (TIs) were purified from the storage roots of sweet potato (Ipomoea batatas [L.] Lam var. Tainong 57) with ammonium sulfate precipitation, Sephadex G-75 gel filtration chromatography, and trypsin- Sepharose 4B affinity chromatography as previously reported [1]. Dehydroascorbate (DHA) was reduced by TIs independent of glutathione to regenerate ascorbate (AsA) contrasting with the function of DHA reductase with glutathione as a cofactor. Intermolecular thiol-disulfide interchanges of TIs were found during DHA reduction. AsA was oxidized by AsA oxidase to generate monodehydroascorbate (MDA) free radicals. MDA was reduced also by TIs to AsA in the presence of NADH which was the function of MDA reductase. The physiological significance of TIs with both DHA reductase and MDA reductase activities are discussed.