In vitro reduction of trypsin inhibitor by purified NADPH/thioredoxin system from sprouts of sweet potato (Ipomoea batatas (L) Lam.) storage roots

Sweet potato (Ipomoea batatas (L.) Lam) storage roots were sprouted in the dark for two months, and the sprouts were used as source materials for purification of thioredoxin reductase successively via the DE-52, Sephadex G-75, and 2′,5′-ADP Sepharose 4B column chromatography. Thioredoxin reductases with a molecular mass of ca 33kDa were obtained. The purification was 813-fold and the yield was 2.5%. The effect of thioredoxin reductase on trypsin inhibitor activity (TIA) was analyzed with a TIA assay method. Native, oxidized form of trypsin inhibitors (TIs) inhibited trypsin activity to 24% of its original value. Trypsin activity recovered to 38 and 46% of its original value, respectively, when TI was reduced in advance by NTR system and DTT. The data indicate that NTR-reduced TIs lost the capacity to inhibit trypsin activity in vitro. mBBr/SDS-PAGE was used to analyze the reduction of TIs on gels. The reduced TIs were more sensitive to heat denaturation, and exhibited increased digestibility by partially purified proteases from sweet potato sprouts. Based on these results, we conclude that the NADPH/thioredoxin system is likely associated with the reduction of TIs and initiation of degradation and re-mobilization of stored TIs during sprouting of sweet potato storage roots. Meanwhile we may expect sprouts to be a better protein source for consumption than SP storage root itself.