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Change in Activity of Soybean Trypsin Inhibitor by Removal of C-terminal Amino Acid Residues during Seed Germination

Abstract Kunitz-type soybean trypsin inhibitor (KSTI) is a major factor lowering the nutritional quality of soybean proteins. KSTI might be degraded during protein processing. In the present work, we determined the effect of removal of C-terminal amino acid residues on the inhibitory activity of KSTI in the cotyledons during germination in Japanese soybean cultivars, Toyokomachi and Wasekogane. The type of KSTI in both cultivars was determined by analysis of the complete nucleotide sequences of PCR products amplified from genomic DNA and partial amino acid sequences of both cultivars. These KSTI were identical to Tib in Toyokomachi and Tia in Wasekogane. KSTI from cotyledons at 4 days after germination in both cultivars was separated clearly into two bands Tl and T2 in Toyokomachi and Wl and W2 in Wasekogane upon native-PAGE. Based on the C-terminal amino acid sequences, KSTIs with higher mobility (Tl, Wl) were found to lack 13 amino acid residues at the C-terminus. The KSTI with lower mobility (T2, W2) lacked 14 or 15 amino acid residues at the C-terminus. Further, KSTI with lower mobility showed high inhibitory activity compared with that of KSTI with higher mobility. Thus, the changes in the form of KSTI from Tl or Wl to T2 or W2 by removal of C-terminal amino acid residues during seed germination may change the structure at the active site and consequently increase the inhibitory activity of KSTI.