Purification, amino acid sequence, and cDNA cloning of trypsin inhibitors from onion (Allium cepa L.) bulbs.

Three protease inhibitors (OTI-1-3) have been purified from onion (Allium cepa L.) bulbs. Molecular masses of these inhibitors were found to be 7,370.2, 7,472.2, and 7,642.6 Da by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), respectively. Based on amino acid composition and N-terminal sequence, OTI-1 and -2 are the N-terminal truncated proteins of OTI-3. All the inhibitors are stable to heat and extreme pH. OTI-3 inhibited trypsin, chymotrypsin, and plasmin with dissociation constants of 1.3 x 10(-9) m, 2.3 x 10(-7) m, and 3.1 x 10(-7)m, respectively. The complete amino acid sequence of OTI-3 showed a significant homology to Bowman-Birk family inhibitors, and the first reactive site (P-1) was found to be Arg(17) by limited proteolysis by trypsin. The second reactive site (P-1) was estimated to be Leu(46), that may inhibit chymotrypsin. OTI-3 lacks an S-S bond near the second reactive site, resulting in a low affinity for the enzyme. The sequence of OTI-3 was also ascertained by the nucleotide sequence of a cDNA clone encoding a 101-residue precursor of the onion inhibitor.