Primary structure and specificity of the major serine proteinase inhibitor of amaranth (Amaranthus caudatus L.) seeds.

A novel member of the potato inhibitor I family of serine proteinase inactivating proteins has been isolated from seeds of grain amaranth (Amaranthus caudatus L.) and characterized. The mature form of the amaranth trypsin/subtilisin inhibitor (ATSI) with pI approximately 8.3 and molecular mass 7887 Da contains 69 amino acids in a sequence showing 33-51% identity with members of the inhibitor I family from other plant families. A minor form with pI approximately 7.8 and same inhibitory properties lacked the N-terminal dipeptide Ala-Arg. In accordance with the reactive-site bond Lys45-Asp46, which was identified by specific cleavage on a subtilisin column, ATSI is a potent inhibitor of trypsin (Ki approximately 0.34 nM) and more weakly of plasmin (Ki approximately 38 nM) and Factor XIIa (Ki approximately 440 nM). However, ATSI also inactivates chymotrypsin (Ki approximately 0.41 nM), cathepsin G (Ki approximately 122 nM) and several alkaline microbial proteinases, including subtilisin NOVO (Ki approximately 0.37 nM). Interestingly, ATSI contains a Trp residue instead of the highly conserved Arg in position 53 (P8'), which is assumed to play a central role in stabilization of the active-site loop during complex formation. ATSI was immediately inactivated by pepsin and hardly represents an antinutritional component in foods or feeds.