Inhibition of bovine α-chymotrypsin by cyclic trypsin inhibitor SFTI-1 isolated from sunflower seeds and its two acyclic analogues

Trypsin inhibitor SFTI-1 isolated from sunflower seeds ( comprising 14 amino acid residues and two cycles: head-to-tail cyclisation and disulfide bridge) is the smallest naturally occurring plant serine proteinase inhibitor. In our recent paper we have shown that the elimination head-to-tail cyclisation did not change trypsin inhibitory activity as judged by measured by association equilibrium constants K-a. The removal of disulfide bridge produced 2.4-fold lower activity. In the present paper we described chymotrypsin inhibitory activity. SFTI-1 inhibits significantly lower bovine a-chymortypsin (K-a = (5.20 +/- 1.56) x 10(6) M-1). The activity of the analogue with disulfide bridge only was practically the same, whereas the K-a value determined for homodetic peptide was almost 3-fold lower. Considering the results obtained and the recent literature data we postulate the lower inhibitory activity against both enzymes of the analogue with head-to-tail cyclisation only reflect its lower proteolytic stability.