First cyclic scorpion trypsin inhibitor (Kd~0.5 nM). Does not inhibit chymotrypsin. Is very stable at high temperature. Expressed by the venom gland. Has the structural arrangement of an alpha-helix connected to antiparallel beta-sheets by disulfide bonds (CS-alpha/beta). This is a cyclic peptide. Trypsin inhibitor cleavage and cyclization are only observed in the presence of trypsin. It is suggested that the inhibitor Lys-55 is placed in the S1 pocket of trypsin, its C-terminal residue Ser-56 is cleaved, and the cyclization occurs via a peptide bond between residues Ile-24 and Lys-55. Does not show activity on voltage-gated potassium channels (hKv1.1/KCNA1, hKv1.4/KCNA4, Kv10.1/KCNH1/EAG1, hKv11.1/KCNH2/ERG1, hKv11.2/KCNH6/ERG2, and hKv11.2/KCNH6/ERG2).