Primary structure of ascidian trypsin inhibitors in the hemolymph of a solitary ascidian, Halocynthia roretzi

The amino acid sequences of trypsin inhibitors I and II from the hemolymph of a solitary ascidian, Halocynthia roretzi, were determined after reduction and S-pyridylethylation. The results indicated that inhibitor I consists of a single polypeptide chain with 55 amino acid residues and four intramolecular disulfide bridges, whereas inhibitor II is composed of two polypeptide chains corresponding to a form derived from inhibitor I by cleavage at the Lys16-Met17 bond. Lys16 may be the reactive-site residue of these inhibitors, because carboxypeptidase B treatment destroys most of the inhibitory activity of inhibitor II but not that of inhibitor I.