We purified a new trypsinϪchymotrypsin inhibitor, designated tessulin, from the rhynchobdellid leech Theromyzon tessulatum. This 9-kDa peptide was purified to apparent homogeneity by gel-permeation and anion-exchange chromatographies followed by reverse-phase HPLC. The structure of tessulin was determined by reduction, S-β-pyridylethylation, trypsin digestion, automated Edman degradation and matrix-assisted laser desorption mass spectrometry (m/z 8985 Da). The 81-amino-acid peptide possesses 16 cysteines and exhibits a 16% sequence similarity with antistasin-type inhibitors. Tessulin inhibits trypsin (K i 1 pM) and chymotrypsin (K i 150 pM) and exhibits no activity with thrombin, factor Xa, cathepsin G and elastase. This is the first trypsinϪchymotrypsin inhibitor isolated from leeches that does not inhibit elastase or cathepsin G, except for cytin and therin. Furthermore, tessulin, in conjunction with other serine-protease inhibitors isolated from Theromyzon (therin, theromin), significantly diminishes the level of human granulocyte and monocyte activation induced by lipopolysaccharides (10 µg). The combined level of inhibition is higher than that of aprotinin, another serine-protease inhibitor used biomedically. Thus, tessulin may be clinically significant in reducing inflammatory events.Keywords : protease inhibitors; trypsin-chymotrypsin inhibitor.In numerous pathological processes, the crucial role played by proteases is becoming more apparent. This critical role creates a need for the development of non-toxic protease inhibitors for in vivo clinical applications. Because of the specific recognition by proteases of defined amino acid sequences, it is possible to inhibit these enzymes when they are involved in pathological processes [1]. The involvement of proteolytic enzymes in a number of degenerative diseases, e.g., pancreatitis [zymogenic forms of proteolytic enzymes (trypsin 1 and 2, chymotrypsin A [2, 3] and elastase [4])], is becoming more evident as is the need for curtailing this activity in uncalled-for situations [5Ϫ7]. In this regard, aprotinin, a 6.5-kDa trypsin and kallikrein inhibitor isolated from bovine organs [8] has been used for more than three decades as an intensive care drug for acute pancreatitis [9], demonstrating the significance of this class of enzyme.In hematophageous leeches, animals that must escape hostimmune surveillance, studies on serine-protease inhibitors have yielded significant findings relating to the clinical importance of these enzyme inhibitors. Of two groups of serine-protease [19]. However, some of these inhibitors exhibit different amino acid sequences and inhibitory activities; their three-dimensional structures share the same structural motif with the leech antihemostatic protein (L.A.P.) [20]. Interestingly, their mechanisms of action and epitopes important for binding to their respective targets are distinct as well [20], suggesting that through evolution different strategies have emerged to deal with a similar problem.Among the second group of serine-protea...