Amino acid sequence determination and biological activity of therin, a naturally occuring specific trypsin inhibitor from the leech Theromyzon tessulatum

We purified a trypsin inhibitor, designated therin, from the rhynchobdellid leech Theromyzon tessulatum. Therin was purified to apparent homogeneity by gel‐permeation and anion‐exchange chromatography followed by reverse‐phase HPLC. By a combination of reduction and S‐β‐pyridylethylation, Edman degradation and electrospray mass spectrometry measurement, the complete sequence of therin (48 amino acid residues; m/z, 5376.35 ± 0.22 Da) was determined. Therin exhibits an approximately 30 % sequence similarity with peptides of the antistasin‐type inhibitors family, i.e. the first and second domains of antistasin, hirustasin, ghilanthen and guamerins (I, II). Therin is a tight‐binding inhibitor of trypsin (Ki, 45 ± 12 pM) and has no action towards elastase or cathepsin G. Furthermore, therin (10−6 M) in conjunction with theromin, a Theromyzon thrombin inhibitor (10−6 M) significantly diminish the level of human leucocytes activation induced by lipopolysaccharide (10 μg) in a manner similar to that of aprotinin. These data suggest a leech trypsin inhibitor with possible biomedical signifance.