Theromin, a novel leech thrombin inhibitor

We purified the most potent thrombin inhibitor described to date from the r hynchobdellid leech Theromyzon tessulatum. Designated theromin, it was puri fied to apparent homogeneity by gel permeation and anion exchange chromatog raphy followed by two reverse-phase steps of high performance liquid chroma tography. The primary sequence of theromin (a homodimer of 67 amino acid re sidues including 16 cysteine residues) was determined by a combination of r eduction and s-beta-pyridylethylation, Edman degradation, trypsin enzymatic digestion, and matrix-assisted laser desorption mass spectrometry measurem ent. Theromin exhibits no sequence homology with any other thrombin inhibit ors. Furthermore, theromin significantly diminishes, in a dose-dependent ma nner, the level of human granulocyte and monocyte activation induced by lip opolysaccharides. In summary, this potent thrombin inhibitor promises to ha ve high biomedical significance.