ECOTIN IS A POTENT INHIBITOR OF THE CONTACT SYSTEM PROTEASES FACTOR XIIA AND PLASMA KALLIKREIN

Ecotin, a serine protease inhibitor found in the periplasm of Escheric hia coli, has been characterized as a potent reversible tight-binding inhibitor of the human contact activation proteases factor XIIa (FXIIa ) and plasma kallikrein, having K-i values of 89 pM and 163 pM, respec tively. Ecotin also inhibited human leukocyte elastase (HLE) with high affinity (K-i = 55 pM). The association rate constants k(on) for FXII a and kallikrein were 5.3 x 10(5) M(-1). s(-1) and 2.9 x 10(5) M(-1). s(-1), respectively, The dissociation rate constant k(off) for kallikr ein, measured in the presence of HLE to prevent reassociation, was 6.3 x 10(-5) s(-1); the k(off) for ecotin with FXIIa was 4.7 x 10(-5) s(- 1). Both FXIIa and kallikrein cleaved ecotin slowly at pH 5.0, identif ying Met-84 as the P-1 residue. The potent anticoagulant effect by eco tin is explained by the coincident inhibition of FXIIa, kallikrein, an d FXa and suggests that it may be useful in the study of inflammatory or thrombotic disorders such as sepsis or cardiopulmonary bypass.