ABSTRACT
Binding of human leukocyte elastase (HLE) to human plasma fibronectin (Fn) was determined by its competition with an oligopeptide chromogenic substrate. Kinetic curves of accumulation of the substrate hydrolysis product in the presence of Fn were specific for slow-binding one-step inhibitors of the enzyme. Values of rate constants for HLE association with Fn and for the dissociation of the complex were, respectively, 2.2.10(3) M(-1).sec(-1) and 1.4.10(-3) sec(-1) at pH 7.5 and 25 degreesC. The dissociation constant of the HLE--Fn complex determined independently by titration of the enzyme with the protein substrate and neglecting the hydrolysis was 3.9.10(-7) M. The resulting values were suggested to describe a high-affinity site in the Fn molecule which was subjected to the primary attack by HLE. The soybean Bowman--Birk protease inhibitor (BBI) efficiently inhibited the HLE-induced degradation of Fn under conditions of both inhibitor preincubation with the protease and its addition into the reaction mixture of the protease with Fn.
