Cathepsin G, a cationic serine proteinase present in neutrophils and monocytes, is able to cleave biologically important proteins and may thus participate in tissue destruction during inflammation. Its activity is physiologically controlled by the fast-acting serpins, alpha 1-anti-chymotrypsin (ka = 5 x 10(7) M-1 S-1) and alpha 1-proteinase inhibitor (ka = 2.7 x 10(5) M-1 S-1). We have shown that cathepsin G forms a tightly bound 1:1 complex with a 5-kDa heparin fragment (Kd = 1.9 x 10-8 M). The partial enzymatic activity retained by this complex is inhibited extremely slowly by the above 68- and 53-kDa serpins. The activity of the complex is also virtually resistant to inhibition by eglin c, and 8-kDa non-serpin inhibitor. A detailed kinetic investigation showed that the inhibition of heparin-bound cathepsin G by the three proteins proceeded via a two-step mechanism. [formula: see text] The three inhibitors have widely different Ki* values (0.18-13 microM) (Ki* = k-1/k1). Their isomerization constants k2 are, however, all in the same range and their extremely low values (0.7-3 ms-1) account for the very low rate of cathepsin G inhibition. The second-order inhibition rate constants k2/Ki* were 4300, 700, and 52 M-1 S-1 for alpha 1-antichymotrypsin, alpha 1-antitrypsin, and eglin c, respectively, indicating that, if heparin is present in vivo, the two former physiological inhibitors will be unable to prevent cathepsin G-mediated proteolysis. Neutrophil elastase binds the 5-kDa heparin fragment with an affinity identical to that of cathepsin G. alpha 1-Proteinase inhibitor reacts, however, much faster with heparin-elastase (ka = 1.8 x 10(6) M-1 S-1) than with heparin-cathepsin G (k2/Ki* = 700 M-1 S-1).