Mechanism-based inhibition of proteinases by serpins involves enzyme acylation and fast insertion of the reactive center loop (RCL) into the central -sheet of the serpin, resulting in mechanical inactivation of the proteinase. We examined the effects of ligands specific to ␣-helix F (␣HF) of plasminogen activator inhibitor-1 (PAI-1) on the stoichiometry of inhibition (SI) and limiting rate constant (k lim ) of RCL insertion for reactions with -trypsin, tissue-type plasminogen activator (tPA), and urokinase. The somatomedin B domain of vitronectin (SMBD) did not affect SI for any proteinase or k lim for tPA but decreased the k lim for -trypsin. In contrast to SMBD, monoclonal antibodies MA-55F4C12 and MA-33H1F7, the epitopes of which are located at the opposite side of ␣HF, decreased k lim and increased SI for every enzyme. These effects were enhanced in the presence of SMBD. RCL insertion for -trypsin and tPA is limited by different subsequent steps of PAI-1 mechanism as follows: enzyme acylation and formation of a loop-displaced acyl complex (LDA), respectively. Stabilization of LDA through the disruption of the exosite interactions between PAI-1 and tPA induced an increase in the k lim but did not affect the SI. Thus it is unlikely that LDA contributes significantly to the outcome of the serpin reaction. These results demonstrate that the rate of RCL insertion is not necessarily correlated with SI and indicate that an intermediate, different from LDA, which forms during the late steps of PAI-1 mechanism, and could be stabilized by ligands specific to ␣HF, controls bifurcation between the inhibitory and the substrate pathways.Serpin (1) plasminogen activator inhibitor-1 (PAI-1), 2 the major endogenous inhibitor of tissue-(tPA) and urokinase-type (uPA) plasminogen activators (2-5) is a well characterized thrombotic risk factor. High levels of PAI-1 inhibit the fibrinolytic system by preventing the formation of plasmin from plasminogen and promote thrombosis (6, 7). An elevated concentration of PAI-1 correlates with an increased rate of ischemic arterial disease (8) as follows: atherosclerosis, angina pectoris (9 -14), and myocardial infarction (15)(16)(17)(18)(19)(20)(21)(22), as well as with deep vein thrombosis and disseminated intravascular coagulation (23-25). Because complete inactivation of PAI-1 is not lifethreatening (26 -28), the development of new approaches to the therapeutic neutralization of PAI-1 is a subject of many studies (for review see Refs. 29,30).Like other serpins, PAI-1 (Scheme 1, I) (31) inactivates a target proteinase (E) through a unique conformational mechanism (Fig. 1). In this mechanism, the formation of the acylenzyme intermediate (EϳIЈ) triggers the spontaneous insertion of a reactive center loop (RCL) as a strand 4 into the central -sheet A, the 70 Å translocation of the acyl-enzyme to the opposite pole of the PAI-1 molecule, and enzyme inactivation because of the mechanical distortion of its catalytic site ( Fig. 1) (32-34). Despite the fact that the structures of both the ...