alpha 1-Antitrypsin (alpha 1-AT) was recently identified as a major physiologic plasma inhibitor of activated protein C. The reaction with activated protein C of recombinant alpha 1-AT containing amino acid substitutions at the reactive center was studied. The substitution of Arg358 for Met, as observed in a patient with a severe bleeding disorder with the mutant alpha 1-AT Pittsburgh, increased the association rate constant for activated protein C from 1.1 x 10(1) to 4.9 x 10(4) M-1 s-1. The association rate constant of activated protein C with protein C inhibitor, a native plasma serpin that contains Arg354 at the reactive site, is 6 x 10(3) M-1 s-1 in the absence of heparin. Plasma containing 4 microM [Arg358]alpha 1-AT inhibited activated protein C activity by greater than 95% in 15 s, and the inhibited activated protein C was shown by immunoblotting to exist as activated protein C-inhibitor complexes. In controls 50% loss of activated protein C activity in normal plasma occurred in 19 min. Double-substituted [Pro357,Met358]alpha 1-AT----[Ala357,Arg358]alpha 1-AT had similar reactivity toward activated protein C as the single-substituted [Arg358]alpha 1-AT. Thus, replacement of the reactive center Met358 of alpha 1-AT by Arg358, analogous to Arg354 of protein C inhibitor, results in an activated protein C inhibitor that is more potent than either of the native inhibitors. Comparison of the association rate constant of the [Arg358]alpha 1-AT for activated protein C to that for thrombin (4 x 10(4) versus 3 x 10(5) M-1 s-1) suggests that thrombin would be more effectively inhibited than activated protein C, thereby giving an explanation for bleeding rather than thrombosis in the alpha 1-AT Pittsburgh patient.