Serpins inhibit serine proteinases through formation of stable H complexes. In this study we have evaluated the effects of PAI-1 neutralizing antibodies (MA) on the stability of PAI-1/proteinase complexes, partially destabilized through prolongation of the reactive center loop. MA-8H9D4, reacting with residues Arg(300), Gln(303), and Asp(305), had no effect on the stability. In contrast, MA-33H1F7 and MA-55174C12, reacting with alpha-helix F and the turn connecting hF with s3A, affected significantly and proteinase-dependently formed PAI-1/protemase complexes. That is, MA-33H1F7 increased the stability of both PAI-1/t-PA and u-PA complexes (7- and 3-fold, respectively) whereas MA-55F4C12 stabilized PAI-1/t-PA complexes (3-fold) but destabilized PAI-1/u-PA complexes (2-fold). It is concluded that interference with the docking site of the cognate proteinase in the preformed serpin/proteinase complex may affect the intrinsic stability. We hypothesize that this is the consequence of a decreased or increased torsion of the RCL on the catalytic triad in the proteinase. (C) 2003 Elsevier Inc. All rights reserved.