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The Conversion of Active to Latent Plasminogen Activator Inhibitor-1 Is an Energetically Silent Event

Christian Boudier ^*, Ann Gils , Paul J. Declerck  and Joseph G. Bieth ^*

^* Laboratoire d'Enzymologie, INSERM U392, Faculté de Pharmacie, Université Louis Pasteur de Strasbourg, Illkirch, France; and Laboratory for Pharmaceutical Biology and Phytopharmacology, Faculty of Pharmaceutical Sciences, Katholieke Universiteit, Leuven, Belgium

Correspondence: Address reprint requests to Dr. Christian Boudier, INSERM U392, Faculté de Pharmacie, Université Louis Pasteur de Strasbourg, 67400 Illkirch, France. Tel.: 33-3-90-24-41-83; Fax: 33-3-90-24-43-08; E-mail: christian.boudier{at}pharma.u-strasbg.fr.

PAI-1 is a proteinase inhibitor, which plays a key role in the regulation of fibrinolysis. It belongs to the serpins, a family of proteins that behave either as proteinase inhibitors or proteinase substrates, both reactions involving limited proteolysis of the reactive center loop and insertion of part of this loop into ß-sheet A. Titration calorimetry shows that the inhibition of tissue-type plasminogen and pancreatic trypsin are exothermic reactions with H = –20.3, and –22.5 kcal.mol^–1, respectively. The Pseudomonas aeruginosa elastase-catalyzed reactive center loop cleavage and inactivation of the inhibitor is also exothermic (H = –38.9 kcal.mol^–1). The bacterial elastase also hydrolyses peptide-bound PAI-1 in which acetyl-TVASSSTA, the octapeptide corresponding to the P₁₄-P₇ sequence of the reactive center loop is inserted into ß-sheet A of the serpin with H = –4.0 kcal.mol^–1. In contrast, H = 0 for the spontaneous conversion of the metastable active PAI-1 molecule into its thermodynamically stable inactive (latent) conformer although this conversion also involves loop/sheet insertion. We conclude that the active to latent transition of PAI-1 is an entirely entropy-driven phenomenon.