Plasminogen Activator Inhibitor-Type1 (PAI-1)

PAI-1 is a glycoprotein with an approximate Mr of 50,000, consisting of 379 amino acids, lacking cysteine residues. It is a member of the serine protease inhibitor (Serpin) superfamily with homology with antithrombin III, PAI-2, alpha-2-antiplasmin, C1-inhibitor, and alpha-1-protease inhibitor.

PAI-1 reacts about equally well with single and double chain t-PA, with second order rate constants in the range of 10⁷ to 10⁸ M^-1s^-1, but not with single chain u-PA. The interaction of PAI-1 and t-PA is reported to be about 5-10-fold slower in the presence of fibrin. The interaction of plasminogen activator with PAI-1 probably results first in the formation of a reversible complex which, in a second step, becomes covalent after the cleavage of the peptide bond Arg 346 - Met 347 has taken place. Finally, a 33 amino acid stretch (Met 347 - Pro 379) is released from the complex reducing the Mr of activator-inhibitor complex by approximately 4.2kD. The plasma concentration of PAI-1 is in the range of 1nM, but the plasma compartment accounts only for  three quarters of the total PAI-1 content in blood. A large amount of   PAI-1 is contained in platelets, which upon physiological stimuli, e.g. thrombin, releases PAI-1.

Purified plasminogen activator inhibitor 1 from the human melanoma cell line MJZJ according to the    method of Wagner et al. (O.F.Wagner, B.R. Binder: Purification of an active plasminogen activator inhibitor immunologically related to the endothelial type plasminogen inhibitor from conditioned media of a human melanoma cell line. J. Biol. Chem. 261: 14474-14481, 1986).