The suicide substrate reaction between plasminogen activator inhibitor 1 and thrombin is regulated by the cofactors vitronectin and heparin.
The interaction of thrombin with plasminogen activator inhibitor 1 (PAI-1) is shown to result in the simultaneous formation of both cleaved PAI-1 and a sodium dodecyl sulfate-stable thrombin-PAI-1 complex. The kinetics of this reaction can be described by a "suicide substrate" mechanism that includes a branched reaction pathway, which terminates in either the stable inhibitor-enzyme complex or the cleaved inhibitor plus free enzyme. Because of the branched pathway, approximately three moles of PAI-1 are needed to completely inhibit one mole of thrombin. Heparin and vitronectin enhance the rate of inhibition from 9.8 x 10(2) L mol(-1) s(-1) to 6.2 x 10(4) L mol(-1) s(-1) and 2.1 x 10(5) L mol(-1) s(-1), respectively, under optimal conditions. In addition to enhancing the rate of inhibition, both cofactors increase the apparent stoichiometry of the PAI-1-thrombin interaction, with cofactor concentration dependencies similar to the inhibition reaction. Thus, at 37 degrees C approximately six cleavage reactions occur per inhibition reaction. Therefore, thrombin will efficiently inactivate PAI-1 in the presence of either vitronectin or heparin, unless a sufficient excess of the inhibitor is present. These results show that physiological cofactors are able to switch a protease-serpin inhibition reaction to a substrate reaction, depending on the local concentrations of each of the components.
van Meijer
M
M
Department of Biochemistry of the Academic Medical Center, University of Amsterdam, the Netherlands.
Smilde
A
A
Tans
G
G
Nesheim
M E
ME
Pannekoek
H
H
Horrevoets
A J
AJ
D000818:Animals; D006493:Heparin / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D017395:Plasminogen Activator Inhibitor 1 / Q000378:metabolism*; D013379:Substrate Specificity / Q000187:drug effects; D013552:Swine; D013917:Thrombin / Q000378:metabolism*; D019096:Vitronectin / Q000378:metabolism* / Q000494:pharmacology
D017395:Plasminogen Activator Inhibitor 1; D019096:Vitronectin; D006493:Heparin; D013917:Thrombin