Vitronectin functions as a cofactor for rapid inhibition of activated protein C by plasminogen activator inhibitor-1. Implications for the mechanism of profibrinolytic action of activated protein C.
Activated protein C (APC) is a natural anticoagulant in plasma that down-regulates the coagulation cascade by degrading factors Va and VIIIa. In addition to its anticoagulant function, APC is also known to possess a profibrinolytic property. This property of APC has been attributed to its ability to neutralize PAI-1, thereby increasing the concentration of tissue plasminogen activator in plasma leading to up-regulation of the fibrinolytic cascade. This hypothesis, however, has not been well established, since the concentration of PAI-1 in plasma is low, and its reactivity with APC is very slow in a purified system. Here we demonstrate that vitronectin enhances the reactivity of PAI-1 with APC approximately 300-fold making PAI-1 the most efficient inhibitor of APC thus far reported (k(2) = 1.8 x 10(5) m(-)1 s(-)1). We further show that PAI-1 inhibition of the Glu(192) --> Gln mutant of APC is enhanced approximately 40-fold, independent of vitronectin, suggesting that vitronectin partially overcomes the inhibitory interaction of PAI-1 with Glu(192). Additionally, we show that PAI-1 inhibition of the Lys(37)-Lys(38)-Lys(39) --> Pro-Gln-Glu mutant of APC is severely impaired, suggesting that, similar to tissue plasminogen activator, the basic 39-loop of APC plays a critical role in the reaction. Together, these results suggest that vitronectin functions as a cofactor to promote the profibrinolytic activity of APC.
Edward A. Doisy Department of Biochemistry and Molecular Biology, Saint Louis University School of Medicine, Saint Louis, Missouri 63104, USA. rezaiear@slu.edu
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D019943:Amino Acid Substitution; D004789:Enzyme Activation; D005342:Fibrinolysis; D006801:Humans; D007700:Kinetics; D017395:Plasminogen Activator Inhibitor 1 / Q000494:pharmacology*; D011486:Protein C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011994:Recombinant Proteins / Q000494:pharmacology; D019096:Vitronectin / Q000378:metabolism*
D017395:Plasminogen Activator Inhibitor 1; D011486:Protein C; D011994:Recombinant Proteins; D019096:Vitronectin