Abstract

The interaction between type 1 plasminogen activator inhibitor (PAI-1), a serine protease inhibitor, and the three serine proteases generated during contact activation of plasma was studied using functional and immunologic approaches. Incubation of Factor XIIa, Factor XIa, and plasma kallikrein with either purified PAI-1 or platelet-derived PAI-1 resulted in the formation of sodium dodecyl sulfate-stable complexes as revealed by immunoblotting techniques. Functional assays indicated that Factor XIa and, to a lesser extent, Factor XIIa and plasma kallikrein neutralized the ability of purified PAI-1 to bind to immobilized tissue-type plasminogen activator (t-PA). Immunoblotting demonstrated that these enzymes also neutralized the ability of PAI-1 to form complexes with fluid-phase t-PA. Clot lysis assays employing purified proteins and 125I-fibrinogen were used to investigate the profibrinolytic effect of these contact activation enzymes. At enzyme concentrations that did not result in direct activation of plasminogen, only Factor XIa was capable of stimulating the lysis of clots supplemented with both t-PA and PAI-1. As a consequence of their interactions with PAI-1, the amidolytic activity of Factor XIIa, Factor XIa, and plasma kallikrein was neutralized by this inhibitor in a time-dependent and concentration-dependent manner. Minimum values estimated for the apparent second-order rate constant of inhibition were 1.6 x 10(4), 2.1 x 10(5), and 6.0 x 10(4) M-1 s-1 for Factor XIIa, Factor XIa, and plasma kallikrein, respectively. These data define new reactions between coagulation and fibrinolysis proteins and suggest that a major mechanism for stimulation of the intrinsic fibrinolytic pathway may involve neutralization of PAI-1 by Factor XIa.

Highlights

  • Two immunologically distinct classes of plasminogen activaactivation of plasma was studied using functional and tor (PA)’ exist,the tissue-type (t-PA) and theurokinase-like

  • Immunoblotting dem- brinolysis, is dependent on the activation of the plasma proenonstrated that thesenzymes neutralizedthe abil- zymes that participate in the contactphase of blood coagulaity of PAI-1 to formcomplexes with fluid-phase t-PA. tion (i.e.Factor XII, Factor XIa,nd plasmaprekallikrein)(5)

  • We present studies demonstrating complex formation and reciprocal inhibition between PAI-1 and each of the enzymes of the contact system (i.e. F.XIIa, F.XIa, and plasma kallikrein)

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Summary

Introduction

Two immunologically distinct classes of plasminogen activaactivation of plasma was studied using functional and tor (PA)’ exist,the tissue-type (t-PA) and theurokinase-like. We present studies demonstrating complex formation and reciprocal inhibition between PAI-1 and each of the enzymes of the contact system (i.e. F.XIIa, F.XIa, and plasma kallikrein).

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