Plasma carboxypeptidases as regulators of the plasminogen system.

Abstract

Carboxy-terminal lysine residues on the surface of cells and fibrin bind plasminogen and control its activation. Since plasma contains basic carboxypeptidases, which remove carboxy-terminal lysines from protein substrates, we investigated if these enzymes are involved in the regulation of plasminogen binding sites. Plasma reduced plasminogen binding to cells, and this effect could be ascribed to the activity of the plasma carboxypeptidases. Purified carboxypeptidase N, which is constitutively active, and plasma carboxypeptidase B, which circulates as a zymogen, were both capable of significantly reducing plasminogen binding to cells. Dose titration experiments verified that plasma concentrations of either carboxypeptidase were sufficient to maximally affect plasminogen binding to cells. Furthermore, plasma carboxypeptidase B, but not carboxypeptidase N, reduced the rate of whole blood clot lysis induced by tissue-type plasminogen activator. These findings establish that plasma carboxypeptidases can modulate plasminogen binding to cells and control the rate of fibrinolysis. These functions delineate a novel role for the plasma carboxypeptidases in the regulation of the plasminogen system. (J. Clin. Invest. 1995Invest. . 96:2534Invest. -2538

Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator

“…For example, freshly isolated monocytes, exposed to CPN in the plasma, bind about 30-fold less plasminogen than monocytes cultured for 18 h or longer [82]. As described above, plasmin can cleave both subunits of CPN, causing increased activity; proteolytically cleaved CPN is also more effective in reducing cellular plasminogen binding than native CPN [78]. This may serve as a feedback mechanism to limit cellular plasminogen activation.…”

“…CPU (also called TAFI) exists in blood as a proenzyme and once activated by thrombin/thrombomodulin during coagulation has a short half-life [79,80]. Although CPU reduces plasminogen binding to both cells and in fibrin clots, the effect of CPN is largely restricted to removal of C-terminal Lys residues of plasma membrane proteins [77,78]. By decreasing cellular plasminogen binding [78], CPN can reduce plasmin-dependent extracellular matrix degradation and cellular migration [77,81].…”

“…Although CPU reduces plasminogen binding to both cells and in fibrin clots, the effect of CPN is largely restricted to removal of C-terminal Lys residues of plasma membrane proteins [77,78]. By decreasing cellular plasminogen binding [78], CPN can reduce plasmin-dependent extracellular matrix degradation and cellular migration [77,81]. For example, freshly isolated monocytes, exposed to CPN in the plasma, bind about 30-fold less plasminogen than monocytes cultured for 18 h or longer [82].…”

“…The binding of plasminogen to C-terminal Lys residues on cell surface proteins and fibrin clots increases its activation up to 1000-fold [75][76][77][78]. It was shown that plasma CPU and CPN cleave C-terminal Lys residues on these plasminogen "receptors" and thereby downregulate plasminogen activation [77,78]. CPU (also called TAFI) exists in blood as a proenzyme and once activated by thrombin/thrombomodulin during coagulation has a short half-life [79,80].…”

The topology of plasminogen binding and activation on the surface of human breast cancer cells

“…Glu-plasminogen binding to cells is decreased after treatment of the cells with carboxypeptidase B suggesting that a component of binding is Cterminal lysine-dependent (Redlitz et al, 1995a). The low affinity of glu-plasminogen binding has led to suggestions that binding is not important for activation.…”