Regulation of the Activities of Thrombin and Plasmin by Cholesterol Sulfate as a Physiological Inhibitor in Human Plasma

: Thrombin and plasmin, both of which are serine proteases in the plasma of vertebrates, play essential roles in blood clotting and fibrinolysis, respectively, and regulation of their activities is important to suppress the excessive reactions within the vascular network and to prevent tissue injury. Along with the peptidic inhibitors belonging to the serpin family, we found that cholesterol sulfate (CS), which is present at the concentration of 2.0+/-1.2 nmol/ml in human plasma, was a potent inhibitor of both plasma thrombin and plasmin. Thrombin, as determined both using a chromogenic substrate and the natural substrate, fibrinogen, was inactivated upon reaction with CS in a dose-dependent manner, but not in the presence of the structurally related steroid sulfates, I3SO3-GalCer and II3NAalpha-LacCer, suggesting that both the sulfate group and the hydrophobic side chain of CS are necessary for the inhibitory activity of CS. Preincubation of thrombin with CS at 37 degrees C for 10 min was required to achieve maximum inhibition, and virtually complete inhibition was achieved at a molar ratio of CS to thrombin of 18:1. CS-treated thrombin had the same Km and a lower Vmax than the original enzyme, and a higher molecular weight. The molecular weight and activity of the original enzyme were not observed on the attempted separation of the CS-treated enzyme by gel permeation chromatography and native PAGE, indicating that the inactivation of thrombin by CS is irreversible. In contrast, CS was readily liberated from the enzyme by SDS-PAGE, suggesting that hydrophobic interactions are involved in the CS-mediated inactivation of thrombin. When acidic lipids were reacted with thrombin after dissolving them in DMSO, I3SO3-GalCer, steroid sulfates and II3NAalpha-LacCer, as well as CS, but not SDS and sodium taurocholate, exhibited inhibitory activity, probably due to micellar formation facilitating interaction between thrombin and negatively charged lipids. On the other hand, plasmin, as determined using a chromogenic substrate, was more susceptible to acidic lipids than thrombin. CS, I3SO3-GalCer and II3NAalpha-LacCer, all of which are present in serum, inhibited the activity of plasmin in aqueous media, as well as in DMSO-mediated lipid solutions. Thus, acidic lipids in plasma were demonstrated to possess regulatory activity as endogenous detergents toward both enzymes for blood clotting and fibrinolysis.