Ionic modulation of the effects of heparin on plasminogen activation by tissue plasminogen activator: The effects of ionic strength, divalent cations, and chloride

Abstract

Ionic strength, divalent cations, and Cl − modulate the ability of the glycosaminoglycan heparin to stimulate the activation of human plasminogen (Pg) by tissue-type Pg activator. Kinetic analysis of Pg activation indicates that heparin is inhibitory, stimulatory, or nonstimulatory as a function of ionic strength. While increasing ionic strength inhibits Pg activation in the absence of heparin, in its presence an activation phase followed by an inhibitory phase is observed. Divalent cations, inhibitors of activation in the absence of heparin, increase the rate of activation in its presence. Kinetic analysis demonstrates that divalent cations augment the heparin stimulatory effect a maximum of 60-fold due to increases in k cat without changes in K m of the reaction. This effect is heparinspecific, since activation is not affected by Ca 2+ in the presence of heparan sulfate or de-N-sulfated heparin. Also, Cl − inhibits Pg activation in the presence of heparin by acting as a competitive inhibitor ( K ic of 100 m m). Furthermore, inhibition by Cl − reduces the overall magnitude of heparin stimulation of Pg activation. These results suggest that physiologic ions in combination with heparin may be significant effectors of Pg activation in the vascular microenvironment.