Calcium-independent activation of prothrombin on membranes with positively charged lipids

The activation of prothrombin by factor Xa is strongly accelerated by negatively charged phospholipids plus calcium ions. In this paper we report that positively charged membranes can also stimulate prothrombin activation provided that the activation reaction is carried out in the absence of calcium ions. Membranes composed of a mixture of phosphatidylcholine (PC) and positively charged lipids like stearylamine, sphingosine, or hexadecyltrimethylammonium bromide caused a more than 1000-fold increase of the rate of prothrombin activation. Prothrombin activation by the factor Xa-factor Va complex was also considerably stimulated by such membranes. Stimulation of prothrombin activation by positively charged membranes was suppressed at high ionic strength. This suggests that electrostatic attraction of negatively charged proteins by positively charged membranes is the major driving force in the association of prothrombin and factor Xa with the lipid surface. Calcium ions strongly inhibited prothrombin activation on vesicles composed of PC and stearylamine (80/20 M/M), which indicates that the regions of prothrombin and/or factor Xa containing gamma-carboxyglutamic acid (gla) are important for the interaction of these proteins with positively charged membranes. The importance of the gla domain was confirmed by the observation that PC/stearylamine vesicles had much less effect on the reactions between proteins that lack gla residues gla-domainless (des-1-45) prothrombin, prethrombin 1, prethrombin 2, or gla-domainless (des-1-44) factor Xa]. The efficiency of prothrombin and prothrombin derivatives to act as substrate decreased in the order prothrombin greater than des-1-45-prothrombin = prethrombin 1 greater than prethrombin 2, while prothrombin activation by gla-domainless (des-1-44) factor Xa was hardly stimulated by positively charged membranes.(ABSTRACT TRUNCATED AT 250 WORDS)