Factor V: an activator of factor Xa in the absence of prothrombin

Activation of prothrombin as measured by the conversion of Nalpha-benzoylarginine ethyl ester

The activation of prothrombin has been studied by using highly purified preparations of activated factor X1 and activated factor X2, factor V and prothrombin. The rate of prothrombin activation was followed using an esterase assay involving the conversion of N alpha-benzoylarginine ethyl ester (BAEE) by thrombin generated in the course of prothrombin activation. The rate of thrombin generation increased by about 26000-fold when factor V and phospholipid were added to prothrombin, factor Xa and calcium. A comparison of the rates of thrombin formation obtained with activated factor X1 and activated factor X2 showed that activated factor X1 had only 70% of the biological activity of activated factor X2. Attempts to explain the rate of prothrombin activation and the difference between the activity of activated factor X1 and activated factor X2 are discussed.

The effect of gamma-carboxyglutamate residues on the enzymatic properties of the activated blood clotting factor X. I. Activity towards synthetic substrates

The esterolytic and amidolytic properties of activated blood coagulation factor X (factor Xa) and the analogous decarboxy species were compared in order to find out if the gamma-carboxyglutamic acid residues influence the function of the active centre. It was found that the two proteins (1) showed similar kinetic parameters when titrated with p-nitrophenyl-p'-guanidinobenzoate hydrochloride (2) had a similar Km and kcat for various synthetic chromogenic tri- and tetrapeptides and (3) were inhibited in the same way by benzamidine. Further it was observed that (4) Ca2+ inactivates factor Xa, but has no influence on the amidase activity of decarbyxyfactor Xa (5) factor V prevents Ca2+-induced inactivation of factor Xa but does not influence the amidase activity of both factor Xa and decarboxyfactor Xa. We conclude that the interaction of the gamma-carboxyglutamic acid residues with Ca2+ in factor X has no measurable influence on the properties of the active site per se.

Potentiation of the function of Hageman factor fragments by high molecular weight kininogen

Patients lacking high molecular weight (HMW) kininogen have profound abnormalities of the Hageman factor-dependent pathways of coagulation, kinin formation, and fibrinolysis. The ability of HMW kininogen to potentiate the Hageman factor fragments (HFf) activation of prekallikrein and Factor XI in plasma was studied. HFf only partially converted Factor XI to XIa and prekallikrein to kallikrein in plasma deficient in HMW kininogen (Williams trait), while enhanced activation of Factor XI and prekallikrein by HFf resulted after reconstitution with HMW kininogen. In a system using highly purified components, HMW kininogen increased the initial rate of prekallikrein activation whether the kallikrein formed was assayed by arginine esterase activity or kininforming ability. The potentiation of prekallikrein activation occurred over a 12-fold range of enzyme (HFf) concentration and was nonhyperbolic with respect to substrate (prekallikrein). HMW kininogen exerted its effect even in the absence of prekallikrein since the hydrolysis of acetylglycyl-lysine methyl ester by HFf was increased by HMW kininogen. These results suggest that one of the functions of HMW kininogen is to augment the catalytic action of HFf.

Interactions of the components of the prothrombinase complex

The interaction of components of the prothrombinase complex, i.e. bovine Factor X or Factor Xa, bovine Factor V or Factor Va, phospholipid, and Ca2+, in various combinations was studied primarily by a gel filtration technique. In experiments, in which phospholipids ranging from those isolated from naturally occurring sources to those long chain (18 : 1) as well as short chain 6 : 0 and 7 : 0 fatty acids prepared by chemical and enzymatic synthesis were used, it was evident that a net negative surface charge on the lipid dispersions was one of the important requirements for interaction. Though the short chain fatty acid phospholipids interacted with the proteins of the prothrombinase complex, there was invariably a diminution in the activity of the enzyme complex. It was established that Factor V or Va did not bind Ca2+ and that the binding of either of these factors with phospholipids (with a net negative charge) was not dependent on Ca2+. However, the interaction of Factor X or Factor Xa with phospholipids with a negative charge required Ca2+. It was shown that Factor X could bind to the same type of lipid of lipid surface as that noted for Factor Xa. Of interest was the apparent difference in the phospholipid binding characteristics of the two variant forms of bovine plasma Factor X, i.e. X1 and X2, which might in part explain the differences in their specific activities. Of importance was the lack of demonstrable complex formation between Factors II, X and V in the absence of phospholipids and/or in the presence or absence of Ca2+. The significance of these results as they might apply to the configuration of the prothrombinase complex and its interaction with prothrombin plus the usefulness of the short chain fatty acid phospholipid in exploring these lipid-protein interactions are discussed.