Bovine factor VII

The propeptide region of clotting factor IX is a signal for a vitamin K dependent carboxylase: evidence from protein engineering of amino acid -4

Homologous "propeptide" regions are present in a family of vitamin K-dependent mammalian proteins, including clotting factors II, VII, IX, X, protein C, protein S and bone "gla" proteins. To test the hypothesis that the propeptide is a signal for the correct gamma-carboxylation of the adjacent gamma-carboxy region, we have mutated amino acid -4 of human factor IX from an arginine to a glutamine residue, by M13-directed site-specific mutagenesis of a cDNA clone. After expression of mutant factor IX in dog kidney cells, we find that it is secreted into the medium in a precursor form containing the propeptide, and is inefficiently gamma-carboxylated compared to the control, wild-type, recombinant factor IX. This result supports the hypothesis that the propeptide region is required for efficient gamma-carboxylation of factor IX in dog kidney cells. Furthermore, it confirms previous results that arginine at amino acid -4 is required for correct propeptide processing.

Membrane-dependent coagulation reaction is independent of the concentration of phospholipid-bound substrate: fluid phase factor X regulates the extrinsic system

Negatively charged phospholipids accelerate blood coagulation; one suggestion is that the binding of the gamma-carboxyglutamic acid-containing zymogens to these lipid surfaces increases their effective concentration as substrates. Alternatively, the charged phospholipids could enhance the direct interaction of substrate with the catalytic complex, which is localized at the membrane surface. We distinguished the alternatives by using prothrombin fragment 1 to compete with the substrate for membrane binding sites without interfering with the direct enzyme-substrate interaction. In a tissue factor-factor VIIa system containing neutral phospholipids (to which the substrate does not bind), prothrombin fragment 1 had no significant effect on factor X activation (Km, 877 +/- 111 nM and 791 +/- 103 nM, with and without prothrombin fragment 1, respectively). In contrast, in a system containing 30% phosphatidylserine, prothrombin fragment 1 displaced phospholipid-bound factor X, increasing the free factor X concentration and the reaction velocity in all 22 determinations. As the velocity increases correlated only with the free factor X concentration, we conclude that the free factor X concentration controls this reaction velocity. The Km in the 30% phosphatidylserine system, calculated using free factor X concentration, was 41 +/- 5 nM and 63 +/- 9 nM, with and without prothrombin fragment 1, respectively. Thus, the negatively charged lipids decreased the intrinsic Km by over 90%. The methodology employed should be applicable to ligand-receptor systems in which ligand binds nonspecifically to the membrane surface.

Studies on the procoagulant activity of human amniotic fluid. I. Stability and coagulation factor requirements

A fraction of human amniotic fluid possessing procoagulant activity was purified 35-70 fold by gel filtration on Sepharose 4B. The activity eluted in the void volume indicating a particle size in excess of 5 X 10(6) daltons. The amniotic fluid factor (AFF) activity is stabilized on storage in the presence of Ca++ ions which could not be replaced by Mg++. Addition of phospholipids resulted in accelerated loss of activity. Steps taken to remove factor VII did not affect the activity, but factor X and V are required.