Determinants of prothrombinase activity and modification of prothrombin conversion by thrombin-treated factor V

Progress Curve Analysis of the one stage chromogenic assay for ecarin

Snake venom prothrombin activators such as Ecarin are readily assayed by continuous spectrophotometric monitoring of p-nitroaniline production in a one step assay containing prothrombin and a p-nitroanilide peptide substrate for thrombin. The coupled reactions result in accelerating p-nitroaniline (pNA) production over the course of the assay giving non-linear progress curves, from which initial velocities are not readily obtained. Most studies therefore resort to approximate estimates of activity, based on the absorbance reached at an arbitrary time. A simple kinetic analysis of the coupled reactions shows that the early points of such curves should be fitted by second order polynomials, representing the accelerating reaction rate in μmol pNA/min/min. The first derivative of the polynomial then gives the increasing velocity of pNA production in μmol pNA/min over the time course of the assay. We demonstrate here that, with the substrate S2238, these rates can be converted to absolute thrombin concentrations using the Michaelis-Menten equation, substituted with values for kcat and Km. These thrombin concentrations increase linearly over the time course of the assay allowing the activity to be expressed in units, defined as μmol product/min, most commonly used to report enzyme activity.

Generation of coagulation factor V activity by cultured rabbit alveolar macrophages

Alveolar macrophages are cell's important in immune defense and inflammation in the lung, and the coagulation system participates in these reactions. In earlier experiments, it was found that alveolar macrophages contain and produce tissue factor, the extrinsic clotting pathway activator. The present experiments explore possible production by alveolar macrophages of the sequence of the clotting proteins that interact to form thrombin following initiation of coagulation by tissue factor. In studies using alveolar macrophages purified from rabbits, factor V activity was not detected in cell preparations assayed directly after isolation. However, after short-term culture, we found generation and release of factor V activity by these cells, which was predominantly from subpopulations with densities of 1.060-1.068 g/ml, corresponding to intermediate stages of alveolar macrophage maturation. Cell viability and protein synthesis were required for generation of the activity as shown by inhibitory effects of cell lysis before culture and by effects observed after including puromycin in cultures with viable cells. The activity generated was characterized as factor V by demonstrating specific functional requirements in one- and two-stage coagulation tests. There was no detectable generation in these cultures of factors II, VII, X, or the more recently described factor X-independent monocyte/macrophage prothrombinases. Factor V activity generated by alveolar macrophages may contribute to prothrominase assembly, activation of clotting, and fibrin formation within the alveolus.

The role of platelet factor V in prothrombin conversion

The contribution of platelet factor V to prothrombin conversion was studied in a purified two-stage system designed to measure the ability of factor V to accelerate prothrombin conversion. When unstimulated gel-filtered platelets (GFP) were substituted for both factor V and phospholipid, thrombin evolution was linear following a long lag time. Gel filtration resulted in considerable phospholipid availability with minimal factor V release. Incubating platelets with collagen in increasing concentrations resulted in marked shortening of the lag time, an increase in the initial rate of thrombin formation, and release of platelet factor V. The inhibition of thrombin formation by preincubation of the platelets with metabolic inhibitors is consistent with previous observations that factor V is released from alpha-granules by collagen in a process requiring metabolic energy. Released platelet factor V added to metabolically inhibited platelets reproduces the acceleration of prothrombin conversion demonstrated in GFP incubated with collagen. Furthermore no acceleration of the clotting time at collagen concentrations used in this study was demonstrated in an assay designed to measure available platelet phospholipid in the presence of excess factor V. The rate of increased thrombin generation produced by collagen stimulation is primarily due to released platelet factor V in the system employed.

The assay of factor V in plasma using a synthetic chromogenic substrate

An analytical procedure using a synthetic substrate was devised for the determination of factor V in plasma. Factor V was activated by the reaction of immunobilized thrombin and incubated with an optimal concentration of a factor Xa, phospholipid, calcium and prothrombin. The thrombin generated was determined with the chromogenic substrate (Testzym S-2238). The initial rate of hydrolysis of the substrate was directly proportional to the factor V concentration. Effectors in plasma, such as prothrombin, fibrinogen, antithrombins and bilirubin, were controlled to minimize their effects. The normal value of factor V was from 3.3 to 5.5 units.

Molecular changes associated with proteolysis of bovine factor V by thrombin

Native factor V contains two major polypeptide chains, h and 1. The molecular weights determined by gel electrophoresis in the presence of sodium dodecylsulfate and dithiothreitol (125 000 and 73 000) are in reasonable agreement with those obtained by gel filtration in 5 M guanidine-HC1 (125000 and 64000). Exposure of factor V to thrombin results in cleavage of the heavier chain to an altered form with a molecular weight of 87000. The other fragment of this proteolytic reaction appears to be a carbohydrate-rich piece, which migrates abnormally slowly on gel electrophoresis conducted under denaturing and reducing conditions. Both proteolytic cleavage products remain associated with the light chain during the purification of factor V. The 87000-Mr fragment is present in samples of factor V which are isolated by immunoprecipitation of blood obtained from a single animal by venous catheter. This finding suggests that some proteolysis may occur in vivo. In contrast, the molecular weight of the light chain is unaltered after thrombin proteolysis of either purified factor V or thrombin-treated plasma.

Factor-V activation by thrombin and its role in prothrombin conversion

Purified coagulation factors and specific antibodies to factor V and factor X were used to investigate the action of thrombin on factor V and the mechanism by which thrombin-treated factor V influences prothrombin activation. The formation of a complex or complexes between phospholipid, factor V, factor Xa and calcium was demonstrated by column chromatography on Sephadex gel, and by immunological analysis of the column fractions including the use of solid-phase antibodies. Kinetic experiments demonstrated that generation of thrombin from purified prothrombin was accomplished by this complex. Pre-treatment of factor V with trace quantities of thrombin resulted in increased yield and rate of thrombin generation. It was shown that phospholipid became saturated when incubated with increasing concentrations of factor V and that the initial saturating concentration of the latter was reduced by pre-treatment with thrombin. The findings confirm that optimum conversion of prothrombin to thrombin is accomplished by a complex or complexes of phospholipid, factor V, factor Xa and calcium and it is suggested that thrombin plays an autocatalytic role in these reactions.