Mechanism of action of vitamin K: demonstration of a liver precursor of prothrombin

Bothrops jararaca envenomation: Pathogenesis of hemostatic disturbances and intravascular hemolysis

To attain fully functional biological activity, vitamin-K dependent coagulation factors (VKDCF) are γ-carboxylated prior to secretion from liver. Warfarin impairs the γ-carboxylation, and consequently their physiological function. Bothrops jararaca snake venom (BjV) contains several activators of blood coagulation, especially procoagulant enzymes (prothrombin and factor X activators) and thrombin-like enzymes. In order to clarify the relative contribution of prothrombin and factor X activators to the hemostatic disturbances occurring during experimental B. jararaca envenomation, warfarin was used to deplete VKDCF, prior to BjV administration. Male Wistar rats were pretreated with saline (Sal) or warfarin (War) and inoculated subsequently with BjV or saline, thus forming four groups: Sal + Sal (negative control), Sal + BjV (positive control), War + Sal (warfarinization control), and War + BjV. Three hours after inoculation, prothrombin and factor X levels fell 40% and 50%, respectively; levels of both factors decreased more than 97% in the War + Sal and War + BjV groups. Platelet counts dropped 93% and 76% in Sal + BjV and War + BjV, respectively, and plasma fibrinogen levels decreased 86% exclusively in Sal + BjV. After 6 and 24 h, platelet counts and fibrinogen levels increased progressively. A dramatic augmentation in plasma hemoglobin levels and the presence of schizocytes and microcytes in the Sal + BjV group indicated the development of intravascular hemolysis, which was prevented by warfarin pretreatment. Our findings show that intravascular thrombin generation has the foremost role in the pathogenesis of coagulopathy and intravascular hemolysis, but not in the development of thrombocytopenia, in B. jararaca envenomation in rats; in addition, fibrinogenases (metalloproteinases) may contribute to coagulopathy more than thrombin-like enzymes.

Synthesis of gamma-carboxylated polypeptides by alpha-cells of the pancreatic islets

gamma-Carboxylated proteins were detected in normal human pancreas by immunohistochemistry with a monoclonal antibody (M3B) specific for gamma-carboxyglutamyl residues. Staining appeared to be localized to the glucagon-secreting alpha-cells in the islets of Langerhans. Consistent with this, sections from a glucagonoma were stained much more intensely with the M3B antibody than those from an insulinoma. A murine alpha-cell line (alphaTC1 Clone 9) was cultured and gamma-carboxylated polypeptides, identified immunologically as prothrombin, protein S and (tentatively) Gas6, were isolated from the intracellular compartment by chromatography on an M3B-coupled resin. As in liver, prothrombin is synthesized by alpha-cells as a gamma-carboxylated zymogen that can be cleaved by ecarin to form an active serine protease that is inhibited by hirudin. The pancreas thus appears to be a novel site of synthesis for certain vitamin K-dependent proteins.

Vitamin K-dependent carboxylase: partial purification of the enzyme by antibody affinity techniques

The vitamin K-dependent carboxylase activity of bovine liver microsomes has been purified 500-fold by adsorption to an antiprothrombin column and elution with a dodeca peptide which competes with a prothrombin precursor enzyme recognition site. The purified enzyme is devoid of bound precursors, and has the same ratio of vitamin K epoxidase activity to carboxylase activity as the crude microsomal preparation.

Biosynthesis and clearance of prothrombin in warfarin-treated rats

The steady-state concentration of abnormal plasma prothrombin in warfarin-treated rats (10 mg/kg) was found to be approx. 6% of the plasma prothrombin level in normal rats. The clearance of abnormal plasma prothrombin in warfarin-treated rats was studied using either cycloheximide, to inhibit the synthesis, or vitamin K, to block the appearance of abnormal prothrombin in plasma. The clearance of abnormal plasma prothrombin corresponded to a half-life of approx. 6 h, which is similar to the half-life of normal plasma prothrombin. The de novo synthesis of prothrombin in warfarin-treated and normal rats was compared by measuring the incorporation of [3H]leucine into plasma prothrombin 90 min after an intravenous injection of the isotope. In warfarin-treated rats, accumulated prothrombin precursor was carboxylated and transported into circulation by injecting vitamin K 30 min after isotope administration. On comparing the incorporation of [3H]leucine into plasma prothrombin in warfarin-treated and normal rats, no significant difference in the de novo synthesis was detected. Our results suggest that the secretion of prothrombin in warfarin-treated rats is decreased to 6% of the normal rate. As the de novo synthesis is not affected by warfarin treatment, more than 90% of the newly synthesized prothrombin appears to be degraded intracellularly.

A study on the intracellular transport of prothrombin, albumin and transferrin in rat

The intracellular transport of prothrombin in rat has been studied and compared with the transport of albumin and transferrin. The proteins were immunoisolated from plasma samples after pulse labelling with [3H]leucine and the secretion kinetics were determined. The half-times for secretion (t1/2) were approx. 30, 53 and 75 min for albumin, prothrombin and transferrin, respectively, whereas the minimal transit time for prothrombin was approx. 30 min, and those for albumin and transferrin 15-20 min. After injection of vitamin K-1 into warfarin-treated rats, the accumulated prothrombin precursor was gamma-carboxylated and secreted with a t1/2 of 37 min. This indicates that the gamma-carboxylation of prothrombin in rough endoplasmic reticulum cannot account for the delay in the transport of prothrombin as compared to albumin. Comparison of the incorporation of [3H]leucine and [3H]glucosamine into plasma prothrombin and transferrin suggested that transferrin is secreted randomly from an intracellular pool, whereas prothrombin is transported in a more orderly sequence. Moreover, treatment of rough microsomes with 0.05% sodium deoxycholate indicated that prothrombin is more tightly associated with the membranes of rough endoplasmic reticulum than albumin and transferrin.

Evidence for the glycoprotein nature of vitamin K-dependent carboxylase from rat liver

The ability of the rat liver microsomal vitamin K-dependent carboxylase and microsomal precursors of prothrombin and other vitamin K-dependent proteins to bind to lectin gels has been determined. Under denaturing conditions which dissociate precursor substrates from the carboxylase enzyme, prothrombin precursors and microsomal proteins labeled in gamma-carboxyglutamate residues with [14C]bicarbonate were nearly quantitatively bound to concanavalin A gels. When lentil lectin gels were used, only about one third of these proteins were bound, suggesting a heterogeneity of this glycoprotein pool. Under non-denaturing conditions, both precursor proteins and vitamin K-dependent carboxylase activity were retained on either concanavalin A or lentil lectin gels. These data are consistent with an increase in lectin binding determinants in the precursor-carboxylase complex and are evidence for the glycoprotein nature of this microsomal enzyme.

Heterogeneity of Echis venoms from different sources

The heterogeneity of separate Echis carinatus and Echis coloratus venom lots were evaluated by high performance size-exclusion chromatography, as well as by clotting and esterolytic activities. Each lot of E. carinatus and E. coloratus venoms appeared to be unique. The variation in E. carinatus venom activities from snakes within the same species could lead to contradictory biochemical and/or physiological test results.

On the quantification of prothrombin from different species using Echis carinatus as activator

The generation of thrombin-like activity from rat, human, bovine and mouse prothrombin by Echis carinatus venom (ECV) treatment was compared using a partially purified system (i.e. whole ECV and isolated prothrombin). A rapid increase in coagulant activity was obtained within 0.5 to 2 min., being constant upon further incubation for 60 min. A large variation in coagulant activity of the ECV generated thrombin from the four species was found, whereas no differences were found for the amidolytic activities. The coagulant activities of the ECV generated thrombin was also low compared with the corresponding thrombin activities obtained by physiological activation. Coagulant activity of the ECV generated thrombin levelled off at increasing concentration of prothrombin in the sample as measured by the one-stage coagulation assay. By measuring amidolytic activity a linear relationship to the concentration of prothrombin was found, however. These findings indicate that ECV converts prothrombin from the four different species to a thrombin-like protein with properties distinct from alpha-thrombin. The lack of linearity in the ECV generated clot activity with increasing concentration of prothrombin could be explained by assuming a dimerization of the thrombin-like protein molecules making them less accessible to fibrinogen. The significance of these observations for the quantification of prothrombin from different species is discussed.

Liver and plasma levels of descarboxyprothrombin (PIVKA II) in vitamin K deficiency in rats

Descarboxyprothrombin (PIVKA II) is a precursor of prothrombin without biological activity, and it increases with vitamin K deficiency. We studied the time course changes in liver and plasma levels of PIVKA II during the progress of vitamin K deficiency in rats. Good correlation was observed between liver PIVKA II and plasma PIVKA II and between liver or plasma PIVKA II and plasma prothrombin in experiments in which rats were fed a vitamin K-deficient diet. Feeding of a vitamin K-deficient diet or fasting caused marked increases in liver and plasma PIVKA II in male rats and a weaker response in female rats. Warfarin, a vitamin K antagonist, caused an abrupt increase in liver PIVKA II, but the increase in plasma PIVKA II was delayed about 3 hr. Plasma prothrombin decreased from about 30 min later. Factor VII decreased similarly to prothrombin, and changes in the prothrombin time and activated partial thromboplastin time were slower than the changes in these substances. Sex differences were not seen in these warfarin actions. These observations indicate that liver and plasma PIVKA II are sensitive markers of vitamin K deficiency in rats, and assay of PIVKA II can be useful for analyzing the action mechanism of drugs which influence blood coagulation.

[Preprothrombin in acute viral hepatitis B]

In vitamin K deficiency or treatment with vitamin K antagonists, a precoagulant of prothrombin (Factor II) called preprothrombin has been established. We measured preprothrombin with Clarke-Freeman electrophoresis in 26 patients with acute viral hepatitis (21 HBS-AG positive) who did not suffer from vitamin K deficiency. Prothrombin and the vitamin-K dependent Factors VII, IX, and X were determined by standard coagulometric methods. Prothrombin was additionally estimated by immunoelectrophoresis according to Laurell. Three patients with acute HBS-AG positive hepatitis showed preprothrombin in their plasma. The activity of Factors II, VII, IX, and X was slightly below normal with normal concentration of Factor II in the immunoelectrophoresis. Liver parenchymal damage and cholestasis were slight; the pseudocholinesterase showed subnormal levels in all three patients. Possible causes for the appearance of preprothrombin in the peripheral blood in acute viral hepatitis and the possible connections with liver cell damage are discussed.

Cleavage and activation of human prothrombin by Echis carinatus venom

The cleavage of human prothrombin by partially purified Echis carinatus venom (ECV) was investigated in the present report. Incubation of prothrombin with ECV resulted in the rapid cleavage of prothrombin to alpha-thrombin, with the release of fragment-1 and fragment-2. When dansyl arginine-N-(3-ethyl-1,5-pentanediyl) amide (DAPA), a very effective inhibitor of thrombin, was included in the ECV-prothrombin solution, meizothrombin was rapidly formed. Only small amounts of meizothrombin-1 could be detected. Prolonged incubation (23 h) in the presence of DAPA, however, resulted in nearly quantitative conversion of meizothrombin to meizothrombin-1 and fragment-1. Kinetic studies strongly suggested that the conversion of meizothrombin to meizothrombin-1 was due to ECV and not meizothrombin autolysis. In addition, EDTA, which inhibits ECV, blocked the cleavage of meizothrombin. Amino terminal sequence analysis indicated that ECV cleaves human prothrombin at two sites; Gly158-Ser159 and Arg322-Ile323. The former site differs from the site of autolytic cleavage of meizothrombin which occurs at Arg155-Ser156. In contrast to reports in the literature, the results of the present study indicate that the release of fragment-1 does not precede activation of human prothrombin by ECV.

Characterization of vitamin K-dependent carboxylase from the livers from the adult ox and dicoumarol-treated calf

The properties of the microsomal vitamin K-dependent carboxylase from the livers of the adult ox and dicoumarol-treated calf were investigated. The enzymes from both sources utilized glutamic residues of synthetic peptides as substrates and could be solubilized with Triton X-100 similarly to the enzyme from vitamin K-deficient rat liver. Under the optimal assay conditions, the microsomes from calf liver had peptide carboxylase activity comparable with that of the rat liver microsomes and 6.5-fold that of adult ox liver microsomes. The apparent Km for reduced vitamin K and the ionic strength optima of the calf and adult ox enzyme clearly differ from those of the rat enzyme. Pyridoxal phosphate activated the adult ox carboxylase only slightly, whereas the calf enzyme was activated by pyridoxal phosphate as effectively as was the enzyme from the vitamin K-deficient rat. Mn2+ activated the adult ox enzyme 9-fold and calf enzyme 22-fold under optimal conditions (no KCl). Three other divalent metal cations (Ca2+, Ba2+, and Mg2+) activated the adult ox and calf enzymes to about half the extent caused by Mn2+, KCl inhibited this activation. The vitamin K-dependent carboxylase from the dicoumarol-treated calf is apparently more tightly bound to the microsomal membrane than is the adult ox enzyme. In many other respects (pH optimum), temperature optimum, Km values for peptide substrate, substrate specificity, inhibitor effects), the properties of the adult ox and calf enzymes resemble closely those of the rat enzyme.

Gamma-carboxyglutamic acid

Gamma-carboxyglutamic acid is an amino acid with a dicarboxylic acid side chain. This amino acid, with unique metal binding properties, confers metal binding character to the proteins into which it is incorporated. This amino acid has been discovered in blood coagulation proteins (prothrombin, Factor X, Factor IX, and Factor VII), plasma proteins of unknown function (Protein C, Protein S, and Protein Z), and proteins from calcified tissue (osteocalcin and bone-Gla protein). It has also been observed in renal calculi, atherosclerotic plaque, and the egg chorioallantoic membrane, among other tissues. Gamma-carboxyglutamic acid is synthesized by the post-translational modification of glutamic acid residues. This reaction, catalyzed by a hepatic carboxylase, requires reduced vitamin K, oxygen, and carbon dioxide. The function of gamma-carboxyglutamic acid is uncertain. In prothrombin gamma-carboxyglutamic acid residues bound to metal ions participate as an intramolecular non-covalent bridge to maintain protein conformation. Additionally, these amino acids participate in the calcium-dependent molecular assembly of proteins on membrane surfaces through intermolecular bridges involving gamma-carboxyglutamic acid and metal ions.

Post-translational carboxylation of preprothrombin

In summary, in this review on the function of vitamin K in post-translational modification of precursor proteins by carboxylation of certain glutamyl residues, I have tried to cover in particular the recent work on the reaction, the enzymes involved and the mechanisms being considered. In doing this I have also considered vitamin K, its discovery, its functional form and the possible relation of its metabolism to the carboxylation reaction. Equally the various vitamin K-dependent gla-containing proteins currently known have been described. The carboxylation of synthetic small molecule exogenous substrates and the synthesis and metabolism of the products of carboxylation are of great help in studying the reaction. Structural specificity of vitamin K analogs in vivo and in vitro has been compared and the use of various antagonists in vivo and in vitro considered in attempts to gain an understanding of the overall reaction. The reactions subsequent to carboxylation, e.g., the activation of prothrombin to thrombin via serine proteases and the related activation of the other vitamin K-dependent proteins have not been considered in this review. The review has not covered prothrombin or other vitamin K-dependent protein isolation, nor the determination of these proteins. As the vitamin K-dependent protein carboxylation story has developed over the past six years, a number of reviews have been written which help in keeping up with the various aspects of the field as it has expanded. These reviews refer to many of the papers I have had to eliminate due to space limitations. They are referenced as 469-489. The review is in no sense comprehensive and many papers have been missed or only mentioned. I have tried to concentrate on the more recent work and, thus, much of the very fine work of the 1940's on vitamin K chemistry is hardly mentioned. Some redundancy has been built into the organization of the review so that a reader can obtain a reasonable view of any one section without having to search the whole review for all possible relevant information on any particular part of the field.

Characteristics of an acetone powder preparation of the vitamin K-dependent gamma-glutamyl carboxylase

An acetone powder, prepared from the liver microsomes of vitamin K-deficient rats, retains an active vitamin K-dependent gamma-glutamyl carboxylase. While the basic requirements of the enzyme are similar to those of the carboxylase of either resuspended microsomes or detergent-solubilized microsomes, the acetone powder preparation reveals some additional properties of the carboxylase. Carboxylation of the synthetic pentapeptide substrate phenylalanylleucyl-glutamyl-glutamyl-valine can occur in the absence of nonionic detergent; however, when vitamin K hydroquinone drives the acetone powder carboxylation nonionic detergent is require for maximal activity. Experiments are described in which the acetone powder is incubated with the pentapeptide, pelleted by centrifugation, resuspended with fresh reactants, and incubated again. They suggest that the low V for the carboxylase, observed by all investigators, is, at least in part, not the result of irreversible enzyme inactivation nor depletion of reactants, but rather accumulation of a yet to be identified inhibitor(s). The acetone powder prepared from microsomes derived from livers of nutritionally normal cows contains vitamins vitamin K-dependent gamma-glutamyl carboxylase. This enzyme can be solubilized from the powder using Triton X-100 and could provided a large supply of starting material for enzyme purification.

Factor II antigen in liver disease and warfarin-induced vitamin K deficiency: correlation with coagulant activity using Echis venom

Four coagulation factors (II, VII, IX, X) are synthesized in the liver as precursor forms. Vitamin K is necessary for their conversion to functional factors. Factor II precursor is antigenically similar to factor II and can be activated to form thrombin by Echis carinatus venom. Forty-one patients with liver disease and 30 patients with vitamin K deficiency (Warfarin treated), were compared with 51 normal patients. Rats with CCl4-induced hepatic necrosis and animals given Warfarin were also studied. The following measurements were made: standard prothrombin time; Echis clotting time; factor II coagulant assay with thromboplastin; factor II assay with Echis venom; and factor II antigen (human) by electroimmunoassay. In animals and humans with liver disease, factor II was reduced, as measured by all techniques. With vitamin K deficiency functional factor II was reduced, but factor II antigen and Echis factor II activity were normal. THe data suggest that the prothrombin time and Echis coagulation methods can be used to differentiate whether the coagulopathy in liver disease is due to reduced precursor levels of factor II, vitamin K deficiency (ie, impaired formation of functional from precursor factor II), or both.

Characterization of a variant prothrombin in a patient congenitally deficient in factors II, VII, IX and X

An 18-month-old child, who had no evidence of liver disease, malabsorption, or chronic ingestion of coumarin compounds, was found to have plasma deficiencies of factors II, VII, IX and X. Assays for factor II and X by immunological techniques (antibody neutralization and immunoelectrophoresis) revealed normal or elevated antigenic activity of these factors, suggesting the presence of abnormal protein variants in the patient's plasma. On two-dimensional immunoelectrophoresis of the patient's plasma in calcium, a normal and an abnormal population of prothrombin were seen. The abnormal prothrombin had a mobility more anodal than that of normal prothrombin, but less anodal than that of acarboxyprothrombin. The abnormal prothrombin, in contrast to acarboxyprothrombin, adsorbed readily to both aluminum hydroxide and barium citrate, and could be identified by two-dimensional immunoelectrophoresis of a barium citrate eluate. We suspect that the abnormal variant represents a partially carboxylated prothrombin.

Mechanism of action of vitamin K: synthesis of gamma-carboxyglutamic acid

Vitamin K (2-methyl-3-phytyl-1,4-naphthoquinone) is required for the synthesis of prothrombin, Factor VII, Factor IX, Factor X, and a number of newly discovered proteins. These plasma proteins participate in calcium-dependent phospholipid membrane interactions which are mediated through the presence of gamma-carboxyglutamyl residues in their amino-terminal region. Vitamin K is required for the postribosomal conversion of glutamyl residues in liver precursors of these proteins to gamma-carboxyglutamyl residues in the completed plasma proteins. In the absence of vitamin K, or in the presence of vitamin K antagonists, animals produce plasma forms which lack the carboxylated residue. These proteins are nonfunctional because of their lack of phospholipid interaction. The vitamin K-dependent carboxylase which carries out this reaction has been studied in rat liver microsomal preparations where it will carboxylate the endogenous precursor proteins. Low-molecular-weight glutamyl-containing peptide substrates, such as Phe-Leu-Glu-Glu-Leu, which are homologous to regions of the prothrombin precursor, will also serve as substrates for the detergent-solubilized enzyme. This enzyme has been shown to require the reduced form of the vitamin and O2 but no ATP or a biotin-containing protein for its activity. The same microsomal preparations will also convert vitamin K to its 2,3-epoxide, and it is possible that activity may be related to the role of the vitamin in driving the carboxylase reaction.

Investigation and standardization of prothrombin times in chickens

This investigation was undertaken to standardize the determination of the one-stage prothrombin time for use with chickens. Homologous thromboplastin was essential and the most active thromboplastin was obtained from chickens four-weeks old or younger. Acetone-dried brain powder could be stored for at least 4 months at -15 degrees C. without loss of activity. Extraction of brain powder with 0.025 M CaCl2 at 42 degrees C. gave better thromboplastic activity than the standard extraction with physiological saline at room temperature. Thromboplastin solutions could be stored in ice water for only 6 hours without loss of activity. Citrate concentration had to be increased from the usual 0.10 M to 0.18 M to prevent premature clotting of plasma. Plasma donor age had no effect on the prothrombin times. Freezing and thawing as well as storage of plasma in the frozen state increased the prothrombin times. Using the best conditions, the mean prothrombin time for 1200 birds determined over a 6-month period was 9.4 sec. with an individual range of 7.18-11.4 sec. This represents a significantly lower prothrombin time with lower variability than that reported in the literature.