Vitamin K dependent

Work in the mid-1970s demonstrated that the vitamin K-dependent step in prothrombin synthesis was the conversion of glutamyl residues to y-carboxyglutamyl residues. Subsequent studies more cleady defined the role of vitamin K in this conversion and have led to the current theory that the vitamin K-dependent carboxylation reaction is essentially a two-step process which first involves generation of a carbanion at the y-position of the glutamyl (Gla) residue. This event is coupled with the epoxidation of the reduced form of vitamin K and in a subsequent step, the carbanion is carboxylated (77—80). Studies have provided thermochemical confirmation for the mechanism of vitamin K and have shown the oxidation of vitamin KH2 (15) can produce a base of sufficient strength to deprotonate the y-position of the glutamate (81—83). 

H. Taber, "Vitamin KMetab.—Vitamin K-Dependent Proteins," ia J. W. Suttie, ed., Proc. Steenhock Sjmp. 8th, University Park Press, Baltimore, Md., 1980, for a review of function of Vitamin K. ... 

PPTA = degree of saturation of (NH 2S04 solution necessary for precipitation of factor. Synthesis is vitamin K-dependent. 

Factor II. Prothrombin is a vitamin K-dependent compound synthesized by the Hver. When prothrombin is activated it is cleaved at two sites, resulting in a two-chain molecule linked by a disulfide bond that has a molecular weight of 37,000 daltons. Thrombin is the serine protease that initiates the conversion of soluble fibrinogen into fibrin. 

Factor VII. This is a vitamin K-dependent serine protease that functions in the extrinsic coagulation pathway and catalyzes the activation of Factors IX and X. Factor VII is present constitutively in the surface membrane of pericytes and fibroblasts in the adventitia of blood vessels, vascular endothehum, and monocytes. It is a single-chain glycoprotein of approximately 50,000 daltons. 

Protein G. This vitamin K-dependent glycoproteia serine protease zymogen is produced ia the Hver. It is an anticoagulant with species specificity (19—21). Proteia C is activated to Proteia by thrombomodulin, a proteia that resides on the surface of endothefial cells, plus thrombin ia the presence of calcium. In its active form, Proteia selectively iaactivates, by proteolytic degradation. Factors V, Va, VIII, and Villa. In this reaction the efficiency of Proteia is enhanced by complex formation with free Proteia S. la additioa, Proteia activates tissue plasminogen activator, which... 

Protein S. Protein S is a single-chain molecule of approximately 78,000 daltons that contains 10 y-carboxy glutamic acid residues in the NH -terminal portion of the molecule. Protein S is a regulatory vitamin K-dependent protein. In plasma 40% of this protein circulates free and 60% circulates bound to C4b binding protein. Free Protein S functions as a nonenzymatic cofactor that promotes the binding of Protein C to membrane surfaces (22—25). 

FIGURE 18.40 The glutamyl carboxylase reaction is vitamin K-dependent. This enzyme activity is essential for the formation of 7-car-boxyglutamyl residues in several proteins of the blood-clotting cascade (Figure 15.5), accounting for the vitamin K dependence of coagulation. 

Scheme 10.24 Reaction catalyzed by vitamin K-dependent glutamate carboxylase.

Protein C is a vitamin K-dependent natural anticoagulant activated by thrombin to form APC in the presence of the endothelial receptor, TM. APC proteolyzes factors Va and Villa, thus downregulating thrombin generation. APC may also have anti-inflammatory... 

Vitamin K carboxylase is a transmembraneous protein in the lipid bilayer of the endoplasmatic reticulum (ER). It is highly glycosilated and its C-terminal is on the luminal side of the membrane. Besides its function as carboxylase it takes part as an epoxidase in the vitamin K cycle (Fig. 1). For the binding of the y-carboxylase the vitamin K-dependent proteins have highly conserved special recognition sites. Most vitamin K-dependent proteins are carboxy-lated in the liver and in osteoblasts, but also other tissues might be involved, e.g., muscles. 

Although vitamin K is a fat soluble vitamin, only little stores are found in the body which have to be refilled permanently via dietary input. The role of vitamin K derived from bacteria in the colon is controversely discussed, as the concentration of biliary acids for the resorption the fatsoluble vitamin K is very low in the colon. In addition, only diseases of the small intestine lead to a deficit in vitamin K concentration which cannot be restored by K2 production of colonic bacteria. However, watersoluble vitamin Ks can be resorbed by the colonic mucosa. Maybe because of the little stores for vitamin K, the process of vitamin K-dependent carboxylation of proteins is part of a cycle with several steps during which vitamin K normally is regenerated (see Fig. 1) and thus can be used several times. 

In bone, three proteins have been described which are vitamin K-dependent, osteocalcin (bone Gla protein), matrix Gla protein (MGP), and protein S. Osteocalcin is synthetized by osteoclasts, regulated by the active form of vitamin D, calcitriol. Its capacity to bind calcium needs a vitamin K-dependent y-carboxylation of three glutamic acid residues. The calcium binding capacity of osteocalcin indicates a possible role in bone mineralization, but its exact function is still unclear. However, it is widely used as a serum marker for bone mineralization. Protein S, mainly a coagulant, is also vitamin-K dependent and synthesized in the liver. Children with... 

This illness is mainly characterized by an age-related bone loss. The detection of osteocalcin in bone was the starting point for a series of studies on the role of vitamin K-dependent proteins in bone development and maintenance and on possible supplemantation therapies. Epidemiologic studies found differences in the risk of hip fractures depending on the dietary vitamin K... 

All anticoagulants interfere with the clotting mechanism of the blood. Warfarin and anisindione interfere with the manufacturing of vitamin K-dependent clotting factors... 

A number of iron-containing, ascorbate-requiring hydroxylases share a common reaction mechanism in which hydroxylation of the substrate is linked to decarboxylation of a-ketoglutarate (Figure 28-11). Many of these enzymes are involved in the modification of precursor proteins. Proline and lysine hydroxylases are required for the postsynthetic modification of procollagen to collagen, and prohne hydroxylase is also required in formation of osteocalcin and the Clq component of complement. Aspartate P-hydroxylase is required for the postsynthetic modification of the precursor of protein C, the vitamin K-dependent protease which hydrolyzes activated factor V in the blood clotting cascade. TrimethyUysine and y-butyrobetaine hydroxylases are required for the synthesis of carnitine. 

Factor XIa in the presence of activates factor IX (55 kDa, a zymogen containing vitamin K-dependent y-carboxyglutamate [Gla] residues see Chapter 45), to the serine protease, factor IXa. This in turn cleaves an Arg-Ile bond in factor X (56 kDa) to produce the two-chain serine protease, factor Xa. This latter reaction requires the assembly of components, called the tenase... 

Coumarin Anticoagulants Inhibit the Vitamin K-Dependent Carboxylation of Factors II, VII, IX, X... 

For activity, factors II, VII, IX, and X and proteins C and S require vitamin K-dependent y-carboxylation of certain glutamate residues, a process that is inhibited by the anticoagulant warfarin. 

Warfarin exerts its anticoagulant effect by inhibiting the production of the vitamin K-dependent coagulation factors... 

Factor IX Replacement Hemophilia B therapy may include recombinant (produced via transfection of mammalian cells with the human factor IX gene) or plasma-derived (concentrate from pooled plasma) factor IX (see Table 64-2). Guidelines for choosing the factor-concentrate formulation for hemophilia B are similar to the guidelines for hemophilia A. However, older-generation factor IX concentrates containing other vitamin K-dependent proteins (e.g., factors II, VII, and IX), called prothrombin complex concentrates (PCCs), have been associated with thrombogenic side effects. Consequently, these products are not first-line treatment for hemophilia B.11... 

PCCs contain the vitamin K-dependent factors II, VII, IX, and X. These agents represent another attempt to bypass the factor at which the antibody is directed (see Fig. 64-2). However, PCCs carry the risk of serious thrombotic complications. Porcine factor VIII is most useful when the inhibitor titer is less than 50 BU (see Fig. 64-2 for dose and frequency). Owing to its similarity to human factor VIII, porcine factor VIII participates in the coagulation cascade. However, most inhibitors have very weak neutralizing activity against it. Porcine factor VIII is a third-line agent (only after factor Vila and a PCC have failed) owing to a 15% incidence of cross-reactivity.15... 

Vitamin K is a fat-soluble vitamin cofactor for the activation of factors II, VII, IX, and X in the liver. Almost all neonates are vitamin K-deficient at as a result of (1) insignificant transplacental vitamin K crossover, (2) lack of colonization of the colon by vitamin K-producing bacteria, and (3) inadequate dietary vitamin K intake (especially in breast-fed infants because human milk contains less vitamin K than infant formula or cow s milk). Vitamin K-deficiency bleeding (VKDB) refers to bleeding attributable to vitamin K deficiency within first 6 months of life. It occurs in three general time frames early (0-24 hours), classic (1-7 days), and late (2-12 weeks). Early onset occurs rarely and usually is associated with maternal ingestion of anticonvulsants, rifampin, isoniazid, and warfarin. Classic vitamin K-dependent bleeding usually results from the lack of prophylactic vitamin K administration in... 

The levels of vitamin K-dependent coagulation factors are physiologically low in neonates. Absence of vitamin K impairs y-carboxylation, and the inactive precursors of factors II, VII, IX, and X accumulate in the plasma, unable to bind calcium and cell membranes. Consequently, the precursor levels may decline further, impairing coagulation and potentially leading to VKDB. 

Prothrombin time PT is performed by adding thromboplastin (tissue) factor and calcium to citrate-anticoagulated plasma, recalcifying the plasma, and measuring the clotting time. The major utility of PT is to measure the activity of the vitamin K-dependent factors II, VII, and X. The PT is used in evaluation of liver disease, to monitor warfarin anticoagulant effect, and to assess vitamin K deficiency. 

Drugs absorbed by active transport mechanisms appear to have a delayed rate, but not extent of absorption, in the neonatal period [20]. The absorption of vitamin K depends, to some extent, on the development of intestinal flora. 

The antithrombotic factors produced by endothelial cells are thrombomodulin (TM) and protein S (PS), components of the vitamin K-dependent protein C (PC) anticoagulant pathway, inhibiting F-Va-F-Villa (E15) tissue plasminogen activator (tPA), responsible for fibrinolysis (N2, LI8) and the lipoprotein-associated coagulation inhibitor (LACI), which inhibits F-VIIa-TF complex and F-Xa (B51). 

Kalafatis M., Swords N. A., Rand M. D Mann K. G. Membrane-dependent reactions in blood coagulation Role of Vitamin K-dependent enzyme complexes. Biochim Biophys Acta 1994 1227,113-29. 

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