VITAMIN K AND ANTICOAGULANT POISONING

在肝脏中，凝血系统中的四种凝血因子 （因子II，VII，IX和X）依赖于维生素K， 生物合成 维生素是酶复合物的必要辅因子， 维生素K依赖性羧化酶，将维生素K的非活性前体 将凝血转化为活性凝血因子 这是通过 前体中特定谷氨酸残基的γ-羧化 proteins. 为了作为羧化酶的辅因子，维生素 K必须以还原形式存在，即维生素K对苯二酚。 似乎在 肝脏有两个独立的途径减少维生素K。 这些途径是：还原途径I;一种由以下组成的途径： 香豆素敏感酶维生素K-环氧化物还原酶和还原途径 II;一种由一类吡啶核苷酸依赖性 能够减少维生素K的维生素A酶。 途径二 酶不易受香豆素的抑制， 凝血因子合成增强的重要途径 香豆素抗凝剂中毒 拟议的工作将侧重于 这条重要的临床途径 通路中的单个酶 将被纯化和鉴定，它们对抑制的敏感性 研究了不同的抗凝药物。 此外，实验 提出将在体外测量凝血因子合成的恢复 当途径I被不可逆地阻断时， 抗凝血药物。 结果应提供详细的体外 作为救助系统的途径二的运作模式， 抗凝剂中毒时凝血因子的合成。 模型 将通过给予抗凝大鼠维生素K进行体内试验。 第二条工作线集中在内源性蛋白质底物上， 羧酸盐跟随酶在整个广泛的 纯化程序。 提出的方法将比较的肽 绘制纯化的底物蛋白和凝血因子II和X。 它 将有可能确定内源性底物是否是前体 因子II或X或不相关蛋白质的形式。

In the liver, four of the blood clotting factors in the coagulation system (factor II, VII, IX and X) are dependent on vitamin K for their biosynthesis. The vitamin is a necessary cofactor for an enzyme complex, the vitamin K-dependent carboxylase, that converts inactive precursors of the clotting to active clotting factors. This is achieved by Gamma-carboxylation of specific glutamic acid residues in the precursor proteins. In order to function as a cofactor for the carboxylase, vitamin K must be in its reduced form, vitamin K hydroquinone. It appears that in the liver there are two independent pathways for vitamin K reduction. These are: reduction pathway I; a pathway consisting of the coumarin-sensitive enzyme vitamin K-epoxide reductase and reduction pathway II; a pathway consisting of a class of pyridine nucleotide-dependent dehydrogenases that are capable of reducing vitamin K. The pathway II enzymes are less susceptible to inhibition by coumarins and constitute an important pathway for enhanced clotting factor synthesis in cases of coumarin anticoagulant poisoning. The work that is proposed will focus on this clinically important pathway. The individual enzymes in the pathway will be purified and identified and their susceptibility to inhibition by different anticoagulant drugs studied. In addition, experiments are proposed that will measure in vitro recovery of clotting factor synthesis by high concentrations of vitamin K when pathway I is irreversibly blocked by anticoagulant drugs. The results should provide a detailed in vitro model system for the functioning of pathway II as a salvage system for clotting factor synthesis in cases of anticoagulant poisoning. The model will be tested in vivo by administering vitamin K to anticoagulated rats. A second line of work concentrates on an endogenous protein substrate for the carboxylate that follows the enzyme throughout an extensive purification procedure. Methods are proposed which will compare by peptide mapping the purified substrate protein and clotting factors II and X. It will be possible to determine if the endogenous substrate is a precursor form of factor II or X or an unrelated protein.