The oxidation of phylloquinone to the 2,3-epoxide (by phylloquinone epoxidase) was studied in liver from control and warfarin-resistant rats. The reaction requires microsomal fraction, soluble protein, a heat-stable soluble factor and O2. It is not inhibited by CO or CN−. Epoxidase activity was stimulated if plasma prothrombin was lowered either by anticoagulants or the absence of vitamin K. The activity of the enzyme rapidly returned to normal values after the administration of vitamin K to hypoprothrombinaemic rats. These differences in the activity of the enzyme occur in the microsomal fraction and not the cytosol. A thrombin-generating polypeptide that accumulates in microsomal fraction of hypothrombinaemic rats correlated directly with epoxidase activity. These data support the view that enzymic interconversion of phylloquinone and its 2,3-epoxide participates in the biological activity of vitamin K.
Anticoagulant compounds, i.e., derivatives of either 4-hydroxycoumarin (e.g., warfarin, bromadiolone) or indane-1,3-dione (e.g., diphacinone, chlorophacinone), have been in worldwide use as rodenticides for >50 years. These compounds inhibit blood coagulation by repression of the vitamin K reductase reaction (VKOR). Anticoagulant-resistant rodent populations have been reported from many countries and pose a considerable problem for pest control. Resistance is transmitted as an autosomal dominant trait although, until recently, the basic genetic mutation was unknown. Here, we report on the identification of eight different mutations in the VKORC1 gene in resistant laboratory strains of brown rats and house mice and in wild-caught brown rats from various locations in Europe with five of these mutations affecting only two amino acids (Tyr139Cys, Tyr139Ser, Tyr139Phe and Leu128Gln, Leu128Ser). By recombinant expression of VKORC1 constructs in HEK293 cells we demonstrate that mutations at Tyr139 confer resistance to warfarin at variable degrees while the other mutations, in addition, dramatically reduce VKOR activity. Our data strongly argue for at least seven independent mutation events in brown rats and two in mice. They suggest that mutations in VKORC1 are the genetic basis of anticoagulant resistance in wild populations of rodents, although the mutations alone do not explain all aspects of resistance that have been reported. We hypothesize that these mutations, apart from generating structural changes in the VKORC1 protein, may induce compensatory mechanisms to maintain blood clotting. Our findings provide the basis for a DNA-based field monitoring of anticoagulant resistance in rodents.
抗凝血化合物,即4-羟基香豆素(例如华法林、溴乙酰胺)或1,3-茚二酮(例如地非辛酮、氯非辛酮)的衍生物,已经在全球使用超过50年作为杀鼠剂。这些化合物通过抑制维生素K还原酶反应(VKOR)来抑制血液凝固。抗凝血剂耐药的啮齿动物种群已经在许多国家报告,并对害虫控制构成了重大问题。耐药性以常染色体显性遗传,尽管直到最近,基本的遗传突变还不清楚。在这里,我们报道了VKORC1基因中8种不同突变体在耐药实验室品系的棕色大鼠和家鼠以及欧洲各地捕获的野生棕色大鼠中的发现,其中5种突变只影响两个氨基酸(Tyr139Cys、Tyr139Ser、Tyr139Phe和Leu128Gln、Leu128Ser)。通过在HEK293细胞中重组表达VKORC1构建物,我们证明Tyr139处的突变体在不同程度上对华法林具有耐药性,而其他突变体则显著降低了VKOR活性。我们的数据强烈支持棕色大鼠至少发生了七次独立的突变事件,而家鼠则发生了两次。它们表明VKORC1的突变是啮齿动物野生种群抗凝血剂耐药性的遗传基础,尽管这些突变本身并不能解释所有已报道的耐药性方面。我们假设这些突变除了在VKORC1蛋白中生成结构变化外,还可能诱导代偿机制来维持血凝。我们的发现为基于DNA的野外监测啮齿动物抗凝血剂耐药性提供了基础。
Coumarin and homologous compounds are the most widely used anticoagulant drugs worldwide. They function as antagonists of vitamin K, an essential cofactor for the posttranslational γ-glutamyl carboxylation of the so-called vitamin K-dependent proteins. As vitamin K hydroquinone is converted to vitamin K epoxide (VKO) in every carboxylation step, the epoxide has to be recycled to the reduced form by the vitamin K epoxide reductase complex (VKOR). Recently, a single coumarin-sensitive protein of the putativeVKOR enzyme complex was identified in humans (vitamin K epoxide reductase complex subunit 1, VKORC1). Mutations inVKORC1 result in two different phenotypes: warfarin resistance (WR) and multiple coagulation factor deficiency type 2 (VKCFD2). Here, we report on the expression of site-directed VKORC1 mutants, addressing possible structural and functional roles of all seven cysteine residues (Cys16, Cys43, Cys51, Cys85, Cys96, Cys132, Cys135), the highly conserved residue Ser/Thr57, and Arg98, known to cause VKCFD2 in humans. Our results support the hypothesis that the C132-X-X-C135 motif inVKORC1 comprises part of the redox active site that catalyzes VKO reduction and also suggest a crucial role for the hydrophobicThr-Tyr-Ala motif in coumarin binding. Furthermore, our results support the concept that different structural components of VKORC1 define the binding sites for vitamin K epoxide and coumarin.