vitamin-k-semiquinone-radical and hydroquinone

Vitamin K1, K2, and K3 are essential nutrients associated with blood clotting and bone metabolism. Quinone oxidoreductases [NAD(P)H:quinone oxidoreductase 1 (NQO1) and NRH:quinone oxidoreductase 2 (NQO2)] are among the selected enzymes that catalyze reduction of vitamin K to vitamin K hydroquinone. NQO1 catalyzes high affinity reduction of vitamin K3 but has only weak affinity for reduction of vitamin K1 and K2. Vitamin K hydroquinone serves as a cofactor for vitamin K gamma-carboxylase that catalyzes gamma-carboxylation of specific glutamic acid residues in Gla-factors/proteins leading to their activation and participation in blood clotting and bone metabolism. Concomitant with Gla modification, a reduced vitamin K molecule is converted to vitamin K epoxide, which is converted back to vitamin K by the enzyme vitamin K epoxide reductase to complete vitamin K cycle. Vitamin K is also redox cycled. One-electron reduction of vitamin K3 leads to the formation of semiquinone that in the presence of oxygen is oxidized back to vitamin K3. Oxygen is reduced to generate reactive oxygen species (ROS) that causes oxidative stress and cytotoxicity. Vitamin K is used as radiation sensitizer or in mixtures with other chemotherapeutic drugs to treat several types of cancer. ROS generated in redox cycling contributes to anticancer activity of vitamin K. NQO1 competes with enzymes that redox cycle vitamin K and catalyzes two-electron reduction of vitamin K3 to hydroquinone. This skips formation of semiquinone and ROS. Therefore, NQO1 metabolically detoxifies vitamin K3 and protects cells against oxidative stress and other adverse effects. On the contrary, NQO2 catalyzes metabolic activation of vitamin K3 leading to cytotoxicity. The role of NQO1 and NQO2 in metabolic detoxification and/or activation of vitamin K1 and K2 remains to be determined. Future studies are also required to identify the enzymes that catalyze high affinity reduction of vitamin K1 and K2 to hydroquinone for use in gamma-carboxylation reactions.

Topics: Animals; Antineoplastic Agents; Humans; Hydroquinones; NAD(P)H Dehydrogenase (Quinone); Oxidation-Reduction; Vitamin K

Ferroptosis, a non-apoptotic form of cell death marked by iron-dependent lipid peroxidation

Topics: Antidotes; Antioxidants; Carbon-Carbon Ligases; Coenzymes; Ferroptosis; Hydroquinones; Lipid Peroxidation; Oxidation-Reduction; S100 Calcium-Binding Protein A4; Vitamin K; Warfarin

The vitamin K oxidoreductase (VKORC1) recycles vitamin K to support the activation of vitamin K-dependent (VKD) proteins, which have diverse functions that include hemostasis and calcification. VKD proteins are activated by Glu carboxylation, which depends upon the oxygenation of vitamin K hydroquinone (KH2). The vitamin K epoxide (KO) product is recycled by two reactions, i.e. KO reduction to vitamin K quinone (K) and then to KH2, and recent studies have called into question whether VKORC1 reduces K to KH2. Analysis in insect cells lacking endogenous carboxylation components showed that r-VKORC1 reduces KO to efficiently drive carboxylation, indicating KH2 production. Direct detection of the vitamin K reaction products is confounded by KH2 oxidation, and we therefore developed a new assay that stabilized KH2 and allowed quantitation. Purified VKORC1 analyzed in this assay showed efficient KO to KH2 reduction. Studies in 293 cells expressing tagged r-VKORC1 revealed that VKORC1 is a multimer, most likely a dimer. A monomer can only perform one reaction, and a dimer is therefore interesting in explaining how VKORC1 accomplishes both reactions. An inactive mutant (VKORC1(C132A/C135A)) was dominant negative in heterodimers with wild type VKORC1, resulting in decreased KO reduction in cells and carboxylation in vitro. The results are significant regarding human VKORC1 mutations, as warfarin-resistant patients have mutant and wild type VKORC1 alleles. A VKORC1 dimer indicates a mixed population of homodimers and heterodimers that may have different functional properties, and VKORC1 reduction may therefore be more complex in these patients than appreciated previously.

Topics: Amino Acid Substitution; Anticoagulants; Drug Resistance; HEK293 Cells; Humans; Hydroquinones; Mutation, Missense; Oxidation-Reduction; Protein Multimerization; Protein Processing, Post-Translational; Vitamin K; Vitamin K Epoxide Reductases; Warfarin

We studied proposed steps for the enzymatic formation of gamma-carboxyglutamic acid by density functional theory (DFT) quantum chemistry. Our results for one potentially feasible mechanism show that a vitamin K alkoxide intermediate can abstract a proton from glutamic acid at the gamma-carbon to form a carbanion and vitamin K epoxide. The hydrated carbanion can then react with CO2 to form gamma-carboxyglutamic acid. Computations at the B3LYP/6-311G** level were used to determine the intermediates and transition states for the overall process. The activation free energy for the gas-phase path is 22 kcal/mol, with the rate-limiting step for the reaction being the attack of the carbanion on CO2. Additional solvation studies, however, indicate that the formation of the carbanion step can be competitive with the CO2 attack step in high-dielectric systems. We relate these computations to the entire vitamin K cycle in the blood coagulation cascade, which is essential for viability of vertebrates.

Topics: 1-Carboxyglutamic Acid; Carbon Dioxide; Carbon-Carbon Ligases; Epoxy Compounds; Hydroquinones; Models, Biological; Molecular Structure; Quantum Theory; Vitamin K

Incorporation of 32P from [gamma-32P]ATP into endogenous proteins, added histone and the copolymers Glu 80 Tyr 20 by rat liver plasma membranes was markedly increased by several naphthoquinones, including menadione. This stimulation was most marked with Glu 80 Tyr 20, has an absolute requirement for either dithiothreitol or reduced glutathione, and was inhibited by superoxide dismutase, catalase, and desferrioxamine to varying degrees depending on the quinones used. Their effectiveness in stimulating the apparent tyrosine-specific protein phosphorylation correlated with the rates of DTT-dependent redox cycling measured by oxygen consumption. Increased protein phosphorylation was also seen with particulate fractions isolated from hepatocytes incubated with quinones. A free radical-mediated mechanism is suggested for the quinone stimulation of protein phosphorylation.

Topics: Adenosine Triphosphate; Animals; Benzoquinones; Cell Membrane; Deferoxamine; Free Radicals; Hydroquinones; Liver; Naphthoquinones; Oxidation-Reduction; Oxygen; Oxygen Consumption; Protein-Tyrosine Kinases; Quinones; Rats; Structure-Activity Relationship; Superoxide Dismutase; Vitamin K

Topics: Hydroquinones; Liver; Mitochondria; Naphthoquinones; Oxidative Phosphorylation; Retinoids; Ubiquinone; Vitamin K