Stabilization of Extracellular Ascorbate Mediated by Coenzyme Q Transmembrane Electron Transport

Effects of coenzyme Q10 supplementation on oxidative stress and antioxidant enzyme activity in glazers with occupational cadmium exposure: A randomized, double-blind, placebo-controlled crossover clinical trial

Recently, there is a growing interest for the use of antioxidants as a preventive agent against cadmium (Cd) intoxication. The current study aimed to investigate the effects of dietary coenzyme Q10 supplementation on oxidative stress and antioxidant enzymes activity in Cd-exposed glazers. In a randomized, double-blind, placebo-controlled 2-month crossover clinical trial, coenzyme Q10 (60 mg twice daily) or placebo was administrated to 40 male glazers occupationally exposed to Cd. The subjects were randomly assigned to the placebo and coenzyme Q10 groups ( n = 20 in each group). Total antioxidant capacity (TAC), superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) as well as malondialdehyde (MDA) in the serum of glazers were measured at baseline and at the end of both intervention phases. The primary outcomes were the changes in serum levels of MDA, TAC and the activities of SOD, CAT, and GPx during administration with coenzyme Q10 versus placebo. Compared with placebo, treatment to coenzyme Q10 was associated with a significant reduction in serum MDA levels ( p < 0.001) While, there was no significant effect on the serum TAC levels ( p = 0.096). Also, the activity antioxidant enzymes of SOD ( p < 0.001) and GPx ( p = 0.003) were significantly higher and the CAT activity ( p < 0.001) was lower after 2-month coenzyme Q10 administration compared with placebo. Data demonstrated that coenzyme Q10 supplementation at a dose of 60 mg (twice daily) is effective in reducing oxidative stress marker level (MDA) and improving antioxidant enzymes activity in glazers exposed to Cd. However, further research is needed to assess coenzyme Q10 as a possible dietary intervention in Cd-exposed workers.

Trial Registration:

Iranian Registry of Clinical Trials Registration Number: IRCT2016061228407N1 ( www.who.int/ictrp/network/irct/en/ ).

The regulation of coenzyme q biosynthesis in eukaryotic cells: all that yeast can tell us

Coenzyme Q (CoQ) is a mitochondrial lipid, which functions mainly as an electron carrier from complex I or II to complex III at the mitochondrial inner membrane, and also as antioxidant in cell membranes. CoQ is needed as electron acceptor in β-oxidation of fatty acids and pyridine nucleotide biosynthesis, and it is responsible for opening the mitochondrial permeability transition pore. The yeast model has been very useful to analyze the synthesis of CoQ, and therefore, most of the knowledge about its regulation was obtained from the Saccharomyces cerevisiae model. CoQ biosynthesis is regulated to support 2 processes: the bioenergetic metabolism and the antioxidant defense. Alterations of the carbon source in yeast, or in nutrient availability in yeasts or mammalian cells, upregulate genes encoding proteins involved in CoQ synthesis. Oxidative stress, generated by chemical or physical agents or by serum deprivation, modifies specifically the expression of some COQ genes by means of stress transcription factors such as Msn2/4p, Yap1p or Hsf1p. In general, the induction of COQ gene expression produced by metabolic changes or stress is modulated downstream by other regulatory mechanisms such as the protein import to mitochondria, the assembly of a multi-enzymatic complex composed by Coq proteins and also the existence of a phosphorylation cycle that regulates the last steps of CoQ biosynthesis. The CoQ biosynthetic complex assembly starts with the production of a nucleating lipid such as HHB by the action of the Coq2 protein. Then, the Coq4 protein recognizes the precursor HHB acting as the nucleus of the complex. The activity of Coq8p, probably as kinase, allows the formation of an initial pre-complex containing all Coq proteins with the exception of Coq7p. This pre-complex leads to the synthesis of 5-demethoxy-Q6 (DMQ6), the Coq7p substrate. When de novo CoQ biosynthesis is required, Coq7p becomes dephosphorylated by the action of Ptc7p increasing the synthesis rate of CoQ6. This critical model is needed for a better understanding of CoQ biosynthesis. Taking into account that patients with CoQ10 deficiency maintain to some extent the machinery to synthesize CoQ, new promising strategies for the treatment of CoQ10 deficiency will require a better understanding of the regulation of CoQ biosynthesis in the future.

Cell survival from chemotherapy depends on NF-kappaB transcriptional up-regulation of coenzyme Q biosynthesis

BACKGROUND

Coenzyme Q (CoQ) is a lipophilic antioxidant that is synthesized by a mitochondrial complex integrated by at least ten nuclear encoded COQ gene products. CoQ increases cell survival under different stress conditions, including mitochondrial DNA (mtDNA) depletion and treatment with cancer drugs such as camptothecin (CPT). We have previously demonstrated that CPT induces CoQ biosynthesis in mammal cells.

METHODOLOGY/PRINCIPAL FINDINGS

CPT activates NF-kappaB that binds specifically to two kappaB binding sites present in the 5'-flanking region of the COQ7 gene. This binding is functional and induces both the COQ7 expression and CoQ biosynthesis. The inhibition of NF-kappaB activation increases cell death and decreases both, CoQ levels and COQ7 expression induced by CPT. In addition, using a cell line expressing very low of NF-kappaB, we demonstrate that CPT was incapable of enhancing enhance both CoQ biosynthesis and COQ7 expression in these cells.

CONCLUSIONS/SIGNIFICANCE

We demonstrate here, for the first time, that a transcriptional mechanism mediated by NF-kappaB regulates CoQ biosynthesis. This finding contributes new data for the understanding of the regulation of the CoQ biosynthesis pathway.

The importance of plasma membrane coenzyme Q in aging and stress responses

The plasma membrane of eukaryotic cells is the limit to interact with the environment. This position implies receiving stress signals that affects its components such as phospholipids. Inserted inside these components is coenzyme Q that is a redox compound acting as antioxidant. Coenzyme Q is reduced by diverse dehydrogenase enzymes mainly NADH-cytochrome b(5) reductase and NAD(P)H:quinone reductase 1. Reduced coenzyme Q can prevent lipid peroxidation chain reaction by itself or by reducing other antioxidants such as alpha-tocopherol and ascorbate. The group formed by antioxidants and the enzymes able to reduce coenzyme Q constitutes a plasma membrane redox system that is regulated by conditions that induce oxidative stress. Growth factor removal, ethidium bromide-induced rho degrees cells, and vitamin E deficiency are some of the conditions where both coenzyme Q and its reductases are increased in the plasma membrane. This antioxidant system in the plasma membrane has been observed to participate in the healthy aging induced by calorie restriction. Furthermore, coenzyme Q regulates the release of ceramide from sphingomyelin, which is concentrated in the plasma membrane. This results from the non-competitive inhibition of the neutral sphingomyelinase by coenzyme Q particularly by its reduced form. Coenzyme Q in the plasma membrane is then the center of a complex antioxidant system preventing the accumulation of oxidative damage and regulating the externally initiated ceramide signaling pathway.

Vitamin C. Biosynthesis, recycling and degradation in mammals

Vitamin C, a reducing agent and antioxidant, is a cofactor in reactions catalyzed by Cu(+)-dependent monooxygenases and Fe(2+)-dependent dioxygenases. It is synthesized, in vertebrates having this capacity, from d-glucuronate. The latter is formed through direct hydrolysis of uridine diphosphate (UDP)-glucuronate by enzyme(s) bound to the endoplasmic reticulum membrane, sharing many properties with, and most likely identical to, UDP-glucuronosyltransferases. Non-glucuronidable xenobiotics (aminopyrine, metyrapone, chloretone and others) stimulate the enzymatic hydrolysis of UDP-glucuronate, accounting for their effect to increase vitamin C formation in vivo. Glucuronate is converted to l-gulonate by aldehyde reductase, an enzyme of the aldo-keto reductase superfamily. l-Gulonate is converted to l-gulonolactone by a lactonase identified as SMP30 or regucalcin, whose absence in mice leads to vitamin C deficiency. The last step in the pathway of vitamin C synthesis is the oxidation of l-gulonolactone to l-ascorbic acid by l-gulonolactone oxidase, an enzyme associated with the endoplasmic reticulum membrane and deficient in man, guinea pig and other species due to mutations in its gene. Another fate of glucuronate is its conversion to d-xylulose in a five-step pathway, the pentose pathway, involving identified oxidoreductases and an unknown decarboxylase. Semidehydroascorbate, a major oxidation product of vitamin C, is reconverted to ascorbate in the cytosol by cytochrome b(5) reductase and thioredoxin reductase in reactions involving NADH and NADPH, respectively. Transmembrane electron transfer systems using ascorbate or NADH as electron donors serve to reduce semidehydroascorbate present in neuroendocrine secretory vesicles and in the extracellular medium. Dehydroascorbate, the fully oxidized form of vitamin C, is reduced spontaneously by glutathione, as well as enzymatically in reactions using glutathione or NADPH. The degradation of vitamin C in mammals is initiated by the hydrolysis of dehydroascorbate to 2,3-diketo-l-gulonate, which is spontaneously degraded to oxalate, CO(2) and l-erythrulose. This is at variance with bacteria such as Escherichia coli, which have enzymatic degradation pathways for ascorbate and probably also dehydroascorbate.

Adaptations to oxidative stress induced by vitamin E deficiency in rat liver

Vitamin E deficiency in rats led to a sequence of antioxidant defense adaptations in the liver. After three weeks, alpha-tocopherol concentration was 5% of control, but ascorbate and ubiquinol concentrations were 2- to 3-fold greater than control. During the early phase of adaptation no differences in markers of lipid peroxidation were observed, but the activities of both cytochrome b5 reductase and glucose-6-phosphate dehydrogenase were significantly greater in deficient livers. By nine weeks, accumulation of lipid peroxidation end products began to occur along with declining concentrations of ascorbate, and higher NQO1 activities. At twelve weeks, rat growth ceased, and both lipid peroxidation products and cytosolic calcium-independent phospholipase A2 reached maximum concentrations. Thus, in growing rats the changes progressed from increases in both ubiquinol and quinone reductases through accumulation of lipid peroxidation products and loss of endogenous antioxidants to finally induction of lipid metabolizing enzymes and cessation of rat growth.

The plasma membrane redox system in aging

Oxidative stress over time leads to the accumulation of damaged macromolecules and to profound physiological changes that are associated with several age-related diseases. The plasma membrane redox system (PMRS) appears to attenuate oxidative stress acting as a compensatory mechanism during the aging process. The PMRS appears to play a protective role during mitochondrial dysfunction to provide cells with a survival mechanism by lowering oxidative stress. The PMRS accomplishes this by producing more NAD(+) for glycolytic ATP production via transfer of electrons from intracellular reducing equivalents to extracelluar acceptors. Ubiquinone and alpha-tocopherol are key antioxidant molecules in the plasma membrane that are affected by aging and can be up-regulated by dietary interventions such as calorie restriction (CR). Up-regulation of PMRS activity leads to cell survival and membrane homeostasis under stress conditions and during calorie restriction. Further studies of the PMRS may provide not only additional information on the mechanisms involved in aging and CR, but may provide therapeutic targets for the prevention and treatment of age-related diseases.