Cell surface glycoconjugates control the activity of the NADH-ascorbate free radical reductase of rat liver plasma membrane

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.

Dual role of plasma membrane electron transport systems in defense

Because oxidative stress is one of the main sources of severe cellular damage, cells have different defense weapons against reactive oxygen species. Ubiquitous plasma membrane redox systems play a role in defense against oxidative stress damage. On the other hand, a tightly controlled and localized production of reactive oxygen species by a plasma membrane NADPH oxidase can be used as a potent microbicidal weapon. This dual, prooxidant and antioxidant role of plasma membrane electron transport systems in defense is studied and discussed.

Genetic evidence for coenzyme Q requirement in plasma membrane electron transport

Plasma membranes isolated from wild-type Saccharomyces cerevisiae crude membrane fractions catalyzed NADH oxidation using a variety of electron acceptors, such as ferricyanide, cytochrome c, and ascorbate free radical. Plasma membranes from the deletion mutant strain coq3delta, defective in coenzyme Q (ubiquinone) biosynthesis, were completely devoid of coenzyme Q6 and contained greatly diminished levels of NADH-ascorbate free radical reductase activity (about 10% of wild-type yeasts). In contrast, the lack of coenzyme Q6 in these membranes resulted in only a partial inhibition of either the ferricyanide or cytochrome-c reductase. Coenzyme Q dependence of ferricyanide and cytochrome-c reductases was based mainly on superoxide generation by one-electron reduction of quinones to semiquinones. Ascorbate free radical reductase was unique because it was highly dependent on coenzyme Q and did not involve superoxide since it was not affected by superoxide dismutase (SOD). Both coenzyme Q6 and NADH-ascorbate free radical reductase were rescued in plasma membranes derived from a strain obtained by transformation of the coq3delta strain with a single-copy plasmid bearing the wild type COQ3 gene and in plasma membranes isolated form the coq3delta strain grown in the presence of coenzyme Q6. The enzyme activity was inhibited by the quinone antagonists chloroquine and dicumarol, and after membrane solubilization with the nondenaturing detergent Zwittergent 3-14. The various inhibitors used did not affect residual ascorbate free radical reductase of the coq3delta strain. Ascorbate free radical reductase was not altered significantly in mutants atp2delta and cor1delta which are also respiration-deficient but not defective in ubiquinone biosynthesis, demonstrating that the lack of ascorbate free radical reductase in coq3delta mutants is related solely to the inability to synthesize ubiquinone and not to the respiratory-defective phenotype. For the first time, our results provide genetic evidence for the participation of ubiquinone in NADH-ascorbate free radical reductase, as a source of electrons for transmembrane ascorbate stabilization.

Effectors of the mammalian plasma membrane NADH-oxidoreductase system. Short-chain ubiquinone analogues as potent stimulators

In the presence of effectors variations in the two recognized activities of the plasma membrane NADH-oxidoreductase system were studied in separate, specific in vitro assays. We report here that ubiquinone analogues that contain a short, less hydrophobic side chain than coenzyme Q-10 dramatically stimulate the NADH-oxidase activity of isolated rat liver plasma membranes whereas they show no effect on the reductase activity of isolated membranes. If measured in assays of the NADH:ferricyanide reductase of living cultured cells these compounds have only a limited effect; the oxidase activity of whole cells is not measurable in our hands. We have furthermore identified selective inhibitors of both enzyme activities. In particular, the NADH-oxidase activity can be significantly inhibited by structural analogues of ubiquinone, such as capsaicin and resiniferatoxin. The NADH:ferricyanide reductase, on the other hand, is particularly sensitive to pCMBS, indicating the presence of a sulfhydryl group of groups at its active site. The identification of these specific effectors of the different enzyme activities of the PMOR yields further insights into the function of this system.

Extracellular ascorbate stabilization as a result of transplasma electron transfer in Saccharomyces cerevisiae

The presence of yeast cells in the incubation medium prevents the oxidation of ascrobate catalyzed by copper ions. Ethanol increases ascorbate retention. Pyrazole, an alcohol dehydrogenase inhibitor, prevents ascorbate stabilization by cells. Chelation of copper ions does not account for stabilization, since oxidation rates with broken or boiled cells or conditioned media are similar to control rates in the absence of cells. Protoplast integrity is needed to reach optimal values of stabilization. Chloroquine, a known inhibitor of plasma membrane redox systems, inhibits the ascorbate stabilization, the inhibition being partially reversed by coenzyme Q6. Chloroquine does not inhibit ferricyanide reduction. Growth of yeast in iron-deficient media to increase ferric ion reductase activity also increases the stabilization. In conclusion, extracellular ascorbate stabilization by yeast cells can reflect a coenzyme Q dependent transplasmalemma electron transfer which uses NADH as electron donor. Iron deficiency increases the ascorbate stabilization but the transmembrane ferricyanide reduction system can act independently of ascorbate stabilization.

Coenzyme Q reductase from liver plasma membrane: purification and role in trans-plasma-membrane electron transport

A specific requirement for coenzyme Q in the maintenance of trans-plasma-membrane redox activity is demonstrated. Extraction of coenzyme Q from membranes resulted in inhibition of NADH-ascorbate free radical reductase (trans electron transport), and addition of coenzyme Q10 restored the activity. NADH-cytochrome c oxidoreductase (cis electron transport) did not respond to the coenzyme Q status. Quinone analogs inhibited trans-plasma-membrane redox activity, and the inhibition was reversed by coenzyme Q. A 34-kDa coenzyme Q reductase (p34) has been purified from pig-liver plasma membranes. The isolated enzyme was sensitive to quinone-site inhibitors. p34 catalyzed the NADH-dependent reduction of coenzyme Q10 after reconstitution in phospholipid liposomes. When plasma membranes were supplemented with extra p34, NADH-ascorbate free radical reductase was activated but NADH-cytochrome c oxidoreductase was not. These results support the involvement of p34 as a source of electrons for the trans-plasma-membrane redox system oxidizing NADH and support coenzyme Q as an intermediate electron carrier between NADH and the external acceptor ascorbate free radical.

Extracellular ascorbate stabilization: enzymatic or chemical process?

Ascorbate is stabilized in the presence of HL-60 cells. This stabilization has been questioned as a simple chemical effect. Further properties and controls about the enzymatic nature of this stabilization are described and discussed. Our results showed that cAMP derivatives and cAMP-increasing agents stimulated the ability of HL-60 cells to stabilize ascorbate. On the other hand, tunicamycin, a glycosylation-interfering agent, inhibited this ability. These data, together with hormonal regulation, support the hypothesis of an enzymatic redox system located at the plasma membrane as being responsible for the extracellular ascorbate stabilization by HL-60 cells.

The action of ascorbate in vesicular systems

Many effects of ascorbate center on its interactions with membranes from plant and animal cells. These actions can be studied using vesicles produced from phospholipid components (liposomes), by isolating naturally occurring vesicles, or by purifying particular membranes that form vesicles during the extraction process. Liposomes have provided information concerning the anti- and prooxidant properties of ascorbate and about how the water-soluble vitamin can have effects within the phospholipid bilayer. The involvement of ascorbate in transmembrane electron transport has been characterized in vesicles normally found in certain cells, such as, chromaffin granules, synaptosomes, glyoxisomes, peroxisomes, and clathrin-coated vesicles. Redox activity using reducing power associated with ascorbate/ascorbate free radical (AFR) has been characterized in some of these vesicles and it appears to be mediated by a b-type cytochrome. Ascorbate also participates in the reduction of iron within clathrin-coated vesicles. Vesicles appearing during purification of plasma membranes have transmembrane electron transport, oxidoreductase activity with ascorbate/AFR as redox agents, and an ascorbate-reducible b-type cytochrome. It is also possible that ascorbate-related redox activity exists at the tonoplast of plant cells.

Characterization of plasma membrane redox activity from Ehrlich cells

Ferricyanide reductase activity of plasma membranes isolated from Ehrlich ascites tumour cells was very sensitive to trypsin treatment. The decreases of activity observed after treatment with different glycosidases suggests that ferricyanide reductase is a glycoprotein. The opposite effects of phospholipase A2 and phospholipase C on the redox activity indicate that the phospholipidic environment plays an important role in the function of ferricyanide reductase. Sodium ions at millimolar concentrations, and some divalent cations at micromolar concentrations (Ca2+, Mg2+, Sr2+, and Mn2+) behaved as stimulators of ferricyanide reductase activity.

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1:40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.

Ceruloplasmin stimulates NADH oxidation of pig liver plasma membrane

NADH oxidation by pig liver plasma membranes is stimulated by ceruloplasmin (CUP) reaching a maximal value at 50 U/ml of CUP. NADH oxidation activated by CUP is proportional to the amount of protein. Concanavalin A (Con A) which recognizes the glucidic residues of the CUP required for binding to the receptor inhibits the NADH oxidation in a dose-responsive manner. Both adriamycin and bathophenantroline disulfonate (BPS), previously reported as transplasma membrane electron transport inhibitors, also inhibit the CUP-stimulated NADH oxidation of pig liver plasma membranes. Our results show a clear interaction between CUP and the NADH oxidase of plasma membrane, which supports an oxidative role for CUP in its growth effect.

Growth factor-stimulated trans plasma membrane electron transport in HL-60 cells

Electron flow across the plasma membrane of living cells and its rapid modulation by growth factors has been measured continuously through a simple assay procedure whereby the transported electrons are captured by ascorbate free radical to slow the rate of chemical oxidation of ascorbate. The assay provides a direct demonstration of electron transport to an external electron acceptor that is both physiological and impermeant. The reduction of external ascorbate free radical is stimulated by the growth factors, EGF and transferrin, and is inhibited by wheat germ agglutinin. The results demonstrate, under physiological conditions, the operation of a growth factor- and lectin-responsive electron transport system at the cell surface using a cultured human cell line.

Ascorbate is regenerated by HL-60 cells through the transplasmalemma redox system

Ascorbate was maintained in the media during a long-term culture by HL-60 cells. The chemical oxidation of ascorbate was reversed in vitro by living HL-60 cells and was related to the amount of cells added. The increase of NADH concentration by lactate addition to cells was accompanied by an increase of both ascorbate regeneration and ferricyanide reduction. Further, plasma membrane enriched fractions from HL-60 cells revealed enhancement of both ascorbate regeneration and ferricyanide reduction in the presence of NADH when previously treated with detergent. The blockage of cell surface carbohydrates by wheat germ agglutinin (WGA) and Concanavalina ensiformis (Con A) lectins significantly inhibited the regeneration of ascorbate caused by the cells. These results support the idea that ascorbate is externally regenerated by the NADH-ascorbate free radical reductase as a part of the transplasma membrane redox system.