Copper-ascorbic acid interaction

Flavonoids exhibit protective action against LDL oxidation. It has been shown [145] that the pretreatment of macrophages and endothelial cells with tea flavonoids such as theaflavin digallate diminished cell-mediated LDL oxidation probably due to the interaction with superoxide and the chelation of iron ions. Quercetin and epicatechin inhibited LDL oxidation catalyzed by mammalian 15-lipoxygenase, and are much more effective antioxidants than ascorbic acid and a-tocopherol [146], Luteolin, rutin, quercetin, and catechin suppressed copper-stimulated LDL oxidation and protected endogenous urate from oxidative degradation [147]. Quercetin was also able to suppress peroxynitrite-induced oxidative modification of LDL [148],... 

How analytical methods deal with interferences is one of the more ad hoc aspects of method validation. There is a variety of approaches to studying interference, from adding arbitrary amounts of a single interferent in the absence of the analyte to establish the response of the instrument to that species, to multivariate methods in which several interferents are added in a statistical protocol to reveal both main and interaction effects. The first question that needs to be answered is to what extent interferences are expected and how likely they are to affect the measurement. In testing blood for glucose by an enzyme electrode, other electroactive species that may be present are ascorbic acid (vitamin C), uric acid, and paracetamol (if this drug has been taken). However, electroactive metals (e.g., copper and silver) are unlikely to be present in blood in great quantities. Potentiometric membrane electrode sensors (ion selective electrodes), of which the pH electrode is the... 

In the diet and at the tissue level, ascorbic acid can interact with mineral nutrients. In the intestine, ascorbic acid enhances the absorption of dietary iron and selenium reduces the absorption of copper, nickel, and manganese but apparently has little effect on zinc or cobalt. Ascorbic acid fails to affect the intestinal absorption of two toxic minerals studied, cadmium and mercury. At the tissue level, iron overload enhances the oxidative catabolism of ascorbic acid. Thus, the level of dietary vitamin C can have important nutritional consequences through a wide range of inhibitory and enhancing interactions with mineral nutrients. 

The mechanism of this eflFect is not known. Hill and Starcher (49) postulated that reduction of copper from its divalent (cupric) state to its monovalent (cuprous) state accounted for the impaired absorption of copper in the presence of ascorbic acid they produced the same effect with another reducing agent, dimercaptopropanol (BAL). This explanation has been accepted by others (56), although the oxidation state of copper for maximum intestinal absorption has not been established. An intramucosal competition of ascorbic acid for sulfhydryl sites on metallo-thioneins was demonstrated (57). If this ligand has any regulatory role in copper uptake, this alternative mechanism of ascorbic acid-copper interaction could explain the mechanism. Experimental confirmation of an ascorbic-acid-induced inhibition of copper absorption in the human intestine has not been presented. 

Given the eflFects of ascorbic acid on the absorption of iron and copper, investigators have been interested in possibly significant interactions with zinc. The absorption of zinc and other divalent mineral ions was studied using an isolated, filled duodenal loop in situ in the rat (58). A lO -M zinc solution was infused in the presence or absence of 10" M ascorbate or dehydroascorbate. A two-thirds reduction in the... 

Interaction in the Cu -Pb -Na2Ad-H20 system (AdH2 = adipic acid) has been examined by precipitation and residual concentration methods the Cu and Pb adipates cocrystallize. For the copper(ii)-catalysed oxidation of ascorbic acid by dissolved dioxygen in the presence of nitrate at pH 2.0—3.5 it is proposed that cupric ascorbate dimers are the reactive species. Dioxygen binds across a Cu—Cu dinuclear... 

The complexes formed between Cu(II) ions and a number of D-aldonic and D-alduronic acids in aqueous solution has been studied, and the complex obtained from methyl a-D-mannopyranoside and copper(II) hydroxide in the presence of lithium hydroxide has been identified as the square planar structure 17. The determination of stability constants of Ca complexes with /wyo-inositol 1,4,5-trisphosphate is discussed in Chapter 18. The interaction of L-ascorbic acid with some metal ions has b n studied in aqueous solution at pH 6-7, and the solid hydrated salts lithium, sodium, potassium, ammcHiium, rubidium and cesium ascorbate have been isolated and characterised by C n.m.r. and f.t.ix. spectroscopy... 

Poly(ionic liquid) brushes with terminated ferrocene units acted similarly, while the interfacial resistance was probed by hexacyanoferrate [457]. Chemical and electrochemical switching of local pH at an electrode-grafted poly(vinyl pyridine) brush again allowed modulation of hexacyanoferrate chemistiy (Fig. 43) [458]. Octacyanomolybdate was used as catalyst for the oxidation of ascorbic acid [459]. Even heteropolyanions (Keggin ions) could be entrapped in polymer films electrochemicaUy [460]. Further, thermoresponsive or pH-responsive cationic copolymer films modulated the hexacyanoferrate or ferrocenedicarboxyUc acid electrochemistry by temperature or variatimi of pH and perchlorate concentration, respectively [461-463]. Besides these complexes with cationic polyelectrolyte films, electroactive cationic counterions (e.g., the europium couple) interacted with anionic networks [464]. Similarly, copper ions within a PAA matrix [367] allowed the construction of actuators [465]. Besides these binary systems (poly-electrolyte/electroactive counterions), multiresponsive electrode modification with an interpenetrating gel network of poly(acrylic) acid and poly(diethyl acrylamide) allowed the modulation of hexacyanoferrate electrochemistry [368]. 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

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