Ascorbic acid degradation, oxidative

Although it does not effectively protect ascorbic acid against oxidative degradation, colloidal silicon dioxide was found to increase the yield of spray-dried powder.f ... 

Several modifications of the OPDA procedure have been reported for automated analysis. Kirk and Ting (16) described a continuous flow analysis in which DCIP was substituted for Norit in the oxidation step. TAA and DHA can be determined directly RAA is calculated as the difference between TAA and DHA. Good agreement between the manual and automated procedures was achieved, with a considerable decrease in analytical time for the automated assay. Kirk and coworkers (17 19) have used the continuous flow analysis for studies of the kinetics of ascorbic acid degradation in model systems. 

Orr (43) used dimethyl sulfoxide as a free radical sink to inhibit the effect of Cu and ascorbic acid on catalase and 8-glucuronidase as well as the degradation of hyaluronic acid. The formation of a radical from ascorbic acid and Cu in water was detected by EPR (44). Based on an EPR spectroscopic study of ascorbic acid during oxidation of methyl-arachidonate-enriched liposomes, ascorbic acid may be important in preventing free radical damage in the central nervous system (45). 

Figures 5 and 6 illustrate the linear relationship between ascorbic acid oxidation and oxygen concentration, and the correlation between browning and ascorbic acid degradation.

Figure 9. The effect of untreated and oxidized polyethylene on ascorbic acid degradation in a model solution, stored at 35°C.

Schulz et al. [234] used dehydro-Shinoda-ascorbic acid, which is the first characteristic intermediate of L-ascorbic acid degradation via the oxidative route, as a starting material of the reaction to distinguish it from the nonoxidative pathway. They found that dehydro-L-ascorbic acid yielded five a-dicarbonyl compounds, namely glyoxal, methylglyoxal, diacetyl, L-threosone, and 3-deoxy-L-threosone (Figure 11.16). They concluded that these a-dicarbonyl compounds are formed from L-ascorbic acid on the oxidative pathway. But, they pointed out that these products can also be produced via the nonoxidative route. They also found that 3-deoxy-L-pentosone is exclusively formed via the nonoxidative route. 

Sulfur dioxide (bisulfites) also reacts with thiamine yielding inactive compounds (see Section 5.6.6), forms adducts with riboflavin, nicotinamide, vitamin K, inhibits ascorbic acid oxidation (see Section 5.14.6.1.6) and reacts with ascorbic acid degradation products, reduces o-quinones produced in enzymatic browning reactions back to 1,2-diphenols (see Section 9.12.4), causes decolourisation of fruit anthocyanins (see Section 9.4.1.5.7) and reacts with synthetic azo dyes to form coloured or colourless products (see Section 11.4.1.3.2). Sulfur dioxide also reacts with pyrimidine bases in vitro, specifically with cytosine and 5-methylcytosine. Important reactions of sulfur dioxide are shown in Figure 11.4. 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

There is evidence from a number of in vitro studies that the vitamin E peroxyl radical formed during fatty-acid degradation may be converted to vitamin E plus nonradical through the actions of vitamin C (Burton et al., 1985). RA patients have reduced serum ascorbate levels (Situnayake et al., 1991) and potentially a reduced capacity for the regeneration of vitamin E. In vitro studies suggest that vitamin E becomes a pro-oxidant when ascorbate levels are low (Bowry and Stocker, 1993). 

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],... 

Oxidative damage, role of ascorbic acid in preventing, 25 769 Oxidative degradation, 70 682 of gasoline, 72 399-400 Oxidative dehydrogenation, 23 342-343 Oxidative pyrolysis, 27 466 Oxidative stability, of olefin fibers, 77 229 Oxidative stability test, 72 400 Oxide crystal glass-ceramics, 72 641 Oxide-dispersion-strengthened alloys, 77 103-104... 

HPLC on a Cosmosil 5 Cis column, using a perchloric acid-acetonitrile eluent (pH 7.6), followed by CLD in the presence of hydrogen peroxide and bis(2,4,6-trichlorophenyl) oxalate (42), was applied to the determination of 1-aminopyrene (43a) and various diaminopyrenes (43b-d). Ascorbic acid was added to avoid oxidative degradation of the aminopyrenes in the presence of metals LOD in the sub-fmol range (SNR 3)147. A fast (less than 10 min) HPLC-ELCD method was proposed for determination of dopamine (19b) and its metabolites in microdialysates, using packed fused silica capillary columns LOD 0.05 Xg/L of dopamine in a 2 XL sample, RSD 3% (n = 10)148. 

Protons are mainly derived from two sources—free acids in the diet and sulfur-containing amino acids. Acids taken up with food— e.g., citric acid, ascorbic acid, and phosphoric acid—already release protons in the alkaline pH of the intestinal tract. More important for proton balance, however, are the amino acids methionine and cysteine, which arise from protein degradation in the cells. Their S atoms are oxidized in the liver to form sulfuric acid, which supplies protons by dissociation into sulfate. 

The main problem in the vitamin E analysis is that it is easily oxidized, thereby an antioxidant, such as butyl hydroxy toluene (BHT) or ascorbic acid, is added to prevent degradation during the extraction step. The traditional method for extraction of tocopherols and tocotrienols in foods is solvent extraction (like soxhlet) and saponification with KOH [457,458]. Some authors have recently proved that saponification is not necessary [459-462], nevertheless, it has been widely applied until the present day. 

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