Oxidase ascorbic acid

An ascorbic acid oxidase (AO) occurs in wheat flour (Table 15.25), which oxidizes L-threo- and D-erythro-2iSCOYhic acid at comparable rates. In addition, a substance has been found in flour extracts which oxidizes L- /ir 6 -ascorbic acid at pH 10 at a maximal rate. In comparison with AO, this activity does not decrease on incubation with proteases nor is it inhibited by the addition of the AO inhibitors KCN and NaF. It obviously catalyzes a nonenzymatic oxidation of ascorbic acid. 

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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. 

Yahia EM, Contreras M and Gonzalez G. 2001b. Ascorbic acid content in relation to ascorbic acid oxidase activity and polyamine content in tomato and bell pepper fruits during development, maturation and senescence. Lebensm Wiss u-Technol 34 452-457. 

In addition to binding to cytochrome c oxidase, cyanide inhibits catalase, peroxidase, methemoglobin, hydroxocobalamin, phosphatase, tyrosinase, ascorbic acid oxidase, xanthine oxidase, and succinic dehydrogenase activities. These reactions may make contributions to the signs of cyanide toxicity (Ardelt et al. 1989 Rieders 1971). Signs of cyanide intoxication include an initial hyperpnea followed by dyspnea and then convulsions (Rieders 1971 Way 1984). These effects are due to initial stimulation of carotid and aortic bodies and effects on the central nervous system. Death is caused by respiratory collapse resulting from central nervous system toxicity. 

Schmid et al. used the same principle to develop sensors to be incorporated into FI systems for the determination of ascorbic acid in fruit juices [38] and that of lactic acid in dairy products [39]. The membrane used in both applications consisted of decacyclene dissolved in silicone rubber that was treated similarly as the membrane in glucose sensors (Fig. 3.4.B). The oxygen optrode was coated with a sheet of carbon black as optical insulation in order to protect it from ambient light or intrinsic sample fluorescence. Ascorbic acid oxidase or lactic acid oxidase was immobilized by adsorbing it onto carbon black and cross-linking it with glutaraldehyde. The FI system automatically buffered and diluted the food samples, thereby protecting the biosensor from a low pH and interferents. 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

Samples for analysis often contain traces of other compounds, in addition to ascorbic acid, that reduce DCIP. One way to minimize the interference of other substances is to analyze two identical aliquots of the sample. One aliquot is titrated directly and the total content of all reducing substances present is determined. The second aliquot is treated with ascorbic acid oxidase to destroy ascorbic acid and then titrated with DCIP. The... 

To test for the presence of interfering substances in the juice, pipet a 10.0-mL sample of the fresh, undiluted juice into a 50-mL Erlenmeyer. Add a few crystals of ascorbic acid oxidase to destroy the ascorbic acid. Let stand, after gentle mixing, for 10 minutes. Add 40 mL of metaphosphoric acid solution. Titrate three 10.0-mL portions of this diluted juice with DCIP. 

Write the reaction for the oxidation of ascorbic acid catalyzed by ascorbic acid oxidase. 

A number of copper -containing protein compounds are enzymes with an oxidase function (ascorbic acid oxidase, urease, etc 1 and these play an important role in Ihe biological oxidation-reduction system. There is a definite relationship of copper with iron in connection with utilization of iron in hemoglobin function. 

Dichlorophenohndophenol solution in H20, 25 mg/100 mL Ascorbic acid oxidase, lyophihzed powder Buret use a 10-mL microburet Mortar and pestle... 

Trichloroacetic acid-octanol extraction AA oxidized to DHAA using ascorbic acid oxidase total vitamin C as DHAA derivatized with o-phenyl-diamide... 

The first, called the Maillard reaction,1 occurs between a carbonyl compound, which here is usually a reducing sugar, and an amine, which here is usually an amino acid, a peptide, or a protein. The second is caramelisation, a reaction where the sugars react on their own, but normally requires more drastic conditions. (Some discuss this under the heading of active aldehydes.) The third is ascorbic acid oxidation. The last, although it need not involve any enzyme at all, is nearest to enzymic browning, since it often does involve ascorbic acid oxidase, which, however, does not affect the phenols, which are the normal substrate in enzymic browning, but may involve other enzymes, e.g., laccase or peroxidase. 

The Wacker process, of course, gives highly selective oxidation of olefins to aldehydes or ketones (42) the function of the 02 is to reoxidize the catalyst, and again any formation of a dioxygen complex is incidental, although such a species could be involved in the reoxidation step. Reoxidation of Cu(I) to Cu(II)/Cu(III) by 02 appears to be involved in certain Cu-containing oxidase systems, for example, ascorbic-acid oxidase (43, 44). 

They can react with metal-containing enzymes (e.g., polyphenoloxidase and ascorbic acid oxidase) to form a copper-dithiocarbamate complex with high fungicidal activity (thiram and ziram probably owe their fungicidal activity to this type of reaction). 

In addition to those mentioned in Table II, there is a group of enzymes in foods which lower the nutritive content. This group includes thiaminase which acts on thiamine in flesh food, ascorbic acid oxidase in... 

Rice bran contains active enzymes (30). Germ and the outer layers of the caryopsis have higher enzyme activities. Some enzymes that are present include a-amylase, p-amylase, ascorbic acid oxidase, catalase, cytochrome oxidase, dehydrogenase, deoxyribonuclease, esterase, flavin oxidase, a and p-glycosidase, invertase, lecithi-nase, lipase, lipoxygenase, pectinase, peroxidase, phosphatase, phytase, proteinase, and succinate dehydrogenase. 

Dopamine- -hydroxylase is another copper enzyme that plays a major function in the biosynthesis of norepinephrine (40). A protective catalyst, superoxide dismutase, has been described to catalyze the dismu-tation of the superoxide anion free radical (41). Other enzymes known to contain copper are the laccases, the phenol oxidases, and the ascorbic acid oxidases (12). 

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