Enzymes ascorbic acid oxidase

Each enzyme is classified by adopting this system. An example will be analyzed. The enzyme ascorbic acid oxidase catalyzes the following reaction ... 

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. 

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. 

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. 

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). 

A number of enzymes have been characterized that catalyze reactions involving DHA. In addition, other aspects of DHA biochemistry can be deduced from metabolic studies of ascorbic acid. Experiments demonstrating the biological oxidation of AA and reduction of DHA were first made in 1928 (10) and during the next decade several groups studied these reactions. By 1941 Crook (62) was able to separate the ascorbic acid oxidase and DHA reductase activities and to show that glutathione was used in the reductase reaction. 

The best characterized of the enzymes involving DHA is ascorbic acid oxidase (L-ascorbate O2 oxidoreductase, EC 1.10.3.3). This plant enzyme catalyzes the reaction of AA and oxygen to give DHA and 1 mol of water (63). 

Enzyme methods using ascorbic acid oxidase have not been widely used but several versions of reagents based on diazotized nitroanilines have been reported (41). The reaction is complicated and the ascorbic acid molecule is partly destroyed in forming the colored product. A chromatographic separation stage must precede the color development reaction (42). The procedure is time-consuming and there is no provision for reducing any dehydroascorbic acid in the sample extract to ascorbic acid. 

The aerobic oxidation of RAA (Figure 1) occurs rapidly when metal catalysts, particularly copper or iron, or enzymes such as ascorbic acid oxidase, polyphenol oxidase, peroxidase, and cytochrome oxidase are present. The anaerobic destruction of ascorbic acid may proceed by a variety of mechanisms that have been postulated (2,3) but not verified. 

In order to better understand the effect of ascorbic acid on non-enzymatic browning of citrus juices, we oxidized endogenous ascorbic acid in grapefruit with ascorbic acid oxidase. The enzyme oxidized ascorbic acid to dehydroascorbic acid and H2O without forming H2O2 (19). 

The ascorbic acid is oxidized by ascorbic acid oxidase in the flour dough to dehydroascorbic acid. In turn, the reduction of dehydroascorbic acid is coupled with the oxidation of protein thiol groups by the enzyme dehydroascorbic acid reductase known to be in wheat (133). 

In the following discussion we will consider the properties of three different classes of this general type of enzyme laccases, ceruloplasmin, and ascorbic acid oxidase. Pertinent data are assembled in Table 2. Each of these enzymes catalyses the general reactions ... 

This type involves a mixture of types I, II and III coppers. Ascorbic acid oxidase, laccase and celluloplasmin are of this type. These enzymes oxidize substrates with O2, forming two H2O molecules from one O2. 

These observations confirm the view that the copper-protein bond in the enzyme is of a nondissociable nature and are not in agreement with the work of Lampitt and Clayson (1945), who postulated that the catalytic activity of ascorbic acid oxidase was to be attributed to traces of ionic copper resulting from an ionization of the copper bound to nonspecific protein. 

Ascorbic acid oxidase Old yellow enzyme Liver catalase Cytochrome C... 

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