Ascorbic biological function

The multi-copper oxidases include laccase, ceruloplasmin, and ascorbate oxidase. Laccase can be found in tree sap and in fungi ascorbate oxidase, in cucumber and related plants and ceruloplasmin, in vertebrate blood serum. Laccases catalyze oxidation of phenolic compounds to radicals with a concomitant 4e reduction of O2 to water, and it is thought that this process may be important in the breakdown of lignin. Ceruloplasmin, whose real biological function is either quite varied or unknown, also catalyzes oxidation of a variety of substrates, again via a 4e reduction of O2 to water. Ferroxidase activity has been demonstrated for it, as has SOD activity. Ascorbate oxidase catalyzes the oxidation of ascorbate, again via a 4e reduction of O2 to water. Excellent reviews of these three systems can be found in Volume 111 of Copper Proteins and Copper Enzymes (Lontie, 1984). 

Although many in vitro studies on the antioxidative property of food constituents have been reported, little is known about the biological functions of dietary antioxidants in vivo, except for several well-known antioxidants such as tocopherols, (i-carotene, and ascorbic acid. Because the bioavailability of food constituents is limited by their digestibility and metabolic fate, an oral administration trial of a dietary antioxidant is favored to evaluate its biological function. 

Copper proteins are involved in a variety of biological functions, including electron transport, copper storage and many oxidase activities. A variety of reviews on this topic are available (Sykes, 1985 Chapman, 1991). Several copper proteins are easily identified by their beautiful blue colour and have been labelled blue copper proteins. The blue copper proteins can be divided into two classes, the oxidases (laccase, ascorbate oxidase, ceruloplasmin) and the electron carriers (plastocyanin, stellacyanin, umecyanin, etc.). 

Ascorbic acid (1) has many biological functions besides its well-known antioxidant activity. It is a functional component of many pharmaceutical and cosmetic preparations. Antioxidants such as glutathione (45) or sodium metabisulfite (Na2S205) are added... 

Occurrence, Sequences, and Biological Function Molecular and Spectroscopic Properties X-Ray Structure of Ascorbate Oxidase... 

It is believed that ascorbate free radicals have an important biological function in the reaction with destructive free radicals, particularly those associated with oxygen which often cause problems in living cells. They have been shown to quench hydroxyl radical and singlet oxygen, a function that is thought to be particularly important in the eye where comparatively large amounts of vitamin C are frequently found. 

L. Avigliano A. Finazzi-Agro, Biological Function and Enzyme Kinetics of Ascorbate Oxidase. In Multi-Copper Oxidases ... 

Type 1 copper proteins are the class of proteins for which cupredoxins were originally named. Type 1 copper proteins include both proteins with known electron transfer function (e.g., plastocyanin and rusticyanin), and proteins whose biological functions have not been determined conclusively (e.g., stellacyanin and plantacyanin). Although these proteins with unknown function cannot be called cupredoxins by the strict functional definition, they have been classified as cupredoxins because they share the same overall structural fold and metal-binding sites as cupredoxins. In addition, many multidomain proteins, such as laccase, ascorbate oxidase, and ceruloplasmin, contain multiple metal centers, one of which is a type 1 copper. Those cupredoxin centers are also included here. Finally, both the Cua center in cytochrome c oxidase (CcO) and nitrous oxide reductase (N2OR), and the red copper center in nitrocyanin will be discussed in this chapter because their metal centers are structurally related to the type 1 copper center and the protein domain that contains both centers share the same overall structural motif as those of cupredoxins. The Cua center also functions as an electron transfer agent. Like ferredoxins, which contain either dinuclear or tetranuclear iron-sulfur centers, cupredoxins may include either the mononuclear or the dinuclear copper center in their metal-binding sites. 

Metabolic sparing of ascorbic acid by flavonoids. The fourth possible mechanism for the flavonoid-induced increases in tissue ascorbic acid is that flavonoids, by taking over some of the biological functions of the acid, thereby reduce the metabolic demand for the vitamin and permit its increased accumulation in the tissues. The inability of flavonoids to prolong the life of scorbutic guinea-pigs [43,46] implies that if substitution occurs then it must be in an area other than that normally associated with the development of scurvy — namely, the defective hydroxylation of the collagen molecule precursor. A possible corollary of this is that ascorbic acid is a multifunctional vitamin, with functional substitution by flavonoids possible only in certain metabolic areas. Such partial substitution is not unknown in nutrition the partial replacement in animals of the vitamin E function of tocopherols by antioxidants is a case in point. 

The hypothesis that ascorbic acid is required for the synthesis of PHI offers a convincing explanation for its biological function in preventing scurvy, a state of general-... 

Cameron. E. Biological function of ascorbic acid and the pathogenesis of scurvy. Med. Hypotheses, 2 154-163.1976. 

Ascorbate is an unstable molecule, which in aqueous solution in the presence of trace transition metals, can be oxidized, losing sequentially two electrons, yielding first the intermediate AFR, also known as semidehydroascorbate (SDHA), and then dehydroascorbate (DHA) (Bendich et al, 1986). In biological systems, ascorbate usually functions as a one-electron donor yielding AFR, and its full oxidation to DHA is rare due to structural constraints of the DHA molecule (Njus et al, 1990). 

Ascorbic acid oxidase is inhibited by agents which bind copper, including cyanide, diethyldithiocarbamate, and 8-hydroxy quinoline. Purified solutions are blue-green in color. The biological function of this enzyme, like that of the phenol oxidases, is not clear. 

Although ascorbic acid has been shown to have many biologic functions, little is known of its chemical role in metabolic processes. Recently, Sealock and Goodland and Knox and LeMay-Knox have demonstrated that ascorbic acid is a coenzyme in the oxidation of tyrosine. This is the only section of the vitamin s activity in which the molecular processes that depend on it are clearly understood (Chapter 11). This explains the abnormal excretion of tyrosine metabolites in human scurvy and the return to... 

The potential importance of the reactions with ascorbate as a possible indicator of biological function propelled us to study whether other metallotexaphyrins besides those containing lanthanide(III) cations could... 

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