Ascorbic acid oxidase source

For those, undoubtedly the majority even in Western countries, who rely on natural dietary sources of the vitamin, care is needed in culinary practice if much of the ascorbic acid is not to be lost. As seen in Chapter 5 the fine cutting of vegetables releases ascorbic acid oxidase which will destroy the vitamin and the use of excessive water for cooking will leach it out of the food. Overcooking and the addition of sodium bicarbonate, thankfully by now little practised, also destroy the vitamin by oxidation which is particularly rapid in cooking pots made of copper. For the human infant, as for any mammal, mother s milk is a whole food and that includes vitamin C at a level of 3.0-5.5 mg%. Bovine milk is much less rich and needs supplementation for feeding to human infants. 

Oxidations now known to be catalyzed by copper-containing enzymes were noticed over a century ago, when Schoenbein observed that oxidation of natural substrates resulted in pigment formation in mushrooms. Individual enzymes were gradually identified laccase by Yoshida in 1883 and tyrosinase by Bertrand in 1896. However, it was not imtil potato polyphenol oxidase was isolated in 1937 by Kubowitz that the role of copper was defined. The family of copper oxidases includes a number of enzymes of both plant and animal origin that may very probably be found to react through similar mechanisms, but which exhibit a number of individual characteristics. The enzymes to be described in this section include potato phenol oxidase, mushroom polyphenol oxidase (tyrosinase), laccase, mammalian and insect tyrosinase, and ascorbic acid oxidase. Each of these differs in certain respects from the others, and undoubtedly other related enzymes will be described from other sources that resemble these, but also display individualities. In these cases, identities in nomenclature must not be extended to imply identities in enzyme structure or activity. 

L-ascorbic acid Potentiometric Based on ascorbate oxidase of natural source immobilised on ethylene-vinylacetate membrane Fernandes et al. (1999)... 

Uchiyama et al. [36] used cucumber juice (source of ascorbate oxidase) for the first time as carrier in a flow injection system for the determination of L-ascorbic acid. In another work, the same researchers used banana pulp and spinach leaf solution as a source of polyphenol oxidase (PPO) in a flow injection system for the determination of polyphenols [37]. However, the first biosensor based on vegetable crude extract (homogenate) was constructed by Signori and Fatibello-Filho [38]. In this study, an amperometric biosensor for the determination of phenols was proposed using a crude extract of yam (Alocasia macrohiza)... 

Fernandes, J.C.B. Kubota, L.T. Neto, G.d.O. Potentiometric biosensor for L-ascorbic acid based on ascorbate oxidase of natural source immobilized on ethylene-vinylacetate membrane. Anal. Chim. Acta 1999, 385, 3-12. 

Uchida et al. excluded the possible function of ascorbic acid as a peroxide source and considered their enzyme to be an oxidase, not a peroxidase. Their second enzyme converted dihydroxyphenylpyruvic acid to homo-... 

In contrast to ascorbic acid and a-tocopherol amidothionophosphates did not have any prooxidative effects as measured by oxygen consumption from buffer solutions containing the drug and cupric sulphate as a source of redox-active metal ions (Ti-ROSH et al. 1996). Amidothionophosphates reduced significantly and in a dose-dependent manner the oxygen burst in human neutrophils as measured by luminol-dependent luminescence, and they also markedly depressed the killing of human fibroblasts by mixtures of glucose oxidase and streptolysin S. The toxicity of these molecules was tested by intraperitoneal injection of doses up to 1000 mg/kg to white Sabra mice. No mortality was observed 30 d after administration of up to 500 mg/kg. 

This chapter will concentrate mainly on structural and functional aspects of these enzymes with the major emphasis on ascorbate oxidase and laccase. Significant progress has been achieved in the last 10 years the determination of amino acid sequences of all three enzymes, each from several sources, and the X-ray structure of ascorbate oxidase. The new information forms the basis of a much deeper understanding of the function of the enzymes as will be demonstrated in this chapter. 

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