Polyphenol, oxidase

Alcohol dehydrogenase (5) and leucine a-ketoglutarate transaminase (33,34) contribute to the development of aroma during black tea manufacturing. Polyphenol oxidase and peroxidase are essential to the formation of polyphenols unique to fermented teas. 

FIa.VOnoIOxida.tlon, The fermentation process is initiated by the oxidation of catechins (1) to reactive catechin quinones (13), a process catalyzed by the enzyme polyphenol oxidase (PPO) (56). Whereas the gaHocatechins, epigaHocatechin, and epigaHocatechin gaHate, are preferred, polyphenol oxidase can use any catechin (Table 2) as a substrate. This reaction is energy-dependent and is the basis of the series of reactions between flavanoids that form the complex polyphenoHc constituents found in black and oolong teas. 

Tissue and Bacteria Electrodes The limited stability of isolated enzymes, and the fact that some enzymes are expensive or even unavailable in the pure state, has prompted the use of cellular materials (plant tissues, bacterial cells, etc.) as a source for enzymatic activity (35). For example, banana tissue (which is rich with polyphenol oxidase) can be incorporated by mixing within the carbon paste... 

FIGURE 6-12 The mixed tissue (banana) carbon-paste sensor for dopamine. PPO = polyphenol oxidase. (Reproduced with permission from reference 36.)... 

RODRIGUEZ LOPEZ J, FENOLL L G, TUDELA J, DEVECE C, SANCHEZ HERNANDEZ D, DE LOS REYES E and GARCIA CANOVAS F (1999) Thermal inactivation of mushroom polyphenol oxidase employing 2450 MHz microwave radiation , J Agric Food Chem, 47 (8) 3028-35. 

Escribano, J. et al., Characterization of monophenolase activity of table beet polyphenol oxidase determination of kinetic parameters on the tyramine/dopamine pair, J. Agric. Food Ghem., 45, 4209, 1997. 

Escribano, J., Cabanes, J., and Garcfa-Carmona, R, Characterisation of latent polyphenol oxidase in table beet effect of sodium dodecyl sulphate, J. Sci. Food Agric., 73, 34, 1997. 

Zakharova and co-workers studied a red variety in 1997 to gain a closer insight into its polyphenol oxidase activity. More recently, a study addressing the antioxidant properties of a red colored Swiss chard was published. However, the pigments were erroneously addressed as anthocyanins. 

Gandia-Herrero, R, Garefa-Carmona, R, and Escribano, J., Purification and characterization of a latent polyphenol oxidase from beet root Beta vulgaris L.), J. Agric. Food Chem., 52, 609, 2004. 

Figure 1. Schematic outline of various products and associated enzymes from the shikimate and phenolic pathways in plants (and some microorganisms). Enzymes (1) 3-deoxy-2-oxo-D-arabino-heptulosate-7-phosphate synthase (2) 5-dehydroquinate synthase (3) shikimate dehydrogenase (4) shikimate kinase (5) 5-enol-pyruvylshikimate-3-phosphate synthase (6) chorismate synthase (7) chorismate mutase (8) prephenate dehydrogenase (9) tyrosine aminotransferase (10) prephenate dehydratase (11) phenylalanine aminotransferase (12) anthranilate synthase (13) tryptophan synthase (14) phenylalanine ammonia-lyase (15) tyrosine ammonia-lyase and (16) polyphenol oxidase. (From ACS Symposium Series No. 181, 1982) (62).

In addition to all of the expected enzyme systems present in leaf tissue, fresh tea leaves contain a high level of polyphenol oxidase that catalyzes the oxidation of the catechins by atmospheric oxygen. Tea polyphenol oxidase exists as series of copper-containing (0.32%) isoenzymes. The major component has a molecular weight of about 144,000.54 The enzyme is concentrated in the leaf epidermis.55 Soil copper deficiency is sometimes responsible for inadequate oxidation during processing.56... 

The initial oxidation of the flavanol components of fresh leaf to quinone structures through the mediation of tea polyphenol oxidase is the essential driving force in the production of black tea. While each of the catechins is oxidizable by this route, epigallocatechin and its galloyl ester are preferentially oxidized.68 Subsequent reactions of the flavonoid substances are largely nonenzymic. 

The rolling or leaf maceration step is carried out in order to disrupt cell structure and allow contact between tea flavanols and tea polyphenol oxidase. The physical condition of the leaf mass must also facilitate oxygen availability. 

Motoda, S., Formation of aldehydes from amino acids by polyphenol oxidase, J. Ferment Technol., 57, 395, 1979. (CA92 17776e)... 

With the death of the bean, cellular structure is lost, allowing the mixing of water-soluble components that normally would not come into contact with each other. The complex chemistry that occurs during fermentation is not fully understood, but certain cocoa enzymes such as glycosidase, protease, and polyphenol oxidase are active. In general, proteins are hydrolyzed to smaller proteins and amino acids, complex glycosides are split, polyphenols are partially transformed, sugars are hydrolyzed, volatile acids are formed, and purine alkaloids diffuse into the bean shell. The chemical composition of both unfermented and fermented cocoa beans is compared in Table 1. 

Chlamiac, E. B., Polyphenol oxidase in Ilex paraguariensis. Bol Inst Quim Agric, 39, 13, 1955. 

Ar-quinolin-2-yl)imino]propionates by injection or sublimation at 530°C yielded a mixture of 3-amino- and 3-ethoxy-l//-pyrimido[l,2-rz]quinolin-l-ones <2004AJC577>. An oxidative cascade for the biomimetic formation of the pyoverdine chromophore was supported by incubation of 2-[(4-hydroxyphenyl)- and 2-[(2-(3,4-dihydroxipheny-l)ethyl]-l,4,5,6-tetrahydropyrimidines with polyphenol oxidase or Pseudomonas extract to afford a mixture of 8,9-dihydroxy-2,3-dihydro- and -2,3,5,6-tctrahydro-l //-pyrimido[ 1,2- ]quinolines <20030L2215>. Oxidation of 2-[(2-(3,4-dihydroxiphenyl)ethyl]-l,4,5,6-tetrahydropyrimidine with MnOz gave a similar result. 

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