Diphenol oxidase

These copper ion-dependent enzymes [EC 1.10.3.1] (also referred to as diphenol oxidases, O-diphenolase, phe-nolases, polyphenol oxidases, or tyrosinases) catalyze the reaction of two catechol molecules with dioxygen to produce two 1,2-benzoquinone and two water. A variety of substituted catechols can act as substrates. Many of the enzymes listed under this classification also catalyze a monophenol monooxygenase activity [/.c., EC 1.14.18.1]. See also Monophenol Monooxygenase Tyrosine Monooxygenase... 

Dopa is converted by at least some insects into N-P-alanyldopamine, which is a preferred substrate for the o-diphenol oxidase of the insect pupal cuticle. Oxidation of this substrate plays a crucial role in the... 

C4.1 Polarographic and Spectrophotometric Assay of Diphenol Oxidases (Polyphenol Oxidase)... 

The assays presented in this section deal with the measurement of enzyme activity, which is expected to be proportional to the amount of active enzyme present in a sample, food or otherwise, unit ci.i is an overview of the important considerations in performing activity assays unitc 1.2 illustrates how these considerations are applied to the assay of a representative food-relevant glycosyl hydrolase. Chapters C2, C3, and C4 present the first units on particular types of activity assays. In Chapter C2, two units present peptidase activity assays that use either synthetic substrates (UNITC2.1) or common, commercially available protein substrates (unit C2.2). unitC3.i presents three different assays for lipase activity. unitc4.i presents assays for diphenol oxidases, and unitc4.2 for lipoxygenase. 

Basic Protocol Polarographic Assay of Diphenol Oxidases C4.1.2... 

The diphenol oxidases (DPOs) includes two major groups of enzymes the ortho-DPOs (also known as catecholases, polyphenol oxidases, and tyrosinases) and the para-DPOs (more usually known as laccases). The names catecholase and laccase are used in this unit. 

Figure C4.1.1 The basic reactions catalyzed by diphenol oxidases (DPOs).

Polarographic and Spectrophotometric Assay of Diphenol Oxidases Key References Machiex et al., 1990. See above. A comprehensive review of all aspects of the chemistry and biochemistry phenolic compounds in fruits. Mayer, A.M. 1987. Polyphenol oxidases in plants—recent progress. Phytochemistry 26 11-20. A useful review of plant DPOs. 

Walker, J.R.L. and Ferrar, PH. 1998. Diphenol oxidases, enzyme-catalysed browning and plant disease resistance. Biotechnol. Genet. Eng. Rev. 15 457-498. 

Digestibility, protein quality analysis in vitro, 131, 134, 136-139 in vivo, 127-128, 136-139 Dilatometry, to measure fat, 571 -572 Dimethyl sulfoxide (DMSO), plant cell wall, isolation, 706, 708 2,4-Dinitrophenylhydrazine (DNPH), determination of carbonyl compounds, 553-554, 558 (fig.) Diphenol oxidases inhibitors, 392... 

Phenolic acids Tannins Polyphenol oxidase, see Diphenol oxidases Polysaccharides cell wall... 

EC 1.11.1.7) (68) and diphenol oxidase (EC 1.10.3.1) (69) have been identified. The potential role of pyruvic decarboxylase (EC 4.1.1.1) catalyzed reaction as a source of acetaldehyde and other aldehydes in juice was discussed (70). Raymond et al. (71) isolated the decarboxylase from orange juice sections and demonstrated that only 10 to 15% of the enzyme was in an active form. Since the purified enzyme was only active with pyruvic acid and 2-ketobutyric acid of the series of 2-ketoacids examined, they (71) concluded that the direct contribution of orange pyruvic decarboxylase to the orange volatile profile was limited to acetaldehyde and possibly propionaldehyde. 

Horigome and Kandatsu (63) prepared a Fuki (Petasites japon-icus miq. a traditional food plant in Japan) acetone-powder with a high o-diphenol oxidase activity. When the powder was added to a mixture of caffeic acid and casein, casein was gradually pigmented. A feeding test with rats demonstrated that there was a close relationship between the decrease in biological value of the pigmented casein and its color intensity. 

Li et al. (1990) developed an assay to measure the diphenol oxidase activity of tyrosine by following the conversion of 3,4-dihydroxymandelic acid (DHMA) to 3,4-dihydroxybenzaldehyde (DHBZ). Tyrosinase is involved in the formation of melanotic pigments in a wide variety of plants and animals. 

Bocks, S.M. 1967. Fungal mctabolism-rv. The oxidation of psilocin byp-diphenol oxidase Phytochemistry6 i6i.[Pg.561]

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