Phenol oxidases substrates

Moreover, there is considerable evidence that PPO is not active as a phenol oxidase in chloroplasts, but is limited as a phenol oxidase by latency or lack of substrate [18]. The latent form of the enzyme can be activated by a wide variety of treatments, including detergents [19], fatty acids [20] trypsin [21] and Ca2+ [22]. The results of Tolbert [21] indicated that light could activate latent PPO because polyphenols can be oxidized photochemically by chloroplast membranes in the absence of other... 

This phenolic oxidase therefore seems to be a true antihypertensive substance, affecting blood pressure without deleterious effects on kidneys. There is a strong presumption, therefore, that some phenolic substrate important for the maintenance of hypertension... 

The phenol oxidases probably play no important role in the elimination of phenolic pressor amines, in spite of the importance that has been attached to the oxidation of the catechol nucleus in the past. The names phenolase and cresolase, polyphenol oxidase, and catechol oxidase serve to identify the enzyme with its mono- or diphenolic substrate, but they usually occur together and are difficultly separated. The enzymes have been purified and their characteristics have been described (56, 104, 106, 156). Beyer (21), Alles (5), and Randall and Hitchings (129) have described the relationship of structure of the phenolic pressor amines to the rate of oxidation of their nucleus in the presence of these enzymes. 

Humic substances are abundant in DOM as well as POM, but the role of phenol oxidases in DOM metabolism has been little studied (Munster and De Haan, 1998). Foreman (1999) found that phenol oxidase activity in water from five contrasting Michigan streams was correlated with phenol concentration and that bacterial production in systems with significant humic DOC input was stimulated by the addition of phenols, suggesting that phenols may be an important growth substrate for some bacterial guilds. 

A phenol oxidase from the fish cestode, Penetrocephalus ganapatii, has been characterised bio-chemically and has been shown to have a pH optimum of 7.4 (Fig. 7.9) (362). Its activity with three different substrates, dopamine, adrenaline (= epinephrine) and dopa are shown in Table 7.4 and Fig. 7.10 the action of various inhibitors is shown in Table 7.5. The Michaelis constant for dopamine was 189 p.M and the enzyme was stable between 20 and 40°C. The action of various inhibitors was also studied. 

Fig. 7.10. Scanned zymogram pattern of phenol oxidase from the pseudophyllidean, Penetrocephalusganapalii, with three different substrates. The gels were equilibrated in 10 mM sodium phosphate buffer, pH 7.4, stained for activity with 0.5 mM substrates in phosphate buffer for 8-10 h and scanned with an extinction recording densitometer. (After Jayabaskaran Ramalingam, 1985.)...

Phenol oxidases belong to the so-called monooxygenases, since they catalyze the incorporation of one atom of oxygen into the substrate. Phenol oxidases are present in all fruit and vegetable products and can affect the flavour and colour by the well-known browning reaction [23]. The enzymatic browning reaction involves the hydroxylation of plant phenolic compounds into the dihydroxy... 

All these reactions occur in the cell, but on the whole, the cell finds the second and third easier to perform, and, what is more, can perform either of them without the direct intervention of oxygen itself. Reaction (1) does have some importance though. There are enzymes, called oxidases, which catalyse the direct oxidation of their substrates by atmospheric oxygen. They are found in most cells, but are especially common in plants. The effects of one of them, polyphenol oxidase, are familiar to those who peel their apples before eating them. Apples contain traces of a compound called catechol which is oxidized, under the influence of poly phenol oxidase, to a complex, dark brown substance. When an apple is cut its surface is exposed to the air and polyphenol oxidase can begin to work, turning the surface of the apple gradually brown. 

Figure 16.3-12. Substrates for phenol oxidase from Mucuna pruriens. 5-, 6-, or 7-Hydroxylated 2-aminotetralins with R = H or C3H7 and 9-hydroxy-N-(n-propyl)-hexahydronaphthoxazine are substrates for the phenol oxidase.

Phenoloxidase (monophenol monooxygenase, E.C. 1.14.18.1) introduces one atom of molecular oxygen into the substrate and was used in alginate-entrapped cells or in partially purified form. The pharmaceutical 7,8-dihydroxy-N-(di-n-propyl)-2-aminotetralin was produced continuously using a phenol oxidase suspension in dialysis tubing in an airlift fermenter coupled to an aluminium oxide column for selective product isolation (Figure 16.3-13)[6S1. A product concentration of 130 mg/L and a yield of 25 % were reached. 

In some fruits, PPOs use other phenolic substrates for example a relative of DOPA, 3,4-dihy-droxyphenylethylamine (dopamine), is the major substrate in bananas and DOPA is the natural substrate in the leaves of broad beans. The grape PPO acts on p-coumaryl and caffeoyl-tartaric (caftaric) acids while dates contain an unusual combination of diphenol oxidase substrates including a range of caffeoyl-shikimic acids these are analogous to the ubiquitous isomers of chlorogenic acid [45],... 

The phenol oxidase probably plays an important part in the sclerotization. We have purified the enzyme and have studied its action on various substrates. From these studies it appears that monophenols, including tyrosine, are only very slowly attacked by the enzyme. Dopa, dopamine and N-acetyldopamine are the best substrates of the diphenols tested. Other substances such as 3,4-dihydroxyphenylpyTuvicacid, 3,4-dihydroxyphenylacetic acid and protocatechuic acid are not attacked. 

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. 

Potato Phenol Oxidase. The studies of Kubowitz on the copper oxidase of potatoes were part of the efforts of Warburg s institute to find enzyme systems that could catalyze oxygen consumption coupled with substrate oxidation. The enzyme was purified on the basis of an assay involving transfer of electrons from pyridine nucleotides to o-quinone and from the resulting catechol to oxygen (I). The reduction of catalytic quantities of quinone was probably catalyzed by a flavoprotein present in the Zwischenferment preparation used to reduce TPN. In the presence of... 

Laccase. A polyphenol oxidase has been purified from the sap of the lac tree by Keilin and Mann. Laccase differs from the potato and mushroom enzyme in several respects. With regard to substrate specificity, it oxidizes p-phenylenediamine more rapidly than catechol. p-Phenylene-diamine is not a substrate for the other polyphenol oxidases described. Laccase apparently is inert with p-cresol. It is not inhibited by carbon monoxide. Unlike the other phenol oxidases, this enzyme is not a pale yellow, but is blue, as is ascorbic acid oxidase (see below). This enzyme, however, is not an ascorbic acid oxidase. 

A much better case can be made for the possible function of GSH in the hydrogen transport systems of higher plants. Ascorbic-acid oxidase appears to be characteristically a plant enzyme. Phenol oxidases and peroxidase systems are also found in many higher plants. Such enzymes can cause an oxidation of ascorbate indirectly by virtue of the nonenzymatic oxidation of ascorbic acid by compounds of quinoid structure formed from the phenolic substrate (39). Such systems are not universally distributed in plants, but where they are present in sufficient amount it is not unreasonable to suppose that some substrates may be oxidized by way of a respiratory chain consisting of substrate, TPN, GSH, ascorbic acid, and a terminal oxidase. 

Gunata, Y.Z., Sapis, J.C. Moutounet, M. (1987). Substrates and aromatic carboxylic-acid inhibitors of grape phenol oxidases. Plytochemistry, 26,1573-1575. 

The absence of evidence for homogeneity has prevented a clarification of the dual specificity of the phenol oxidases. Most preparations oxidize both monophenols and o-diphenols. With the monophenols, however, the maximum oxidation rate with purified enzymes is always preceded by an induction period which can be eliminated by the addition of traces of diphenol.This autocatalytic behavior with monophenols and the priming effect of diphenols may be due to a catalytic role of diphenols formed in the oxidation of the monophenolic substrate.Thus the oxidation of monophenols appears to follow the pattern ... 

A further prediction from Karlson s model is that acetyldopamine causes sclerotization. Indeed it has been found that even unspecific substrates of phenol oxidase induce sclerotization in explanted insect cuticles. 

The phenol oxidase (catechol Oxidase) of tea leaves provides a classic example of artifactual organelles, and probably of artifactual substrate specificity as well, both produced by protein-phenolic complexing. Li and Bonnerreported that the enzyme is insoluble, tightly bound to the chloroplast grana. Sanderson demonstrated that conventional extracts from tea leaves contained no soluble protein, and that the "chloroplast" fraction consisted largely of protein-tannin complexes. Addition of hydrated nylon as a phenol adsorbent solubilized the enzyme completely. 

In the formation of quinones, oxygen can accept electrons directly, not only in chemical experiments but also biochemically, with the aid of the enzyme phenol oxidase. As far as the reaction of the substrate is concerned, it is immaterial whether the electrons are transferred directly to oxygen or to some prosthetic group. It suffices that they are removed and the substrate thus oxidized. 

EC 3.2.1 glycosidases, EC 4.2.2 lyases, EC 3.1.1 esterases, EC 1.11.1 peroxidases, EC 1.1.3 carbohydrate oxidases, EC 1.10.3 phenol oxidase, and other EC classes, according to their main reactions. Each class and subclass has shared primary enzyme substrates, a feature that may facilitate enzyme selections for fargefed biomass materials. 

Enzymatic electrochemical biosensors are based on immobilized enzymes, whose products can be electrochemically measured after degradation of the substrate at the surface of the biosensor. Many different types of enzymatic biosensors have been developed for environmental monitoring of pesticides, phenols, heavy metals, nitrate, formaldehyde and sulfur oxide, etc. Commonly used enzymes include but are not limited to organophosphorus hydrolase (OPH), AChE, butyrylcholi-nesterase (BChE), horseradish peroxidase (HRP), tyrosinase (phenol oxidases), nitrate reductase, nitrite reductase, formaldehyde dehydrogenase (FDH), and sulfite oxidase (SO). 

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