Phenolase enzymes

The familiar browning of cut friut and vegetables caused by the enzyme phenolase is another aspect of the close proximity of phenolic sysztems in daily life (ref.43). Dopamine I2-(3,4-dihydroxyphenyl)ethylamine] is the causative agent implicated in recent genetic researches examining reasons for human novelty-seeking (ref. 44). 

This copper-dependent enzyme [EC 1.14.18.1] (also known as tyrosinase, phenolase, monophenol oxidase, and cresolase) catalyzes the reaction of L-tyrosine with L-dopa and dioxygen to produce L-dopa, dopaquinone, and water. This classification actually represents a set of copper proteins that also catalyze the reaction of catechol oxidase [EC 1.10.3.1] if only 1,2-benzenediols are available as substrates. 

The products of oxidation are easily visualized shortly after ozone treatments but no claim should be made that phenolics or the oxidative enzymes, polyphenol oxidase, phenolase or peroxidase... 

The involvement of phenols and enzymes of the phenolase complex appears to be secondary to the induction of necrosis. The induction must involve a modification of membrane structure which leads to altered membrane permeability and loss of cell compart-mentalization. If this occurs, regulation of cellular metabolism is lost, enz3mies are activated, and these and their substrates that are normally separated by membranes would react together. 

Phenols are present in chloroplasts (23, 24) and in vacuoles (25) of plant cells. The enzyme polyphenol oxidase and other enzymes of the phenolase complex are bound to the chloroplast lamellae or stroma 27) and in the cytoplasm (26). Although... 

Mason (30) and Pierpoint (31) have described the involvement of o-diphenols in plants and how they contribute to abnormal plant pigmentation. o-Diphenols are oxidized to o-quinones by enzymes of the phenolase complex (o-diphenol O2 oxidoreductase, E.C. 1.10.3.1) and by peroxidase (E.C. 1.11.1.7). o-Quinones react with amino acids, proteins, amines and thiol groups of proteins to polymerize and from reddish-brown pigments. Concentrations of caffeic acid are doubled in both bean (8) and peanut... 

Morphine and codeine biosynthesis (Samuelsson, 1999 Herbert et al., 2000 Novak et al., 2000) Studies on the biosynthesis of morphine have been carried out mainly on cell cultures mainly of Coptis japonica and species of Thalictrum. Two enzymes (tyrosine decarboxylase and phenolase) catalyze the formation of dopamine from one molecule tyrosine. Dopamine is also the key intermediate in the biosynthesis of mescaline. 

Studies on irradiated solutions of phenolase showed that the enzyme was inactivated. The radiation-induced changes were found to be different from those reported to occur upon denaturation of proteins. Infrared absorption spectra revealed that deamination had occurred. Acid- and basebinding groups were reduced in number rather than increased, and the optical rotation became more dextrorotatory than levorotatory. It was fur-... 

The presence of oxygen was shown to enhance inactivation of the enzyme, while the presence of ovalbumin in the phenolase solution protected the activity. This was accepted as evidence that indirect action of the radiation was responsible for the inactivation of phenolase since the presence of dissolved oxygen or protein would have no effect on direct collisions of the 7-rays and the enzyme molecules. 

This class includes enzymes that use diphenols or related compounds as electron donors and oxygen as the acceptor, thereby forming the oxidized donor and water. Members include catechol oxidase (E.C. 1.10.3.1), laccase (E.C. 1.10.3.2), and o-aminophenol oxidase (E.C. 1.10.3.4). Laccase is also known as / -diphenoloxidase. whereas catechol oxidase is also known as diphenoloxidase, phenoloxidase, polyphenoloxidase, o-diphenolase, phenolase and tyrosinase. Many of these names are also used in reference to a different enzyme, monophenol monooxygenase (E.C. 1.14.18.1). This enzyme will be discussed further in Section 1.8.2.2. 

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. 

The enzymes involved in enzymic browning are known by the name polyphenoloxi-dase and are also called polyphenolase or phenolase. It is generally agreed (Mathew... 

Catechol as an Activator of Tyrosinase. The phenolase activity of tyrosinase has been studied less completely than the catecholase activity, partly because of the lack of a satisfactory assay procedure. The phenolase reaction, however, is characterized by a lag time which can be abolished by adding dihydroxyphenylalanine (DOPA), the immediate product of the hydroxylation reaction 29S4, 102, 117), This phenomenon has been described by several investigators (29-34) and is illustrated in Figure 12, from Pomerantz and Warner (117), using the enzyme from Hamster melanoma. The same phenomenon has been analyzed by Duckworth and Coleman (102) for the mushroom enzyme. In the absence of DOPA, maximum velocity of the hydroxylase reaction is not reached for several minutes. Pomerantz and Warner (117) devised a convenient assay for the phenolase reaction by determining the radio-... 

Mechanistic aspects of the action of tyrosinase and the usual transduction schemes have been summarized on several occasions [166,170-173]. In short, this copper enzyme possesses two activities, mono- and di-phenolase. Due to the predominant presence of the mono phenolase inactive form (met-form), the enzyme is inherently inefficient for the catalysis of these monophenol derivatives. However, in the presence of a diphenol, the catalytic cycle is activated to produce quinones and the scheme results in an efficient biorecognition cascade. This activation is achieved more efficiently when combined with electrochemical detection through the reduction of the produced quinones [166], as illustrated in Fig. 10.5. Consequently, a change in the rate-hmiting step can be observed through kinetic to diffusion controlled sensors with a concomitant increase in stability and sensitivity, as depicted in Fig. 10.6. 

Phenolase. An enzyme that promotes the oxidation of phenolic compounds. 

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, collectively known as melanins (11). Maillard reactions, caramelizations and ascorbic acid oxidations can produce similar types of colored compounds (12). For some food processing... 

Another enzyme which will cause loss of ascorbic acid in plant material is phenolase. This is the substance which helps to produce the browning offruit such as apples when polyphenolic species are oxidised by oxygen from the air. The enzyme functions with oxygen and ascorbic acid and reduces or/Ao-quinones back to orfAo-diphenols. This results in the formation of dehydroascorbic acid, which is rapidly converted to 2,3-diketogulonic acid. The process is catalysed by copper(ii) and other transition metal ions, which therefore accelerate the loss of ascorbic acid from vegetables and fruit, when for example they are cooked in copper or iron containers. Of course the major factor in the removal of vitamin C from cooked vegetables is simple dissolution in the cooking water. 

Some enzymes capable of oxidizing C-H bonds contain copper ions [52]. For example, tyrosinase [53a] contains a coupled, binuclear copper active site which reversibly binds dioxygen as a peroxide that bridges between the two copper ions. This enzyme catalyzes the orthohydroxylation of phenols with further oxidation of catechol to an ortho-quimne [53b-d], The mechanism proposed for phenolase activity of tyrosinase is shown in Scheme XI. 13 [521]. 

In this chapter, the dioxygen activation mechanism at the dinuclear copper-active sites of tyrosinase and catechol oxidase has been surveyed. In both enzymes, a (ji-rfirf -peToxo) dicopper(II) complex has been detected and characterized as a common reactive intermediate by several spectroscopic methods. In spite of longstanding efforts in the enzymological studies, mechanistic details of the enzymatic reactions (phenolase and catecholase activities) still remain ambiguous. On the other hand, recent developments in the model chemistry have provided a great deal of information about the structure and physicochemical properties as well as the reactivity of the peroxo intermediate and have advanced our understanding of the enzymatic reactions. 

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