Phenolase complex

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... 

Biochemistry of poppy alkaloid synthesis. Phenolase complex in Papaver somniferum... 

Mason HS, Eowlks WL, Peterson E (1955) Oxygen Transfer and Electron Transport by the Phenolase Complex. J Am Chem Soc 77 2914... 

Mason, H.S. (1955). Comparative biochemistry of the phenolase complex. Adv. Enzym. 16 105... 

Phenolic compounds are also involved in the enzymatic and non-enzymatic browning of food products. The occurrence of browning reactions is often disadvantageous in the majority of fruit processing operations. Chlorogenic acid and catechin serve as the principal substrates for the phenolase complex that is responsible for the enzymic browning exhibited by many fruits and vegetables. 

The various systems containing the phenolase complex in animals and plants (heterotypical aspects of the same enzyme) are described in an excellent review by H. S. Mason (1955) and it will suffice to quote the conclusion of this review ... 

At the phylogenetic level of the plants, it appears to catalyze the formation of intermediates in biosynthetic systems which produce the flower pigments and related flavonoids, the lacs and lacquers, the simple and polymeric tannins and their esters, the phenolic alkaloids, the quinones, tropolones and simple plant melanins, and the lignins. At higher phylogenetic levels the phenolase complex catalyzes intermediate phases in the... 

Enzymes of this class catalyze, in effect, two reactions of oxygen (transfer and reduction), and it is particularly important, if difficult, to establish the number of active centers involved in each case. No enzyme has been shown rigorously to be a mixed-function oxidase because this requirement has not been adequately satisfied, but a number of enzymes are probably members of this class because they possess the remaining required characteristics. It is useful to refer to a complex of activities, e.g., phenolase complex, while the physical relationship of the oxygen-transferring and oxygen-reducing functions is still in question. 

The phenolase complex consists of two activities, phenol -hydroxylase ( cresolase, equation 53) and o-diphenol dehydrogenase ( catecholase, equation 54) (3,111,203,244,352,358,462,579,585,... 

The cresolase function of the phenolase complex requires a source of electrons. The oxidation of o-diphenols is thus required to maintain enzymic hydroxylation of monophenols (34,83,129,289,298,299, 486,517,571,586) but the reaction is also activated by other reducing agents (38,83,298,411,451,488,490). In the presence of purified py-rocatecholase, cresolase activity is inhibited (571). This inhibition can be reversed by addition of o-diphenol the relief lasts as long as diphenol is present. Furthermore, in the case of stereospecific mammalian phenolase complex, dihydroxyphenyl-L-alanine is a more efficient activating agent for hydroxylation of tyrosine than dihydroxy-phenyl-D-alanine (243). 

It is apparent that hypotheses proposed for the mechanism of action of the phenolase complex now comprise the reasonable permitted alternatives. Whichever is correct, one atom of the oxygen molecule consumed during hydroxylation of monophenols by the phenolase complex appears in the resulting o-diphenol and the other atom is reduced. The cresolase activity of this complex is accordingly a mixed function oxidase. The catecholase function, on the other hand, results in the reduction of both atoms of the oxygen molecule to water. It is therefore a four-electron transfer oxidase. 

Phenolase Complex. Phenolase (= tyrosinase, or phenol oxidase) converts tyrosine to dopa (= dihydroxyphenylalanine) and oxidizes the dihydroxy derivative further to the quinone stage. Through a series of subsequent reactions, some of which occur spontaneously and wathout enzymic catalysis, the black or brownish black melanin is finally formed (for a schematic representation of the reactions see Chapt. VIII-11). The phenolase is an oxidase with mixed functions, where the product of oxygenation, the hydroquinone derivative, simultaneously acts as... 

In 1955, Mason and his collaborators, through the use of Oj and found that during the oxidation of 3,4-dimethylphenol to 4,5-dimethylcatechol catalyzed by a phenolase complex, the oxygen... 

It is conceptually very difficult to admit the existence of a direct contact between the monohydroxyphenol and the phenolase (or its hypothetical oxygenated form) in the presence of ascorbic acid or of DPNH, when the existence of any such contact can be excluded in the presence of hydroquinone. But if, for the sake of discussion, this direct contact is admitted, it will be found that the direct enzymic hypothesis still remains inadequate. According to the direct hypothesis, the phenolase complex activated by a reducing agent, or the oxygenated phenolase complex, introduces the oxygen directly into the molecule of the monohydroxyphenol. The simplest formulation of the over-all reaction would be... 

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