Phenol oxidases

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

An oxidative activation of the lignin also can be achieved in a biochemical way by adding enzymes (phenol oxidase laccase) to the spent sulfite liquor, whereby... 

Fetterer, R.H. and Hill, D.E. (1993) The occurrence of phenol oxidase activity in female Trichuris suis. Journal of Parasitology 79, 155—159. 

Gottlieb and Geller (34) reported further to have obtained an enzyme preparation from mushroom spawn, which they claim to catalyze a reaction between native lignin and oxygen. They believe that this enzyme is different from other phenol oxidases heretofore studied. It was later reported that the enzyme acts on low molecular weight water soluble compounds closely related to native lignin. The nature of these compounds was, however, not determined (35). 

Ryan ID, Gregory R, Tingey WH. Phenolic oxidase activities in glandular trichomes of Solarium berthauttii. Phytochemistry 1983 21 1885-1887. 

Felby, C., Pedersen, L.S. and Nielsen, B.R. (1997a). Enhanced auto adhesion of wood fibers using phenol oxidases. Holzforschung, 51(3), 281-286. 

Kirh, K. T., and A. Kelman Lignin Degradation ivs Related to the Phenol Oxidases of Selected Wood-Decaying Basidiomycetes. Phyto-pathol. 55, 739-745 (1965). 

Oxidative coupling involves condensation reactions catalyzed by phenol oxidases. In oxidative coupling of phenol, for example, arloxy or phenolate radicals are formed by the removal of an electron and a proton from an hydroxyl group. The herbicide 2,4-D is degraded (Fig. 15.5) to 2,4 dichlorophenol, which can be oxidatively coupled by phenol oxidases (Bollag and Liu 1990). 

Studies of pathogenic fungi suggested an explanation for the absence of inducer molecules in host root exudate. Quinones such as 2,6-DMBQ have been shown to be released by white rot fungi as terminal oxidation products of lignin model compounds (38). These findings substantiated previous reports (39-41) that laccases, phenol oxidases using 0 as the oxidant, are directly involved in... 

Dr. Flaig. The polymeric products of lignin decomposition formed in the presence of phenol oxidases are not water soluble. 

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... 

Niemetz, R., and Gross, G. G., 2003, Oxidation of pentagalloylglucose to the ellagitannin, tellimagrandin II, by a phenol oxidase from Tellima grandiflora leaves, Phytochem. 62 301-306. 

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. 

In spite of the fact that phenol oxidase probably plays no important role in the inactivation of pressor amines in the body, it has been reported that the injection of the enzyme into hypertensive rats led to a reduction in their blood pressure (141,143)- It is difficult to assess the value of these experiments because of the nonspecific depressor effects of crude protein preparations on blood pressure. For example, Prinzmetal et al. (126) found that their tyrosinase preparations inactivated by boiling decreased the blood pressure of hypertensive patients as well as did their enzymically active preparations. 

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