Insects Phenol oxidase

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. 

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. 

Polyphenol Oxidases. Plant trichomes and their exudates confer resistance to a variety of insects (54-56). In solanaceous plants, such as the tomato and potato, trichomes contain polyphenol oxidases and catecholic phenolics (e.g., caffeic and chlorogenic acids), which contribute to resistance to a variety of insect pests. In the potato plant, the polyphenol oxidases and phenolics are separated in different trichomes. When insects, such as aphids or leaf hoppers, walk across the surface of the plant they break the two types of trichomes. Trichomal fluids are liberated and, upon mixing, polymerize as a result of polyphenol oxidase activity on catechols, forming an often lethal adhesive trap for the insects (52,58) In tomato plants, the polyphenol oxidase and chlorogenic acid are separated by intracellular compartments, but upon breakage of trichomes by insects, polymerization and physical entrapment occurs (54). 

Extrapolation of these results to the real world suggests that the simultaneous use of PPO/phenolics and proteinase inhibitors as bases of resistance against certain insects may be mutually incompatible. If one were to rely on proteinase inhibitors as a basis, the elimination of high levels of polyphenol oxidase in the plant would not guarantee antibiotic activity because high levels of dietary protein can abolish PI toxicity (89) Hence, the activity of PI, PPO and phenolics in situ may require the manipulation of multiple plant factors. 

Peng, Z. Miles, P.W. (1991). Oxidases in the gut of an aphid, Macrosiphum rosae (L.) and their relation to dietary phenolics. Journal of Insect Physiology, yi, 119-1% . 

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