Polyphenol oxidases in plants

Polyphenol oxidases and peroxidases are among the most studied enzymes in fruits and vegetables. Owing to the deleterious effects of discoloration and off-flavor formation induced by their actions, these enzymes have not ceased to be a matter of concern to food technologists, while their versatility as catalyst and their diversity as protein present a challenge to the biochemist. This article gives an account on the present state of knowledge in this field. The occurrence of polyphenol oxidases and peroxidases in food and food raw materials, and their role and importance in food processing are briefly outlined. Results of biochemical research including catalytic properties, substrate specificity, susceptibility towards pH and temperature, action of inhibitors, isolation, purification, and characteristics of the enzymes are given, with special emphasis on recent achievements based on high resolution separation and isoenzyme techniques. Finally, the behavior of polyphenol oxidase and peroxidase in selected major groups of fruits and vegetables is discussed. Some contradictions found in the literature are pointed out and some questions that have not been given the necessary attention by researchers so far are mentioned.

Polyphenol oxidase and peroxidase in fruits and vegetables.

Phenolic-storing cells : keys to programmed cell death and periderm formation in wilt disease resistance and in general defence responses in plants?

Publisher Summary Salivary function is especially interesting in Hemiptera because of the effects the saliva has on the living and surviving organisms, on which many of these insects feed. The saliva of Hemiptera is by no means a simple secretion—in addition to the usual salivary functions of moistening food and mixing it with hydrolytic enzymes before ingestion, the saliva of phytophagous species plays an important physiochemical role during the mechanical penetration of plant tissues by the piercing and sucking mouthparts; in accomplishing this task, the saliva may vary in its chemical composition and physical consistency from one moment to the next. Moreover, deposits of solidifying components of the saliva of many species persist in the food plants, modifying the long term effects of feeding by the insects. This chapter compiles the various types of investigation on salivary functions in the Homoptera and Heteroptera, and suggests profitable lines of future investigation based on analogous functions in different taxonomic groups.

The Saliva of Hemiptera

The Role of Phenolics in Host Response to Infection

Plant preparations that convert tryptophan to indoleacetic acid (IAA) characteristically have low reaction yields. Since such preparations sometimes also contain IAA-oxidase activity, inhibition of the oxidase might raise the yields of IAA. Accordingly, polyphenolic inhibitors (1, 11, 19, 33, 40) were added to incubation mixtures containing tryptophan and a seedling enzyme preparation that was able to convert tryptophan to IAA. More than tenfold increases in auxin yield resulted. However, the presence of auxin in enzymeless controls, plus experiments on the recovery of added IAA, indicated that the enhancements observed might be caused by a polyphenolic oxidation of tryptophan to IAA. We shall present evidence that the sequence

L. G. Paleg

Papers

Formation of auxin from tryptophan through action of polyphenols