Norman F. Haard

Polyphenol oxidases and peroxidases in fruits and vegetables

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 ...

Induction of rishitin and lubimin in potato tuber discs by non-specific elicitors and the influence of storage conditions

Abstract Mercuric acetate, u.v. light and other non-specific elicitors caused accumulation of rishitin and lubimin in potato tuber discs. Terpene accumulation was not directly related to the necrotic reaction, was highly dependent on the concentration of elicitor and on the post-harvest storage history of the tubers and was inhibited by antibiotics. Tubers storred at 4 °C for 1 month after harvest accumulated approximately 5 times more terpenes than those stored at 25 °C. Discs from freshly harvested tubers did not accumulate detectable levels of terpenes as a result of these stress conditions. The data explain the inconsistent reports on the effect of non-specific elicitors on potato phytoalexins and point to a need for careful control of post-harvest storage conditions for such studies.

Isolation and partial characterization of auxin protectors from Synchytrium endobioticum incited tumors in potato

Abstract Auxin protectors were isolated from potato tubers, potato sprouts and Synchytrium endobioticum incited tumors (potato wart). Three groups of protector were distinguished by molecular sieve chromatography. Dormant tubers contained relatively low levels of auxin protector compared to sprouts and tumor tissue. Tumors differed from normal tissues by their relatively high content of high molecular weight protector. The water-soluble high molecular weight protector from tumors was partially purified and shown to be a protein fraction (mol. wt 10 000 to 20 000) containing covalently bound caffeic and ferulic acids. The protein fraction contained unusually high levels of serine (15·4%), glycine (14·5%), glutamic acid (14·1%), proline (9·7%), aspartic acid (9·1%) and alanine (7·3%). A water-insoluble high molecular weight protector fraction was also isolated from wart tissue and shown to also contain covalently bound caffeic and ferulic acids but to differ from the water soluble fraction in its unusually high content of hydroxyproline (18%). It is suggested that cell wall proteins may be an important source of protector activity in potato wart tissue.

Potentiation of wound induced formation of ipomeamarone by cyanide insensitive respiration in sweet potato (Ipomoea batatas) root slices

Summary Ipomeamarone, a furanosesquiterpenoid stress metabolite of sweet potato root, accumulated in slices treated with 5 mM HgCl 2 . A fivefold increase in HgCl 2 -induction of ipomeamarone accumulation was observed in tissue treated with either ethylene (100 ppm), cyanide (10 mM) or carbon monoxide (1 mM) in the dark. Ethylene and its analogues did not induce ipomeamarone accumulation in the absence of mercuric chloride. Potassium isothiocyanate (10 mM) and 8-hydroxyquinoline sulfate (10mM), known inhibitors of cyanide-insensitive respiration, repressed HgCl 2 and HgCl 2 -cyanide induction of ipomeamarone. Also, induction of ipomeamarone formation by HgCl 2 was potentiated by aging tissue slices 24 hours prior to treatment. These data are consistent with the hypothesis that ethylene and ethylene analogues potentiate ipomeamarone accumulation by evoking the cyanide-insensitive respiratory pathway.

Ergothioniene Inhibition of Peroxidase Mediated Indole-3-Acetic Acid Oxidation

Summary Ergothioniene caused a persistent inhibition of indole-3-acetic acid (IAA) oxidation catalyzed by horseradish peroxidase. Micromolar concentrations of ergothioniene inhibited the reaction when dichlorophenol — MnCl 2 were present in the assay as well as in the absence of these cofactors. Lineweaver-Burk analysis indicate the inhibition by ergothioniene is apparently competitive. The peroxidase catalyzed reaction at pH 6.1 was much more sensitive to ergothioniene than the reaction at pH 4.0. These data are discussed with reference to the mechanism of IAA oxidation in the presence of peroxidase.