Release of enzymes from cell walls by an endopectate-trans-eliminase

A cell wall-bound adenosine nucleosidase is involved in the salvage of extracellular ATP in Solanum tuberosum

Extracellular ATP (eATP) has recently been demonstrated to play a crucial role in plant development and growth. To investigate the fate of eATP within the apoplast, we used intact potato (Solanum tuberosum) tuber slices as an experimental system enabling access to the apoplast without interference of cytosolic contamination. (i) Incubation of intact tuber slices with ATP led to the formation of ADP, AMP, adenosine, adenine and ribose, indicating operation of apyrase, 5'-nucleotidase and nucleosidase. (ii) Measurement of apyrase, 5'-nucleotidase and nucleosidase activities in fractionated tuber tissue confirmed the apoplastic localization for apyrase and phosphatase in potato and led to the identification of a novel cell wall-bound adenosine nucleosidase activity. (iii) When intact tuber slices were incubated with saturating concentrations of adenosine, the conversion of adenosine into adenine was much higher than adenosine import into the cell, suggesting a potential bypass of adenosine import. Consistent with this, import of radiolabeled adenine into tuber slices was inhibited when ATP, ADP or AMP were added to the slices. (iv) In wild-type plants, apyrase and adenosine nucleosidase activities were found to be co-regulated, indicating functional linkage of these enzymes in a shared pathway. (v) Moreover, adenosine nucleosidase activity was reduced in transgenic lines with strongly reduced apoplastic apyrase activity. When taken together, these results suggest that a complete ATP salvage pathway is present in the apoplast of plant cells.

Generation of superoxide anion and hydrogen peroxide at the surface of plant cells

In addition to well-known cell wall peroxidases, there is now evidence for the presence of this enzyme at the plasma membrane of the plant cells (surface peroxidase). Both are able to catalyze, through a chain of reactions involving the superoxide anion, the oxidation of NADH to generate hydrogen peroxide. The latter is oxidized by other wall-bound peroxidases to convert cinnamoyl alcohols into radical forms, which, then polymerize to generate lignin. However, there are other enzymes at the surface of plasma membranes capable of generating hydrogen peroxide (cell wall polyamine oxidase), superoxide anion (plasma membrane Turbo reductase), or both (plasma membrane flavoprotein?). These enzymes utilize NAD(P)H as a substrate. The Turbo reductase and the flavoprotein catalyze the univalent reduction of Fe3+ and then of O2 to produce Fe2+ and O2-., respectively. The superoxide anion, in the acidic environment of the cell wall, may then dismutate to H2O2. These superoxide anion- and hydrogen peroxide-generating systems are discussed in relation to their possible involvement in physiological and pathological processes in the apoplast of plant cells.

NAD(P)H oxidation elicits anion superoxide formation in radish plasmalemma vesicles

Radish plasmalemma-enriched fractions show an NAD(P)H-ferricyanide or NAD(P)H-cytochrome c oxidoreductase activity which is not influenced by pH in the 4.5-7.5 range. In addition, at pH 4.5-5.0, NAD(P)H elicits an oxygen consumption (NAD(P)H oxidation) inhibited by catalase or superoxide dismutase (SOD), added either before or after NAD(P)H addition. Ferrous ions stimulate NAD(P)H oxidation, which is again inhibited by SOD and catalase. Hydrogen peroxide does not stimulate NADH oxidation, while it does stimulate Fe2+-induced NADH oxidation. NADH oxidation is unaffected by salicylhydroxamic acid and Mn2+, is stimulated by ferulic acid, and inhibited by KCN, EDTA and ascorbic acid. Moreover, NADH induces the conversion of epinephrine to adrenochrome, indicating that anion superoxide is formed during its oxidation. These results provide evidence that radish plasma membranes contain an NAD(P)H-ferricyanide or cytochrome c oxidoreductase and an NAD(P)H oxidase, active only at pH 4.5-5.0, able to induce the formation of anion superoxide, that is then converted to hydrogen peroxide. Ferrous ions, sparking a Fenton reaction, would stimulate NAD(P)H oxidation.

Polyphenol oxidase and peroxidase 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 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.

Some enzymes present in the walls of mesophyll cells of tobacco leaves

Cell-wall enzymes were assayed by the difference between enzyme activities in the whole cell and the protoplast. Both peroxidase (85.2%) and acid phosphatase (21.9%) were located in the wall. However, malate dehydrogenase was found only in the protoplast. A study of the time-course of the release of peroxidase and malate dehydrogenase into the incubation medium from cells either treated with cellulase or untreated, also indicated that peroxidase and not malate dehydrogenase was located in the wall. Only two anodic isoenzymes of peroxidase were present in the cell wall. These were more negatively charged than those of horseradish peroxidase.