Plant cell ascorbate system

Takahama, U. Oniki, T. A peroxidase/phenolics/ascorbate system can scavenge hydrogen peroxide in plant cells. Physiol. Plantarum 1997, 101, 845-852. 

Ushimara, T., Maki, Y., Sano, S., Koshiba, K., Asada, K., and Tsuji, H., 1997, Induction of enzymes involved in the ascorbate-dependent antioxidative systems, namely ascorbate peroxidase, monodehydroascorbate reductase and dehydroascorbate reductase, after exposure to air of rice Oryza sativa) seedlings germinated under water, Plant Cell Physiol. 38 541n549. 

In order to further understand the functions of ascorbate that may occur at the plasma membrane, we review here the participation of this vitamin as a prooxidant in the electron transport in this organelle and its role in cell growth control. We include here also the special role of the ascorbate system in plant cells. Relationships of both ascorbate function and its recycling in the plasma membrane with other known mechanisms of growth control are also considered in an attempt to present the integration of extracellular ascorbate in cell physiology as a whole. 

A widely distributed group of enzymes known as the tyrosinases or polyphenol oxidases catalyzes the oxidation of phenolic substances by oxygen. Where these have been isolated, they have been shown to be copper proteins. These enzymes are particularly abundant in plant tissues where they may function as terminal oxidases in place of the cytochrome system the relative importance of the phenol oxidases in plant cell respiration, however, has not yet been determined.The oxidation of ascorbic acid in plant tissues is also due to the presence of a copper enzyme. 

D-glucose and the three-enzyme system GOx, mutarotase and invertase for sucrose estimation. A common format was adopted to facihtate design and operation, in this case immobilization method, the fact that all enzymes used were oxidases and that a common detection principle, reoxidation of H2O2 generated product, was chosen (except for ascorbic acid which was estimated directly). Pectin, a natural polysaccharide present in plant cells, was used as a novel matrix to enhance enzyme entrapment and stabilization in the sensors. Interferences related to electrochemi-caUy active compounds present in fruits under study were significantly reduced by inclusion of a suitable cellulose acetate membrane diffusional barrier or by enzymatic inactivation with ascorbate oxidase. Enzyme sensors demonstrated expected response with respect to their substrates, on analyte average concentration of 5 mM. 

In plants, ascorbate is required as a substrate for the enzyme ascorbate peroxidase, which converts H202 to water. The peroxide is generated from the 02 produced in photosynthesis, an unavoidable consequence of generating 02 in a compartment laden with powerful oxidation-reduction systems (Chapter 19). Ascorbate is a also a precursor of oxalate and tartrate in plants, and is involved in the hydroxylation of Pro residues in cell wall proteins called extensins. Ascorbate is found in all subcellular compartments of plants, at concentrations of 2 to 25 mM—which is why plants are such good sources of vitamin C. 

DNA damage in the presence of cytochrome P450s (CYPs) and Cu(II) in vitro or in cells [Yoshino et al., 2004 Ahsan et al., 1999]. At higher concentrations curcumin exhibited genotoxicity and cytotoxicity in different systems [Cao et al., 2007]. At last, in vivo intervention trials using typical antioxidants such as ascorbate or carotenes yielded mostly disappointing results in terms of protection against oxidation-related diseases, sometime even increased the incidences of diseases [Stevenson and Hurst, 2007 Dinkova-Kostova and Talalay, 2008]. Obviously, the direct antioxidant capacity of plant phenolic compounds cannot explain their cytoprotective activities. 

It is clear that ferric chelates present in soil water are the natural electron acceptors for the inducible system (or turbo reductase) responsible for ferric reduction prior to iron uptake by dicotyledoneous and nongrass monocotyledoneous plants (Holden et al., 1991 Lesuisse and Labbe, 1992). In contrast, the natural electron acceptor of the so-called constitutive plasma membrane redox system both in plant and animal cells has not been completely defined. In addition to oxygen and iron-containing compounds, the semioxidized form of ascorbate, AFR, has been proposed as a natural electron acceptor (Goldenberg et al, 1983). 

---