The facts presented in a number of investigations suggest that some well-known redox systems are present in the central vacuole. For example, ascorbic acid (AsA), as also ascorbate-dependent enzymes has been identified in the vacuoles of some plants. These facts allow the authors assume the fact of functioning of the ascorbate system in the vacuolar compartment. Furthermore, in the vacuoles of some plants, glutathione and glutathione-dependent enzymes have also been identified. Therefore, the glutathione system can function within the vacuole. The vacuoles accumulate phenolic compounds (PhO), which are substrates for vacuolar phenolic peroxidases (POXs). These enzymes are known to reduce hydrogen peroxide. Together with AsA and POXs, PhO form the redox system of POX/PhO/AsA (PPA) reducing H2O2. PPA system is considered as typical for the plant cell vacuolar compartment. Obviously, in the plant cell vacuole, there is the system of antioxidant defense including the ascorbate system, the glutathione system and the PPA system.

The clamp rot is one of the usual biotic stress factors for dormant red beetroots (Beta vulgaris L.). The process of root rotting proceeds due to the activity of a complex of microorganisms. A variety of protective systems, including the system of antioxidant defense, help the beetroot cells to resist the pathogenic microorganism action and to maintain their viability. Due to such factors, the activation of the antioxidant defense enzymes is expected. The results obtained demonstrate that antioxidant enzymes were activated in the vacuoles of beetroots affected by root rot. The activity of peroxidase (POX, EC 1.11.1.7), Cu, Zn-superoxide dismutase (Cu, Zn-SOD, EC 1.15.1.1) and glutathione reductase (GR, EC 1.8.1.7) increased markedly. The activity of antioxidant enzymes in the vacuoles has been assessed to be quite high compared to that of tissue extracts. In the tissues of affected beetroots, the activity of tSOD (total SOD) did not increase, while tPOX (total POX) activity reduced. Only the tGR (total GR) activity increased. The isozyme composition of vacuolar enzymes was characterized by stability. The increase in the number of cationic isoforms of tPOX was noted in the tissue of affected beetroots. In general, the changes in the activity of the vacuolar antioxidant enzymes suggest that the central vacuole is an important component of cell defense in biotic stress.

Vacuoles and leucoplasts were isolated from red beet (Beta vulgaris L.) taproots and compared in terms of ascorbic acid concentration and ascorbate oxidase (AO) activity. The total amount of ascorbic acid (AA(tot)) was greater in vacuoles than in the plastids. The content of oxidized form of ascorbic acid, i.e., dehydroascorbic acid (DHA), was also higher in vacuoles than in the leucoplasts. However, the content ratios of reduced ascorbic acid (AA) to its oxidized form (AA : DHA) were similar in vacuoles and leucoplasts. The studied organelles displayed a relatively high AO activity. The oxidation of AA was catalyzed by ascorbate oxidase in vacuoles, whereas it was mediated by ascorbate oxidase and polyphenol oxidase in leucoplasts. The oxidases catalyzing AA oxidation are presumed to regulate the redox state of ascorbic acid in these organelles.

The role of glutathione in the plant vacuole is still being debated. In the present paper, the redox state of glutathione and the activity of glutathione S-transferase (GST, E 2.5.1.18) in the vacuole compared to those in leucoplast have been studied. Organelles were isolated from dormant red beet (Beta vulgaris L) taproots. Two generally used approaches have been applied to quantitatively assess the content of glutathione. Initially, levels of glutathione were measured in isolated organelles after labeling with monochlorobimane (MCB) and imaging with the use of confocal laser scanning microscopy. However, there are factors limiting the specificity of this method, because of which the resulting concentrations of vacuolar GSH have been underestimated. Another approach used was HPLC, which allows to simultaneously quantify the reduced glutathione (GSH) and glutathione disulfide (GSSG). The concentration of the total glutathione (GSHt) and GSSG in vacuoles determined with the aid of HPLC-UV was higher in comparison to that in the leucoplasts. The reduction potential (E-h) for the glutathione couple in the vacuoles was more positive (-163 mV), than that in plastids (- 282 mV). The relatively rapid increase in fluorescence in the isolated vacuoles and plastids during MCB-labeling has indicated to the contribution of GSTs, since the conjugation of GSH to bimane is catalysed by these enzymes. The GST activity in the vacuoles has been assessed to be quite high compared to that of leucoplasts. The number of isoforms of GSTs also differed markedly in vacuoles and plastids. Collectively, our findings suggest the idea that the glutathione accumulated by central vacuole seems to contribute to the redox processes and to the detoxification, which can take place in this compartment.

Proportions between oxidized and reduced glutathione forms were determined in vacuoles isolated from red beet ( Beta vulgaris L.) taproots. The pool of vacuolar glutathione was compared with glutathione pools in isolated plastids and mitochondria. The ratio of glutathione forms was assessed by approved methods, such as fluorescence microscopy with the fluorescent probe monochlorobimane (MCB), high-performance liquid chromatography (HPLC), and spectrophotometry with 5,5′-dithiobis-2-nitrobenzoic acid (DTNB). The fluorescence microscopy revealed comparatively low concentrations of reduced glutathione (GSH) in vacuoles. The GSH content was 104 μM on average, which was lower than the GSH levels in mitochondria (448 μM) and plastids (379 μM). The content of reduced (GSH) and oxidized (GSSG) glutathione forms was quantified by means of HPLC and spectrophotometric assays with DTNB. The glutathione concentrations determined by HPLC in the vacuoles were 182 nmol GSH and 25 nmol GSSG per milligram protein. The respective concentrations of GSH and GSSG in the plastids were 112 and 6 nmol/mg protein and they were 228 and 10 nmol/mg protein in the mitochondria. The levels of GSH determined with DTNB were 1.5 times lower, whereas the amounts of GSSG were, by contrast, 1.5–2 times higher than in the HPLC assays. Although the glutathione redox ratios depended to some extent on the method used, the GSH/GSSG ratios were always lower for vacuoles than for plastids and mitochondria. In vacuoles, the pool of oxidized glutathione was higher than in other organelles.

2 Федеральное государственное бюджетное образовательное учреждение высшего образования «Иркутский государственный университет»

РЕДОКС-АКТИВНЫЕ НИЗКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ ВАКУОЛЕЙ

General principles of organization and distinctive features of the redox processes of biological systems are discussed. We paid special attention to the most examined parts of redox biology. As one of the approaches to the generalization of accumulated knowledge about redox processes, the so-called redox hypothesis of oxidative stress was examined. Extrapolation of this hypothesis on the processes taking place in plant cells, formulated on the basis of thiol-disulfide metabolism of animal cells, may help to systematize the available knowledge about redox processes in plants.

Glutathione transferase (GST) activity revealed in vacuoles of red beetroot (Beta vulgaris L.) cells was investigated in comparison with the GST activity of plastids and extracts of tissues. The level of GST activity determined by spectrophotometric method proved fairly high in water extracts and membrane fractions of isolated vacuoles and plastids, as well as in water extracts of tissues. In the objects studied, pH dependence of the GST activity slightly differed. Optimal pH for the vacuolar GST activity was in the range 7.0–7.5, for the GST of plastids and tissue extracts it was 7.5. The GSTs differed in specificity to the substrates fluorodifen and ethacrynic acid. The activity of the vacuolar and tissue extract GSTs with fluorodifen was significantly higher than that of the GST from plastids. Ethacrynic acid, often used as a competitive inhibitor of GST, almost completely inhibited the GST activity assayed with 1-chloro-2,4-dinitrobenzene as a main substrate. However, ethacrynic acid was a substrate only for the GSTs of vacuoles and tissue extract, but not for the GST of plastids. Using zymography allowing estimation of the GST activity in a gel after electrophoresis of proteins, several zones of enzymatic activity were revealed in all objects that may correspond to different isozymes. It was found that the composition of the vacuolar GST isoforms and their substrate specificity may differ from the GSTs of other cellular structures. It is assumed that vacuole, having quite high activity of GST, should make a significant contribution to intracellular detoxification processes.

The glutathione of the red beetroot vacuoles (Beta vulgaris L.) was measured using three well-known methods: the spectrofluorimetric method with orthophthalic aldehyde (OPT); the spectrophotometric method with 5.5'-dithiobis-2-nitrobenzoic acid (DTNB); the highperformance liquid chromatography (HPLC). The content of reduced (GSH) and oxidized glutathione (GSSG) differed depending on the research method. With OPT the concentration of glutathione was: GSH – 0.059 µmol /mg protein; GSSG – 0.019 µmol/mg protein and total glutathione (GSH total ) – 0.097 µmol/mg protein. In the case of determining with DTNB the concentration of glutathione was: GSH – 0.091 µmol/mg protein; GSSG – 0.031 µmol/mg protein; GSH total – 0.153 µmol/mg protein. HPLC-defined concentration of glutathione was lower: GSH – 0.039 µmol/mg protein; GSSG – 0.007 µmol/mg protein; GSH total – 0.053 µmol/mg protein. Redox ratio of GSH/GSSG was also dependent on the method of determination: with OPT – 3.11; with DTNB – 2.96 and HPLC – 5.57. Redox ratio of glutathione in vacuoles was much lower than the tissue extracts of red beetroot, which, depending on the method of determination, was: 7.23, 7.16 and 9.22. The results showed the vacuoles of red beetroot parenchyma cells contain glutathione. Despite the low value of the redox ratio GSH/GSSG, in vacuoles the pool of reduced glutathione prevailed over the pool of oxidized glutathione.

Glutathione reductase (GR, EC 1.8.1.7) is the enzyme that reduces oxidized glutathione (GSSG) and thus regulates the redox state of glutathione (GSH/GSSG). GR has been studied in most plants. This enzyme has been identified in chloroplasts and cytosol, so these cellular compartments are considered to be the main place of the enzyme localization. In the same time, just a little is known about GR vacuoles. There are no conclusive evidences to prove the presence or absence of this enzyme in the vacuoles. GR activity was found in the vacuoles of red beet root cells (Beta vulgaris L.). The level of activity, the optimum pH and isoenzyme composition of GR were compared in the vacuoles and tissue extract of beet root. Vacuolar GR activity was quite high, it was 1.5-2 times higher than the activity of the tissue extract. Enzyme pH optimum of all the objects were identical. pH-optimum depend on the pyridine nucleotide nature: pH 7.0-8.0 was an optimal range with NADPH; pH 5.0 – with NADH. GR activity of the vacuoles and tissue extracts decreased in the presence of a noncompetitive inhibitor 1-chloro-2.4-dinitrobenzene (CDNB), indicating the specificity of this enzymatic reaction. Two bands with glutathione reductase activity have been identified in the vacuoles and tissue extracts using zymography method to determine the enzymatic activity in PAAG after electrophoresis of proteins. Belonging to the GR isoforms of these bands was confirmed by enzyme immunoassay (Western blotting). The electric mobility of isoforms of the study objects did not differ significantly. It is concluded that the biochemical characteristics of vacuolar glutathione reductase were substantially identical to the biochemical characteristics of other localization GR.

Changes in the enzymatic activity of phenol-dependent peroxidase (PO) of vacuoles and tissue extract of red beet (Beta vulgaris L.) roots in different phases of plant development and in hyperosmotic stress and pathogen infection were found. The highest activity was observed during root growth and the lowest PO activity occurred in dormancy, respectively. Activation of the enzyme was observed in infected roots. The isozyme composition of PO was characterized by lability, and the number of cationic isoforms varied significantly. The optimum pH of the enzyme changed depending on the growth phase and stressor, tending to shift towards low values at rest and in hyperosmotic stress. The shift in the optimum pH coincided with the appearance of additional cationic PO isoforms.

Years of research have shown that some of the redox elements (enzymes, coenzymes, and co-substrate) are isolated from each other kinetic and spatial manner (compartmentalization) in the eukaryotic cells. The redox elements forming the "highly" and "widely" specialized redox system are found in all cell structures: mitochondria, plastids, peroxisomes, apoplast, nucleus etc. In recent years the active involvement of the central vacuole in the maintenance of the plant cell redox homeostasis is discussed, actually the information about the vacuolar redox system is very small. The high-priority redox processes and "redox-specialization" of the vacuolar compartment are not known. We have begun a study of red beet-root vacuole redox systems (Beta vulgaris L.) and have identified redox enzymes such as: phenol peroxidase (EC 1.11.1.7), superoxide dismutase (EC 1.15.1.1) and glutathione reductase (EC 1.8.1.7). This paper presents some of the characteristics of these enzymes and considers the probable ways of their functioning in vacuolar redox chains.