The current status of superoxide dismutase (SOD) is that it is an enzyme with diverse ramifications. This review attempts an understanding of SOD as a structural, functional, and biological entity. Accordingly, the review is in three parts. The first part discusses SOD in terms of protein structure, proceeding from primary to secondary and three-dimensional structure for the three forms of SOD: copper/zinc SOD, manganese SOD, and iron SOD. This is the order of structural knowledge of the enzyme. Iron SOD is an enzyme of prokaryotes and some higher plants. Manganese SOD is an enzyme of prokaryotes and eukaryotes. Copper/zinc SOD is an enzyme of eukaryotes and certain prokaryotes. The evolutionary relationships of the three forms of SOD, the status of the copper/zinc SOD gene in prokaryotes, and the cloning and sequencing of SOD genes are discussed. The second part of the review deals with the catalytic mechanism of SOD in the three forms of the enzyme. Structural and mechanistic conclusions from various spectroscopic studies are critically considered. A detailed picture is given of the active site of copper/zinc SOD. The third part is a review of SOD in the general context of oxygen toxicity. After consideration of the question of superoxide toxicity and superoxide pathology, several areas in which SOD has been investigated or used as a tool in a biochemical, pharmacological, or clinical context are discussed, including population genetics; trisomy 21; development and senescence; the nutritional copper, zinc, and manganese status; hemolysis and anemia; oxygen toxicity in the lung and nervous system; inflammation, autoimmune disease and chromosome breakage, ischemia and degenerative changes; radiation damage; and malignancy. A comprehensive picture is given of measurements of SOD activity in disease states, and the question of superoxide-related disease is considered at several points.

Aspects of the structure, function, and applications of superoxide dismutase

On a global basis, water is a paramount factor in determining the distribution of species, and the responses and adaptations of a species to water stress are critical for its success in any environmental niche. Numerous studies have reported a myriad of changes elicited by water stress. The changes observed are dependent on the species under study and on the severity, duration, and time course of the stress. Before reviewing the changes in detail, we will first present an overview of stress and responses using the stress-strain concept of physics. Next, we will discuss specific water-related parameters for quantifying plant water status and briefly consider how changes in the parameters may affect plant functions. This is followed by the main body which first reviews and analyzes selected responses to water stress and then examines the integrated adaptive behavior of whole plants.

Physiological Responses to Moderate Water Stress

Heterocysts are differentiated cells that are specialized for fixation of N2 in an aerobic environment. In heterocysts in the light, Photosystem I generates ATP, but no photosynthetic production of O2 takes place. Instead, reductant moves into heterocysts from vegetative cells. In return, fixed nitrogen moves from heterocysts to vegetative cells. In neither case is there certainty about the identity of the traffic molecules. Pathways of electron-donation to N2 have been extensively investigated, but their in-vivo importance remains to be critically tested. Nitrogenase in heterocysts is protected from inactivation by O2 by a variety of means, principally by enhanced respiration and by a barrier, the heterocyst envelope, to entry of O2. However, the respiratory apparatus and the biosynthetic processes that result in synthesis of the barrier have been little studied. The detailed mechanisms underlying metabolic, environmental, and developmental control of nitrogenase are under investigation. Studies of heterocyst development are being greatly facilitated by recent advances in the genetics of Anabaena sp. An autoregulated gene, hetR, that is activated shortly after nitrogen-stepdown is critical for the differentiation of heterocysts. Two enigmas remain to be answered: how is it determined which cells will differentiate; and, after differentiation is initiated, what intercellular interactions and intracellular mechanisms regulate the progression of the differentiation process? An evolutionary and biochemical relationship between the processes leading to the formation of heterocysts and akinetes is suggested.

Heterocyst Metabolism and Development

Dating from the Pre-Cambrian era, cyanobacteria have a long history of adaptation to the Earth's environment. By evolving oxygen via photosynthetic reactions similar to those of plants and green algae, these prokaryotes were essential to the evolution of the present biosphere. They continue to make a large contribution to the equilibrium of the Earth's atmosphere by production oxygen and removing carbon dioxide. To survive in extreme or variable environments, cyanobacteria have developed specific regulatory systems, in addition to more general mechanisms equivalent to those of other prokaryotes or photosynthesis eukaryotes. Specific regulatory systems control the differentiation of specialized nitrogen-fixing cells and of cell types facilitating the dispersion of species. In the past decade, considerable progress has been made towards understanding the expression of the cyanobacterial genome in response to variations in the intensity and spectral quality of incident light and in response to nutritional conditions, especially carbon, nitrogen and sulphur sources. These studies have provided insights into the relationships between carbon and nitrogen intermediary metabolism, and a start towards understanding of the interconnected pathways which lead from the perception of environmental signals to the regulation of enzyme activities and gene expression. Cyanobacterial regulatory mechanisms share common features with those of other prokaryotes, but are unique since these essentially photo-autotrophic organisms must maintain a proper cellular C/N balance, in spite of dailty variations in incident light. Thus an appropriate coordination between photosynthesis and other metabolic processes must be achieved through control of the catalytic activity of key enzymes by reducing equivalents and ATP produced by photosynthetic or respiratory electron transport. Recently discovered kinases/phosphatases act by post-translational modification of specific proteins which probably act as signal transducers or modulators of gene expression in a manner similar to the well-known two-component regulatory systems described in other bacteria. In this overview, we present our current knowledge on the molecular aspects of the biology of cyanobacteria, as well as on their mechanisms of resistance to metal ions and their responses to metabolic stress.

Adaptation of cyanobacteria to environmental stimuli: new steps towards molecular mechanisms

Neutral sugar, free amino acid, and anthocyanin levels and vacuole/extravacuole distribution were determined for Hippeastrum and Tulipa petal and Tulipa leaf protoplasts Glucose and fructose, the predominant neutral monosaccharides observed, were primarily vacuolar in location Glutamine, the predominant free amino acid found, was primarily extravacuolar γ-Methyleneglutamate was identified as a major constituent of Tulipa protoplasts Qualitative characterization of Hippeastrum petal and vacuole organic acids indicated the presence of oxalic, malic, citric, and isocitric acids Data are presented which indicate that vacuoles obtained by gentle osmotic shock of protoplasts in dibasic phosphate have good purity and retain their contents

http://www.plantphysiol.org/content/plantphysiol/64/1/88.full.pdf

Content and vacuole extravacuole distribution of neutral sugars, free amino acids, and anthocyanin in protoplasts

Glucose-6-phosphate dehydrogenase (G6PDH) was isolated from heterocysts and vegetative cells of Anabaena sp. strain PCC 7120. Both enzyme preparations proved to be more active in their oxidized than in their reduced forms. At least one protein with thioredoxin activity was found in Anabaena sp. which, if reduced with dithiothreitol, deactivated the G6PDH preparations. The deactivated heterocyst G6PDH could be reactivated neither by O2 nor by oxidized thioredoxin. Reactivation of the enzyme was, however, achieved by oxidized glutathione or H2O2. The active form of Anabaena G6PDH was readily deactivated by heterologous thioredoxin(s). The Anabaena thioredoxin(s) modulated heterologous enzymes.

Thioredoxins and the redox modulation of glucose-6-phosphate dehydrogenase in Anabaena sp. strain PCC 7120 vegetative cells and heterocysts.

Osmotic shock triggers an increase in ribonuclease level in protoplasts isolated from tobacco leaves

In excised Avena leaves, kinetin and benzyladenine decreased, while abscisic acid and benzimidazole increased the over-all nuclease level. Significant effects were observed as early as 2 h after treatment. Not all the nucleases of the Avena leaf were affected by the growth regulators. Changes in over-all nuclease activity were accounted for almost entirely by changes in the amount of a relatively purine-specific endo-ribonuclease, which produces 2', 3'-cyclic phosphates as breakdown products. Slight changes induced by the growth regulators were also detected in the amount of a sugar non-specific endo-nuclease which produces 5'-nucleotides and has a relative specificity for adenylic acid. The level of an alkaline phosphodiesterase, an exo-nuclease which produces 5'-nucleotides, was not affected by any of the growth regulators tested. Gibberellic acid and indol-3yl-acetic acid did not influence the level of Avena nucleases.

Hormonal Control of Nuclease Level in Excised Avena Leaf Tissues1

Is the increase in ribonuclease level in isolated tobacco protoplasts due to osmotic stress

The incorporation of labeled precursors into RNAs and proteins of isolated tobacco (Nicotiana tabacum L.) leaf protoplasts decreases with increasing osmotic pressure in the incubation medium. The incorporation of precursors into RNA and proteins is linear for 15–18 h after the isolation of the protoplasts, irrespective of the osmolarity of the culture media. The uptake of precursors is also affected by the osmolarity of the medium. However, the osmotic stress-induced inhibition of incorporation of precursors into RNA and proteins is also apparent if the differences in uptake are taken into consideration in the calculation. Incorporation of 32P into TMV-RNA is also inhibited by osmotic stress. As assayed by the double labeling ratio technique, osmotic stress has less unequivocal effect on TMV protein synthesis.

G. L. Farkas

Papers

Thioredoxins and the redox modulation of glucose-6-phosphate dehydrogenase in Anabaena sp. strain PCC 7120 vegetative cells and heterocysts.

Osmotic shock triggers an increase in ribonuclease level in protoplasts isolated from tobacco leaves

Hormonal Control of Nuclease Level in Excised Avena Leaf Tissues1

Is the increase in ribonuclease level in isolated tobacco protoplasts due to osmotic stress

Effect of "osmotic stress" on protein and nucleic acid synthesis in isolated tobacco protoplasts.