Revival: Antioxidants in Higher Plants (1993)

Ruth G. Alscher, John L. Hess

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351369145
• Edition: 1
• Topic: Medicine
• Subtopic: Alternative & Complementary Medicine

Antioxidants in Higher Plants provides a unique blend of molecular and biochemical approaches to cover the state of the art in antioxidant function. The chemistry and protective potential of sulfhydryl and hydroxyl compounds are emphasized. Interesting perspectives are presented regarding the response of antioxidant metabolism to interactions among environmental pollutants, illumination, temperature, and water availability. The book also discusses how tools of molecular biology may further clarify antioxidant function and response to stress. Antioxidants in Higher Plants will be an excellent reference for plant physiologists, biochemists, molecular biologists, ecologists, and students.

Chapter 1
Glutathione

Alfred Hausladen and Ruth G. Alscher

Table of Contents

1. I. Introduction
2. II. Properties of Glutathione and Glutathione-Dependent Enzymes in the Antioxidative System
   1. A. Glutathione and Homoglutathione
      1. 1. Properties
      2. 2. Subcellular and Intraorgan Distribution of Glutathione
      3. 3. Assay
   2. B. Dehydroascorbate Reductase
      1. 1. Properties
      2. 2. Enzymatic vs. Nonenzymatic Reduction of Dehydroascorbate
      3. 3. Assay
   3. C. Glutathione Reductase
      1. 1. Overview
      2. 2. Subunit Composition
      3. 3. Isozymes
      4. 4. Light Activation
      5. 5. Subcellular Localization
      6. 6. Assay
   4. D. Other Glutathione-Dependent Enzymes
3. III. Biosynthesis and Metabolism of Glutathione
   1. A. The γ-Glutamyl Cycle
   2. B. Properties and Localization of Enzymes of the γ-Glutamyl Cycle
   3. C. Modification of GSH Concentration
   4. D. Regulation of GSH Biosynthesis
   5. E. Effect of Light on GSH Biosynthesis
4. IV. Active Oxygen in Plants
   1. A. Formation of Active Oxygen
      1. 1. The Role of Oxygen in the Formation of Free Radicals
      2. 2. The Mehler Reaction
      3. 3. The Action of Xenobiotics Involves Oxyradical Production
   2. B. Scavenging of Active Oxygen in the Chloroplast
   3. C. The Cytosol
5. V. Responses and Adaptations to Oxidative Stress
   1. A. Oxidative Stress Leads to Specific Gene Expression in Bacteria
   2. B. Coordinated Changes in Components of the Scavenging Cycle
   3. C. Cross-Resistance
   4. D. Cold Stress Involves Free Radicals
   5. E. The Central Role of Glutathione and Other Components of the Scavenging Pathway in Plant Responses to Air Pollutants and Other Oxidative Stresses
6. VI. Oxidized and Reduced Glutathione as Modulators of Plant Metabolism
   1. A. Effects on Enzymes
   2. B. Effects on Gene Expression
7. VII. Conclusions
8. References

I. Introduction

Glutathione is widely distributed in plant cells. It appears to be synthesized in both the chloroplast and the cytosol, and to occur in both subcellular compartments at relatively high levels. Its function as an antioxidant is reviewed here. In concert with ascorbate, glutathione acts by scavenging free radicals and hydrogen peroxide. The reactive intermediates form as a consequence of oxidative stresses caused by extremes of temperature, drought, herbicides, or air pollutants. An integral part of the response of glutathione metabolism to oxidative stress appears to include an increase in the levels of the reduced form of the molecule. Both altered gene expression and modulation of enzyme functioning are involved. The possible role of glutathione as an elicitor of transcriptional events associated with stress responses is discussed.

IL Properties of Glutathione and Glutathione-Dependent Enzymes in the Antioxidative System

A. Glutathione and Homoglutathione

1. Properties

The tripeptide glutathione (γ-Glu-Cys-Gly, GSH) is the major low-molecular weight thiol in most plants.¹ Some legumes contain the homologous peptide homoglutathione (γ-Glu-Cys-Ala, hGSH), either exclusively or in combination with glutathione. Klapheck² has recently surveyed the occurrence of GSH and hGSH in 13 legume species. Whereas Vicieae and Trifolieae contained both GSH and hGSH, hGSH predominated in Phaseoleae.

The chemical properties of GSH, including its redox chemistry, have been reviewed in detail by Kosower³ and Mannervik et al.⁴ The antioxidant function of GSH is mediated by the sulfhydryl group of cysteine which, upon oxidation, forms a disulfide bond with a second molecule of GSH to form oxidized glutathione (GSSG). GSH can also form mixed disulfides with proteins or other thiols, such as Coenzyme A. The negative redox potential (E′₀ = —0.34 V) of GSH allows for efficient reduction of dehydroascorbate or disulfide bonds in proteins. While α-tocopherol and ascorbic acid are generally considered to be the major free radical scavengers in biological systems, GSH is also capable of forming a thiyl radical upon reaction with other radicals.³ To our knowledge, there are no reports on any differences in redox chemistry between GSH and hGSH.

2. Subcellular and Intraorgan Distribution of Glutathione

The GSH concentration in spinach or pea chloroplasts prepared by aqueous isolation techniques has been determined to be between 1 and 4 mM,^5-8 but the cytosol also appears to contain high levels of glutathione.^6,10 Gillham and Dodge⁸ reported that 90% of total cellular glutathione exists outside the chloroplast in pea leaf cells. Klapheck et al.,⁹ on the other hand, have concluded that chloroplasts isolated by aqueous methods have lost a substantial amount of their complement of glutathione, even those plastid preparations which give high values in the ferricyanide intactness test. Using a nonaqueous isolation method, they obtained a glutathione distribution of 35:65 for chloroplast to cytosol. Smith et al.¹⁰ reported 50 to 60% glutathione in the chloroplast, using nonaqueous isolation methods. No experimentally determined values for in vivo cytosolic concentrations of glutathione are currently available. Smith et al.¹⁰ have fo...

Table of contents

• Cover
• Title
• Copyright
• Introduction
• The Editors
• The Contributors
• Table of Contents
• Chapter 1 Glutathione
• Chapter 2 Ascorbic Acid
• Chapter 3 Carotenoids
• Chapter 4 The Xanthophyll Cycle
• Chapter 5 Vitamin E, α-Tocopherol
• Chapter 6 Plant Phenolics
• Index