Review article
Redox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple

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Abstract

Redox state is a term used widely in the research field of free radicals and oxidative stress. Unfortunately, it is used as a general term referring to relative changes that are not well defined or quantitated. In this review we provide a definition for the redox environment of biological fluids, cell organelles, cells, or tissue. We illustrate how the reduction potential of various redox couples can be estimated with the Nernst equation and show how pH and the concentrations of the species comprising different redox couples influence the reduction potential. We discuss how the redox state of the glutathione disulfide-glutathione couple (GSSG/2GSH) can serve as an important indicator of redox environment. There are many redox couples in a cell that work together to maintain the redox environment; the GSSG/2GSH couple is the most abundant redox couple in a cell. Changes of the half-cell reduction potential (Ehc) of the GSSG/2GSH couple appear to correlate with the biological status of the cell: proliferation Ehc ≈ −240 mV; differentiation Ehc ≈ −200 mV; or apoptosis Ehc ≈ −170 mV. These estimates can be used to more fully understand the redox biochemistry that results from oxidative stress. These are the first steps toward a new quantitative biology, which hopefully will provide a rationale and understanding of the cellular mechanisms associated with cell growth and development, signaling, and reductive or oxidative stress.

Introduction

I often say that when you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.

—Lord Kelvin in Popular Lectures and Addresses, lecture on Electrical Units of Measurement, 1883

It is now realized that the direction of many cellular processes depends on “redox state.” But at present the term redox state is not well defined. The research in this area is mostly observational in that cells or tissues are subjected to an oxidative or reductive stress and then the effects are observed. The research community has not yet related the applied stresses to quantitative changes in redox environment, only to qualitative changes. Thus, we do not know on a quantitative basis the “redox environment” needed to initiate a particular set of cellular signals. In this work we (i) provide a definition for redox environment; (ii) provide a definition of redox state; (iii) show how the Nernst equation can be a tool to provide quantitative estimates of redox state; (iv) review important biological redox couples that play a role in determining the cellular redox environment; (v) illustrate how glutathione uniquely contributes to the cellular redox environment; (vi) examine how protein sulfhydryl groups participate in these processes; and (vii) present a framework for how the redox state of the GSSG/2GSH couple and the biological status of a cell are linked. This framework leads to the proposal that the biological status of a cell is intertwined with its redox environment.

Section snippets

Redox state and redox environment, definitions

Life depends on overcoming entropy. Energy is required to maintain the ordered state of a living organism. For humans, this is achieved by capturing the energy released in oxidation processes to: (i) build cellular and organismic structures, (ii) maintain these structures, and (iii) provide the energy for the processes they support. The energy comes from the movement of electrons from oxidizable organic molecules to oxygen. This results in an overall reducing environment in cells and tissues.

The Nernst equation

In 1889 Walter H. Nernst investigated the theory of galvanic cells and developed what is now known as the Nernst Equation. The Nernst equation allows one to determine the voltage of an electrochemical cell (ΔE) taking the Gibbs energy change (ΔG) and the mass action expression (Q) into account (Reactions , , , ). The Nernst equation has broad applications in biology because much of biology involves electron transfer reactions. These reactions are responsible for producing energy and for

1e-process

The redox reactions of superoxide in typical biological settings are 1e-processes. The Nernst equation for the O2/O2•− redox pair would be: Ehc=E°′−59.1 log [O2•−]/[O2] mV at 25°C, pH 7.0 where, E°′O2/O2•− = −160 mV2 [7], [8], [9] and Ehc is the half-cell reduction potential. For example, if the steady-state level of superoxide in a cell is 10−10 M and dioxygen is 10 μM (10−5 M), then: Ehc=−160 mV−59.1 log (10−10/10−5) at 25°C, pH 7.0 Ehc=+136 mV. This positive potential implies that this

Compartmentation of GSH and redox-environment [29]

When dealing with homogeneous fluids such as plasma, the assessment of the redox environment is relatively uncomplicated because the determination of the molar concentrations of GSH and GSSG is straightforward. But when dealing with cells or tissues, compartmentation of GSH and GSSG may pose a problem, as all compartments may be at a nonequilibrium steady-state with respect to each other. A measurement of total content of GSH and GSSG in cells would represent an overall redox environment, not

Role of protein sulfhydryls in the cellular redox environment

Numerous proteins contain sulfhydryl groups (PSH) due to their cysteine content. In fact, the concentration of PSH groups in cells and tissues is much greater than that of GSH [65]. These groups can be present as thiols (-SH), disulfides (PS-SP), or as mixed disulfides, for example, PS-SG when conjugated with GSH). Proteins can bind GSH, cysteine, homocysteine, and γ-glutamylcysteine to form mixed disulfides, but GSH is the dominant ligand [66]. The oxidation of the thiol form of an enzyme or

The cellular redox environment throughout the life of a cell

Two of the major pathways for signaling in cells involve: (i) phosphorylation of proteins, or (ii) changes in the thiol status of proteins due to changes in the redox environment of the cell. Both oxidative and reductive stress can trigger redox cascades that bring about changes in the thiol status of the cell. Changes in the cellular redox environment can alter signal transduction, DNA and RNA synthesis, protein synthesis, enzyme activation, and even regulation of the cell cycle [31], [52],

Summary

It is now realized that redox changes in the cell will initiate various signaling pathways [144], [145], [146], [147], [148], [149]. Research in this area is in its infancy and is mostly observational, in that cells and tissues are subjected to an oxidative or reductive stress and the effects observed. The research community, in general, has not yet related the applied stress to quantitative changes in cellular redox environment or quantitative changes in the redox status of specific redox

Acknowledgements

This work was supported by NIH grants CA 66081 and CA 81090.

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