Review articleRedox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple
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: 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: 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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