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for example) (Halliwell and Gutteridge, 1999). Direct effects of ROS on specific amino acids in proteins
include oxidations of the reduced sulfur moiety of cysteine and methionine to produce disulfides and
sulfoxides/sulfones, respectively (Dean et al., 1997); the production of carbonyl groups (aldehydes and
ketones) on side chains of some amino acids, particularly of proline, arginine, lysine, and threonine
(Dalle-Donne et al., 2003); and oxidations of the aromatic side chains of phenylalanine, tryptophan,
tyrosine, and histidine (Stadtman and Levine, 2003). Among these effects, the production of carbonyls
is considered the most common effect, and increases in their production have been associated with a
number of human diseases, including Alzheimer’s disease, cystic fibrosis, diabetes, and amyotrophic
lateral sclerosis; consequently, many assays have been developed to detect protein carbonyls (Dalle-
Donne et al., 2003).

Redox Status and Energetics
The redox status of a cell (or cellular compartment, tissue, etc.) refers to the collective ratios of
interconvertible oxidized and reduced forms of redox couples. (For a lucid description and a formal
approach for quantifying “redox environments” in the context of cellular oxidative stress, the reader is
referred to Schafer and Buettner, 2001.) Key redox couples underlying redox status include
+
+
NADH/NAD , NADPH/NADP , and GSH/GSSG. Reduced forms of these couples are critical for a
number of fundamental cellular functions including energy production (e.g., mitochondrial ATP produc-
+
tion linked with electron release by NADH, resulting in oxidation to NAD ), biosynthesis (many pathways
employ NADPH), and protection from ROS (wherein GSH plays a central role as described previously);
therefore, cells and tissues maintain overall reducing environments that favor the reduced forms of most
redox couples, including those just mentioned.
Reactive oxygen species can drive redox status to a more oxidized state through a variety of direct
and indirect mechanisms. GSH can be oxidized directly by ROS, as well as through GPX-catalyzed
reductions of H O and lipid peroxides (Equations 6.15 and 6.16). The reduction of GSSG resulting
2
2
from GSH oxidations back to GSH is catalyzed by glutathione reductase, which requires the oxidation
of NADPH. NADH and NADPH are also oxidized during the activity of quinone reductase (Equation
6.19). Both reduced pyridine nucleotides can be oxidized during the generation of ROS by redox-cycling
chemicals, a phenomenon described later. There is also evidence that NADH and NADPH can act directly
as ROS scavengers, resulting in their oxidation (Kirsch and De Groot, 2001). This activity is suggested
to be important in mitochondria, where their concentrations are similar to that of GSH (~5 mM). In all
of these cases, however, oxidations of reduced intermediates impose an energetic cost.

Deleterious Organismal Effects of Reactive Oxygen Species

The preceding discussions have focused on molecular and cellular aspects of ROS and oxidative stress,
and the complexity and varied nuances of these phenomena should be apparent; therefore, it is not
surprising that a diverse array of diseases and pathologies have been associated with oxidative stress. It
is beyond the scope of this chapter to discuss these in detail; however, a number of diseases (most
described in humans), pathologies, and other effects are noted here to provide a sense of the breadth of
health outcomes that are at least in part associated with oxidative stress.
Oxidative stress, as measured by cellular effects described above (e.g., oxidations of lipids, protein,
DNA, perturbed redox status, and altered antioxidant capacity), has been observed in association with
many disease states and tissue injuries, but it is important to bear in mind that such associations do not
confer causality. Evidence of oxidative stress may be encountered in any tissue undergoing trauma due
to infection, injury, or cell death (apoptosis or necrosis)—such tissues are inherently more prone to
oxidative damage due, for example, to compromised energetics. Determining that oxidative stress is the
cause of a disease or tissue damage rather than the result requires careful analysis. Moreover, many
diseases have complex etiologies, and attempts to pinpoint a single cause, such as ROS, are often
erroneous.