Energy in mammalian cells is derived from aerobic respiration-a process whereby molecular oxygen is reduced to water via a series of enzymatic reactions. Small amounts of superoxide (O₂^-), hydroxyl radical (OH), and hydrogen peroxide (H₂O₂), collectively termed reactive oxygen species (ROS), are generated during this metabolic process. Strenuous exercise increases oxygen consumption and may transiently raise ROS production. Oxidative stress is a term used to describe an excess of these reactive oxygen intermediates, which if not removed by endogenous scavenger mechanisms, can mediate toxic effects on cellular elements, macromolecules (lipids, proteins, and nucleic acids), and adversely modulate intracellular signaling mechanisms.1,2

Living tissues have evolved a series of enzymatic pathways to detoxify ROS before they lead to potentially damaging cellular effects. Examples of antioxidant protection include conversion of O₂^- by superoxide dismutase (SOD) yielding H₂O₂, which in turn can be detoxified to water (H₂O) by catalase or glutathione peroxidase.1,3 Glutathione functions as a principal intracellular redox buffer, acting as a critical cofactor for the enzymatic action of glutathione peroxidase. Glutathione exists both in reduced monomeric form with a free thiol group (GSH) and in an oxidized disulfide state (GSSG). In order for glutathione to function as an effective antioxidant scavenger and protect cells from oxidative stress, it needs to exist predominantly in the reduced form (GSH). The ratio of GSH/GSSG may be a useful barometer of the oxidative stress imposed on a cellular system, with a lower ratio suggesting an unfavorable oxidative milieu. The biological activity of ROS depends on their relative balance in relation to intracellular antioxidant defenses. The enzyme glutathione reductase, together with NADPH (reduced nicotinamide adenine dinucleotide phosphate), converts GSSG to GSH, maintaining glutathione in its reduced (protective) state. However, an overwhelming generation of ROS may exhaust this cellular antioxidant mechanism and potentially lead to loss of intracellular homeostasis.

In this issue of Journal of Cardiopulmonary Rehabilitation, Elokda and colleagues report dynamic changes in blood glutathione levels in 80 young participants without cardiovascular disease in response to an acute bout of maximal graded treadmill exercise test.4 In the present study, blood glutathione was used as a surrogate marker of systemic oxidative burden. Values of GSH, GSSG, and GSH/GSSG ratio were measured at baseline, immediately post exercise, and 60 minutes following recovery. The investigators demonstrated a decrease in GSH immediately post-treadmill exercise, along with a parallel rise in GSSG, thereby producing an overall fall in the GSH/GSSG ratio. By 60 minutes, GSH and GSSG levels recovered to baseline concentrations. As the authors point out, their findings demonstrate an acute effect of exercise on the GSH/GSSG redox status suggesting transient and reversible oxidative stress in association with strenuous exertion, and their results are in agreement with previous work by other investigators in this field.

The challenge of studying free radical systems in vivo lies in their exceptionally short half-life and low concentrations. In particular, reduced glutathione in erythrocytes undergoes rapid conversion to glutathione disulfide. Therefore, the use of an assay that provides prompt measurements of free GSH prior to oxidation could theoretically improve the sensitivity of this methodology as a marker for acute oxidative stress. The GSH/GSSG-412(TM) assay system (OxisResearch(TM), Portland, OR), as used in this study, has the advantage of being able to remove 99% of total glutathione from a sample within the first minute and provides more rapid measurements of GSH and GSSG. Older assays for measuring GSH/GSSH redox status utilized a different and less efficient method, and demonstrating the utility of the current GSH/GSSG-412(TM) assay system in this relatively sizable study represents a featured strength of the present investigation.

There is considerable support from clinical studies that exercise promotes cardiovascular health. Exercise training reduces the risk of traditional cardiovascular risk factors such as diabetes mellitus, hypertension, dyslipidemia, and obesity.5 Regular exercise also prevents the development of coronary artery disease (CAD) and improves ischemic symptoms in patients with established atherosclerosis. These effects are likely partly mediated as a result of improved blood flow and tissue perfusion with chronic training.6 Regular physical exercise has been shown to improve endothelium-dependent vascular dilation likely as a result of enhanced nitric oxide (NO) activity that is sensitive to oxidative degradation. Although exercise-related generation of ROS could theoretically pose a conflict with regard to NO bioaction and favorable mechanisms of physical exercise, there is growing evidence that chronic training, in turn, adaptively bolsters the activity of protective antioxidant enzymes such as catalase, SOD, and glutathione peroxidase.7 Regular exercise also appears to favorably modulate the pro-oxidant phenotype of diseased atherosclerotic blood vessels. In patients with ischemic heart disease, sustained training for several weeks downregulates the gene expression and activity of pro-oxidant NAD(P)H oxidase subunits, and decreases vascular generation of ROS in human arterial tissue.8 These favorable effects are linked to parallel improvement in endothelium-dependent dilation, providing further evidence that antioxidant adaptation to regular exercise may be functionally intertwined with cardiovascular benefit.

While results of this study demonstrate dynamic changes in circulating glutathione levels in relation to acute exercise, questions remain regarding the application of this tool in clinical practice. It remains unclear whether the spot determination of an individual's blood GSH/GSSG status represents a comprehensive assessment of intracellular oxidative stress in all tissues. Whether measuring blood GSH/GSSG ratio offers any additional diagnostic or prognostic information when integrated with other clinical variables in patients with suspected cardiovascular disease remains an open question. The present study examined glutathione levels in a relatively sedentary and slightly overweight group of patients. The effect of variable fitness levels and chronic training regimens on basal and exercise-related changes in glutathione status may be of clinical interest. Finally, changes in GSH in response to pharmacological modulation of cardiovascular risk factors may yield further clinical information.

In summary, the present study by Elokda and colleagues builds on our limited knowledge of assessing glutathione status as a marker for oxidative stress in relation to exercise. Further studies will be needed to establish its role in the evaluation and clinical management of a broad spectrum of individuals.

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