2005
Chad Kerksick and Darryn Willoughby
Abstract
An increase in exercise intensity is one of the many ways in which oxidative stress and free radical production has been shown to increase inside our cells. Effective regulation of the cellular balance between oxidation and antioxidation is important when considering cellular function and DNA integrity as well as the signal transduction of gene expression.
Many pathological states, such as cancer, Parkinson's disease, and Alzheimer's disease have been shown to be related to the redox state of cells. In an attempt to minimize the onset of oxidative stress, supplementation with various known antioxidants has been suggested. Glutathione and N-acetyl-cysteine (NAC) are antioxidants which are quite popular for their ability to minimize oxidative stress and the downstream negative effects thought to be associated with oxidative stress. Glutathione is largely known to minimize the lipid peroxidation of cellular membranes and other such targets that is known to occur with oxidative stress. N-acetyl-cysteine is a by-product of glutathione and is popular due to its cysteine residues and the role it has on glutathione maintenance and metabolism.
The process of oxidative stress is a complicated, inter-twined series of events which quite possibly is related to many other cellular processes. Exercise enthusiasts and researchers have become interested in recent years to identify any means to help minimize the detrimental effects of oxidative stress that are commonly associated with intense and unaccustomed exercise. It is possible that a decrease in the amount of oxidative stress a cell is exposed to could increase health and performance.
Keywords: free radicals, reactive oxygen species, health, supplementation
Reactive Oxygen Species and Oxidative Stress
The presence of oxygen is a fundamental component of cellular metabolism. However, any situation which results in a sudden or chronic overconsumption of oxygen can lead to the production of free radicals, which are more appropriately termed 'reactive oxygen species' (ROS). Production of these reactive oxygen species is known to occur as a result of a few different mechanisms: 1) mitochondrial origin in which free radicals either escape scavenging enzymes or develop due to an error in oxidative processes, 2) inside the capillary endothelium where a hypoxic and reoxygenation process is created during intense exercise as well as during various types of cardiovascular disease, and 3) an oxidative burst from inflammatory cells which are commonly mobilized as a result of the muscle or tissue damage which is well-documented with extended or eccentric-based exercise[1].
Aerobic energy metabolism, or oxidative phosphorylation, is a critical metabolic pathway within cells to provide the energy necessary to complete our daily tasks. In a resting state these processes work slowly while during times of intense exercise, these metabolic processes are increased as much as 100-fold. Inside the mitochondria, the electron transport is responsible for a series of redox reactions which result in the resynthesis of ATP.
In this system, O2 is reduced by cytochrome-c oxidase, which is the terminal enzymatic component of this mitochondrial enzymatic complex. As the demand and subsequent flux for this process increases so does the chance that redox uncoupling will occur and increase the accumulation of free radicals throughout the cell. A free radical is a molecule that contains at least one unpaired electron in its outer spin orbits.
Characterized by their unpaired electron(s), superoxide radicals, hydroxyl radicals, hydrogen peroxide, nitric oxide, lipid alkoxyl and peroxyl radicals are the most common reactive oxygen species in living, aerobic systems[2]. It is estimated that 2–5% of the oxygen that is passed through the electron transport system inside the mitochondria results in superoxide[3]. The complete reduction of oxygen can be seen from the steps outlined below[4].
O2 + e- → O2- Superoxide radical
O2- + H2O → HO2 + OH- Hydroperoxyl radical
HO2 + e- + H → H2O2 Hydrogen Peroxide
H2O2 + e- → OH + OH- Hydroxyl Radical
Superoxide is the most well-known of the free radicals as it is commonly produced during the natural pathway of oxidative phosphorylation. Superoxide is readily dismutated by intracellular superoxide dismutase enzymes (e.g. copper superoxide dismutase [CuSOD], magnesium superoxide dismutase [MgSOD]).
Consequently, these antioxidant enzymes can have many different origins (e.g. endothelial, plasma, tissue) and are commonly used in the literature to assess the amount of oxidative stress that is occurring. Superoxide is converted primarily into hydrogen peroxide; however, from a chemical structure standpoint, hydrogen peroxide is not a free radical. It is considered to be a free radical due to its ability to readily result in the hydroxyl radical.
Hydrogen peroxide, unlike other free radicals, is able to be transported across cellular membranes. The enzyme, catalase, has been shown to effectively dismantle much of the hydrogen peroxide found in our cells with water as a by-product. The hydroxyl radical is the most reactive of the free radicals and in the presence of various transition metals (e.g. Fe3+, Cu2+) it is known to directly target cellular lipids, proteins, nucleic bases, causing DNA base modification or fragmentation[5].
The ability of the hydroxyl radical to remove or add hydrogen molecules to unsaturated aspects of cellular membranes (e.g. lipid peroxidation) makes it one of the most potent free radicals in existence. Its extremely short half-life restricts its diffusion capability to other parts of the cells while also enhancing its potency[4].
To protect against the deleterious effects of ROS, our bodies have a complex system of endogenous antioxidant protection in the form of enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. Under normal, resting conditions reactive oxygen species are removed from the cell preventing any subsequent damage.
However, under more extreme conditions such as: 1) inadequate intake of foodstuffs containing the antioxidants, 2) excessive intake of pro-oxidants, 3) exposure to noxious chemicals or ultraviolet light, 4) injury/wounds, and/or 5) intense exercise, especially eccentric exercise, the body's endogenous antioxidant system is not able to effectively remove excessive ROS production[2].
In situations such as the ones listed above in which the production of pro-oxidant molecules increase to a point where the antioxidant system cannot effectively remove them is when oxidative stress is known to occur. Oxidative stress has been implicated in a number of diseases which include atherosclerosis, pulmonary fibrosis, cancer, Parkinson's disease, multiple sclerosis, and aging[6].
Research on oxidative stress during exercise has begun to indicate that regular training enhances the ability of these mechanisms to effective respond to the increase of oxidative product.
As mentioned before, free radicals have been found to react with macromolecules (lipids, proteins, DNA) within the cell, with one of the most frequent targets being the polyunsaturated fatty acids that largely comprise the cell membranes.
The systematic oxidation of these polyunsaturated fatty acids is called lipid peroxidation. Lipid peroxidation has been found to limit different aspects of muscle or cell function by decreasing the fluidity of the membrane, making it more difficult for proteins/nutrients to pass through.
Further, lipid peroxidation has been found to decrease the membrane's ability to uptake glucose as well as respond to varying levels of immune challenges[7]. Lipid peroxidation is commonly quantified in research studies by measuring the accumulation of the by-products that result from this process. One of these by-products is malondialdehyde (MDA).
In response to various forms of exercise many studies have reported significant increases of malondialdehyde [7-9]. Evidence of lipid peroxidation by increased levels of malondialdehyde and other such substances such as 8-isoprostane and thiobarbarbituric acid-reactive substance levels is one of the primary means by which researchers have associated oxidative processes with an overall decrease of cellular function.
Supplementation with antioxidants, either through an increased consumption in the diet or from supplementation, has become extremely popular as a means to improve one's health or increase physical performance. It has been suggested that increasing the circulating levels of certain antioxidants (e.g. glutathione, n-acetyl-cysteine, α-lipoid acid, vitamin A, vitamin E, vitamin C, etc.) will help to prevent the accumulation of free radicals inside our cells thus reducing oxidative stress[4,10,11] while other studies have suggested the possibly of little to no effect[12].
By decreasing oxidative stress, researchers have suggested that the risk of cancer, parkinson's disease, alzheimer's disease, etc. may all be decreased. Additionally, research has also begun to link excessive oxidative stress with an up-regulation of various proteolytic pathways (e.g. calcium-activated calpains and ubiquitin-proteolytic pathway) as well as apoptosis.
Summary
In conclusion, the development of free radicals and oxidative stress during exercise is an important consideration for optimal performance, recovery, and health. Currently, the relationship between oxidative stress and prolonged, unaccustomed, high-intensity exercise is not fully determined. Even further, research exists which illustrates a possible relationship between free radicals and oxidative stress to other diseases and pathways of cellular destruction.
Systems of proteolysis and apoptosis are two of the primary pathways in which oxidative stress appears to play a substantial role in the extent to which they are active in skeletal muscle. A commonly sought-after approach to oxidative stress is the exogenous administration of compounds that are thought to have antioxidant properties.
Much more research at this time needs to be conducted to determine the changes seen inside skeletal muscle cells after exposure to intense, unaccustomed damaging exercise. From these studies, researchers will be able to more effectively determine what signals or environments are responsible for causing oxidative stress, proteolysis as well as apoptosis.
Additionally, future research should also target on the signal transduction pathways in skeletal muscle upon exposure to oxidative stress in an attempt to identify areas of cross-communication as possible areas for effective intervention.
