The Antioxidant Defense and Pathology of Oxidative Stress as a Result of Lipid Peroxidation

By: Dean L. Hoffmeister

In recent years, the study of spontaneous free radical reactions of organic compounds with oxygen, known as autoxidation, has received considerable attention. Even though this process has been studied for decades, new findings indicate that autoxidation plays a major role in destructive biological processes (Nakazawa, 1996). The specific autoxidation of lipids, also known as lipid peroxidation, is one process in which scientists have taken particular interest. This stems from current findings that link lipid peroxidation with a wide variety of degenerative processes, including atherosclerosis, heart attacks, cancer, ischaemia/reperfusion injury, and even the aging process as a whole (Davies, 1996). For these reasons, obtaining knowledge about lipid peroxidation and mechanisms to control the extent of its biochemical and biophysical damage continue to be pursued by the scientific community.

The origin of the oxidative damage caused by lipid peroxidation lies within the unique chemical nature of oxygen. When molecular oxygen is formed by the joining of two oxygen atoms, the outer valence shell electrons do not spin-pair, but remain as two unpaired electrons (Davies, 1996). Therefore, molecular oxygen is considered to be a true bi-radical. This radical character allows oxygen to perform unique oxidation/reduction chemistry. Aerobic organisms utilize this chemistry to obtain essential energy by oxidizing carbon- and hydrogen-rich molecules using oxygen (Gutteridge, 1995). Unfortunately, this process has its cost. When oxygen is univalently reduced, it generates highly reactive intermediates. The intermediates that are produced include the superoxide anion radical (O2.)-, hydrogen peroxide (H2O2), and the hydroxyl radical (.OH) (Nakazawa, 1996). These agents have been shown to be responsible for oxidative damage by initiating lipid peroxidation.

Lipid peroxidation can be viewed as a classic free radical chain reaction that consists of chain initiation, propagation, and termination steps. Figure 1 illustrates the free radical process of lipid peroxidation. Initiation of lipid peroxidation occurs when a radical species with significant oxidizing character, such as the hydroxyl radical (.OH), removes an allylic hydrogen from a polyunsaturated fatty acid (PUFA). Initiation can also be stimulated by using the ions or chelates of transition metals, like iron, which exacerbate the toxicity of H2O2 by promoting the formation of the hydroxyl radical (Hall, 1997). The removal of the allylic hydrogen from the PUFA forms a lipid radical (L.). An immediate rearrangement occurs, forming a more stable lipid radical, whose dienes are conjugated (Gutteridge, 1995). In an aerobic environment this radical reacts with oxygen, giving rise to a lipid peroxyl radical (LOO.). Propagation reactions can continue this process at this point by the lipid peroxyl radical abstracting an allylic hydrogen atom from another adjacent PUFA, resulting in a lipid hydroperoxide (LOOH) and a second lipid radical (L.). This second lipid radical can proceed through the same reactions as the first, generating additional lipid hydroperoxides. The propagation step goes through approximately eight rounds of peroxide generation on average, before a termination event occurs. The termination event can be the result of any reaction with another radical, protein, or compound that acts as a free radical trap, forming a stable end product (Davies, 1996). The biophysical consequences of peroxidation on membrane phospholipids can be both extensive and highly destructive (Hall, 1997). Membranes altered by peroxidation are known to have a modified fatty acid composition, a disruption of permeability, a decrease in electrical resistance, an increase in flip-flopping between monolayers, and inactivated, cross-linked proteins (Richter, 1987). Clearly, the process of lipid peroxidation and resulting products are detrimental to the viability of the cell in question. It is these collective effects of lipid peroxidation on cellular processes that has been implicated as the underlying mechanism for numerous pathological conditions (Davies, 1996).

However dismal the situation for aerobic organisms may sound, there is a sundry of mechanisms whose sole purpose is to protect against the constant threat that peroxidation poses. A series of primary antioxidant defenses attempt to control the onslaught of the radical species governing peroxidation. Some enzymatic species that act to minimize the detrimental effect of radicals include superoxide dismutase, glutathione peroxidases, and catalase (Davies, 1996). Also, numerous removal and repair enzymes, such as phospholipase A2, are available for the molecules that do get damaged by peroxidation (Nakazawa, 1996). Cells are known to also utilize many non-enzymatic antioxidant compounds which react directly against oxidizing agents. Two of the major antioxidant compounds that have received considerable attention are vitamin E (alpha- tocopherol) and vitamin C (ascorbic acid).

In the last few years, there have been detailed investigations alpha-tocopherol and ascorbic acid, two naturally occurring antioxidant compounds. This has lead to an explosion in the general public's interest in using these compounds as food additives or dietary supplements (Davies, 1996). Alpha-tocopherol, a fat-soluble vitamin, is considered to be the major membrane-bound antioxidant employed by the cell (Van Acker, 1993). On the other hand, ascorbic acid is regarded as the major aqueous-phase antioxidant (Davies, 1996). Recent evidence suggests that alpha-tocopherol and ascorbic acid function together in the cyclic-type of reaction seen in Figure 2 (Buettner, 1993). During this process, alpha-tocopherol is converted to a radical by donating a labile hydrogen to a lipid or lipid peroxyl radical (Van Acker, 1993). The oxidized alpha- tocopherol radical is energetically stable and has low reactivity with other molecules within the membrane. Oxidized alpha-tocopherol can then be re-reduced to its original form by ascorbic acid. This regeneration of reduced alpha-tocopherol presumably occurs at the surface of the membrane where ascorbic acid and alpha-tocopherol can meet (Van Acker, 1997). Along with acting as a reducing agent for alpha-tocopherol, ascorbic acid is also considered a preventative antioxidant because of its ability to scavenge for reactive radicals.

Even with these numerous and effective mechanisms to control for oxidative damage, they are by no means totally efficient. Lipid peroxidation is an inescapable fact that can not be avoided. Scientists are now trying to examine the extent that lipid peroxidation plays in provoking diseased states. Many feel that they have only seen the tip of the iceberg. It is well-known that lipid peroxidation causes havoc in membranes, a key component and building block of life. A pathological state induced by the altering of a membrane due to lipid peroxidation is not just possible; it is a very real phenomenon. One such example is that numerous studies have recently suggested lipid peroxidation may be the major cause of tissue injury seen in heart attacks and strokes. These conditions both involve an ischemic period, where blood flow and oxygenation of tissue is restricted (Davies, 1996). It is thought that damage is caused, not by the ischemic period, but rather during the re-establishment of blood flow and oxygenation. This reperfusion period has been shown to generate a burst of reactive species and oxygen radicals as a result of the conditions originally established by the ischemic period (Nakazawa, 1996). It is during the reperfusion period that extensive lipid peroxidation occurs and irreversible tissue damage is seen. Promising experiments have been conducted that imply such reperfusion damage can be limited by the administration of antioxidants during the post-ischemic state (Hall, 1997). As one can see, interest in oxidative stress as a result of lipid peroxidation will continue as scientists try to pacify the paradoxical nature of oxygen.

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