Free-radical theory

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The free-radical theory of aging is that organisms age because cells accumulate free radical damage with the passage of time . In general, a "free radical" is any molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are stable over eons, most biologically-relevant free radicals are fairly reactive. For most biological structures free radical damage is closely associated with oxidation damage. Oxidation and reduction are redox chemical reactions . Most people can equate to oxidation damage as they are familiar with the process of rust formation of iron exposed to oxygen . Oxidation does not necessarily involve oxygen, after which it was named, but is most easily described as the loss of electrons from the atoms and molecules forming such biological structures. The inverse reaction, reduction , occurs when a molecule gains electrons. As the name suggests, antioxidants like vitamin C prevent oxidation and are often electron donators.

In biochemistry , the free radicals of interest are often referred to as reactive oxygen species ( ROS ) because the most biologically significant free radicals are oxygen-centered. But not all free radicals are ROS and not all ROS are free radicals. For example, the free radicals superoxide and hydroxyl radical are ROS, but the ROS hydrogen peroxide (H 2 O 2 ) is not a free radical species.

Denham Harman first proposed the FRTA in the 1950s [1] and extended the idea to implicate mitochondrial production of ROS in the 1970s [2] . Of all the theories of aging , Harman's has the most consistent experimental support. However models exist (i.e. Sod2+/- mice) that demonstrate increased oxidative stress, without any effect on lifespan. Hence, more data is needed to identify the role of free radicals/oxidative stress in aging.

The Free Radical Theory of Aging

The free radical theory of aging was conceinved by Harman at a time when most scientists still believed that free radicals were too unstable to exist in biological systems and before anybody had invoked them as a cause of degenerative diseases. Harman drew inspiration from two sources: 1) the rate of living theory, which held that lifespan was an inverse function of metabolic rate, oxygen consumption. 2) Rebbeca Gershman's observation that hyperbaric oxygen toxicity and radiation toxicity could be explained by the same underlying phenomenon: oxygen free radicals. Noting that radiation causes "mutation, cancer and aging" Harman argued that oxygen free radicals produced during normal respiration would cause cumulative damage which would eventually lead to organismal loss of functionality, and ultimately, death. In later years, the free radical theory was expanded to not only include aging per se, but also age related diseases. Free radical damage within cells has been linked to a range of disorders including cancer , arthritis , atherosclerosis , Alzheimer's disease , and diabetes . This involvement is not at all surprising as free radical chemistry is an important aspect of phagocytosis , inflammation , and apoptosis . Cell suicide, or apoptosis, is the body's way of controlling cell death and involves free radicals and redox signalling. Redox factors play an even greater part in other forms of cell death such as necrosis or autoschizis .

More recently, the relationship between disease and free radicals has led to the formulation of a greater generalization about the relationship between aging and free radicals. In its strong form, the hypothesis states that aging per se is a free radical process. The "weak" hypothesis holds that the degenerative diseases associated with aging generally involve free radical processes and that, cumulatively, these make you age. The latter is generally accepted, but the "strong" hypothesis awaits further proof. Both models trace back to Harman's work.

Evidence

• Results have demonstrated that the overexpression of catalase , an enzyme involved in the decomposition of hydrogen peroxide , increased both the average lifespan and maximum lifespan of mice by 20% ( PMID 15879174 ). However, the authors of that paper also indicated that the lifespan extension effect had apparently lessened in new generations of these mice.
• Making a well-studied roundworm, Caenorhabditis elegans , more susceptible to free radicals has led to shortned lifespan ( PMID 11237107 ). However, increasing atmospheric oxygen tension above the normal 21% O2, does not meaning fully decrease lifespan of C. elegans. On the other hand, consistent with the free radical theory, it does shorten lifespan of the fruit fly Drosophila.
• While genetic manipulations that increase the levels of oxidative damage generally do shorten lifespan in mice, there is as yet very limited evidence that decreasing free radicals below their normal levels, actually extends lifespan (see above).
• Feeding of antioxidants, which should increase lifespan if the theory is correct, can extend average but not maximum lifespan in mice, even so, this effect is weak when it is observed and overall inconsistent.

Antioxidant therapy

This theory implies that antioxidants (e.g. Vitamin A , vitamin C , and vitamin E ) — which prevent free radicals from oxidizing sensitive biological molecules, or reduce the formation of free radicals — will slow the aging process and prevent disease.

The antioxidant chemicals found in many food-stuffs (such as the well known vitamins A, C and E) are frequently cited as the basis of claims for the benefits of a high intake of vegetables and fruits in the diet. In particular, antioxidant therapy forms the basis of many basic pharmacological interventions and particularly orthomolecular medicine .

One possible strike against the FRT of Aging (but not necessarily the FRT of certain diseases) is that antioxidant supplementation has not yet been convincingly shown to produce a mammalian extension of lifespan. One exception is PBN (phenybutylnitrone), which produces about a 10% extension of maximum lifespan in experimental animals [1] . However, this finding has not been reproduced by other laboratories.

While there is good evidence to support the idea of FRTA in model organisms such as Drosophila melanogaster and Caenorhabditis elegans , [3] [4] recent evidence suggests that oxidative stress may also promote life expectancy of Caenorhabditis elegans by inducing a secondary response to initially increased levels of reactive oxygen species. [5] This process was previously named mitohormesis or mitochondrial hormesis on a purely hypothetical basis. [6] . The situation in mammals is even less clear. [7] [8] [9] Recent epidemiological findings support the process of mitohormesis , and even suggest that antioxidants may increase disease prevalence in humans.