To understand the theories of aging we must first know a little about the nature of aging. It is obvious to a biology student that biological aging can be described at a phenomenological level as a process that is observed in all living things. It consists of a systematic, continuous alteration of individuals, leading last to their death. According to Strehler (1959), The aging process can be characterized by the following four main features:
You can see how varied the theories are and that they extend into every area in biology and even into mathematics (Austad, 1998. and Kirkwood, 1998). Since it would take many books to completely describe in detail each theory and hypothesis, I will only explain the basics of a few theories.
The first theory that I will discuss is called the disposable soma hypothesis. This hypothesis suggests that aging has evolved as a byproduct of optimization of energy and resources for the various works performed by the organism. It assumes that energy resources are better spent for maintenance of reproductive cells which are resposible for species survival. To maintain, repaire, and replace all of the bodies somatic cells would take too much energy and thus it is not cost effective. It is too expensive in terms of energy and thus the nonreproductive cells of the body are expendable (Kirkwood, 1986). Therefore aging results from a progressive accumulation of somatic defects and damage. This theory relates to evolutionary survival and selection in a unique way. Maintenance and repair include the prevention and removal of damaged DNA, accuracy in macromolecular sythesis, and degradation of defective proteins. Life-span of different species depends on the differential level of somatic maintenance and repair. We can conclude that long-lived species in general have a greater level of maintenance and repair systems compared to short-lived species. Scientists have done many studies on this concept. A study don by Tarin (1994) showed that
from several congenic mouse strains and mammalian species suggest that there may be an association between cleavage rate of concepti and longevity, in such a way that concepti from species, strains or the sex (male) with the fastest cleavage rates have shorter life spans.
Tarin goes on to say that a short lifespan my be due to the fact that fast repoducing cells tend to make more mistakes in their DNA and structure. Thus somatic mutations build up and aging occurs at a faster rate. Niedermuller and Hofecker in 1992 did a study which answered the question: Is selection for extended life expectancy possible by regularity in aging? They found that in later evolution appearing species of mammals have longer maximum life span potentials. According to their data, a nonadaptive hypothesis of aging was the most plausible and the best hypothesis that fit was the disposable soma.
Later species are adapted better to changing environmental conditions by their other development, their ecological niches are larger, their environment contains specifically lesser dangers and risks
, because they better cope with them--therefore it is worth while to spend more energy for the maintenance of their soma than for their reproduction…( Niedermuller and Hofecker, 1992).
The disposable soma hypothesis balances the maintenance and repair of somatic cells on one side and the reproduction and fertility on the other. If more energy is used for maintenance of soma, less will be available for reproduction, and vice versa. This hypothesis treats senescence as a price paid for sexual reproduction (Kirkwood, 1981). There is a correlation between time taken to reach reproductive maturity and the species life-span. Experimental animals whose reproductive age is delayed tend to live longer than normal animals (Sengall et al, 1991). As you can see there is considerable support for this hypothesis. Many studies have shown that animals with exhaustive reproductive activity seem to expend much more energy than is allocated for this purpose. They are left with little to maintain and repair the somatic cells and therefore they die soon after their single reproduction. This theory also can draw support from the free radical theory of aging which will be soon discussed. The maintenance and repair of free radical damage to various structural and functional molecules play an important role in determining the aging of an organism. Accumulation of these negative effects occurs because of failure of prevention and repair systems for such damages.
The free radical hypothesis is probably one of the most popular and widely known aging mechanisms. People spend millions of dollars per year on products which are “supposed” to help prevent free radicals from damaging cells and cell components. But what is a free radical? I have asked members of my family and some friends who are not in a science program and they could not give me a reasonable definition. Thus the free radical hypothesis could be one of the least understood mechanism. Free radicals are chemical species which contain an unpaired electron in an outer orbital. This unpaired electron makes them very reactive. Free radicals can be produced as transient intermediates in the course of normal cell metabolism. An example of this is found in oxidative process of the mitochondria. Since free radicals may attack important molecules such as DNA, proteins, and lipids, and since they also tend to be self-propagating, they are capable of generating considerable damage. According to Stanley Pine (1987), free radicals can be formed in just about any possible reactions such as fragmentation, substitution, addition, oxidation, and reduction. The process of oxidation is believed to produce the most free radicals because oxidation-reduction reactions or organic compounds proceed by free-radical pathways.
Examples of reactive free radicals include the following:
The very reactive hydroxyl radical (OH), which is produced when ionizing radiation passes through water.
The moderately reactive thiyl radical (RS), which is formed when reactive radicals like the hydroxyl radical react with sulfhydryl groups, e.g., cysteine side chains in proteins.
The weakly reactive nitric oxide (NO) radical, which is found in polluted urban air.
Free radicals exhibit some of these patters:
Attack other molecules indiscriminately.
Produce oxygen-consuming chain reactions, such that a single free radical effectively damages a large number of other molecules.
Cause fragmentation or cross-linking of molecules, including vital molecules like DNA and critically important enzymes (Mehlhorn, 1994, and Kirkwood, 1998).
Due to the randomness of the reactions that can occur some products of free radical chemistry are completely foreign to repair or for reuse into the cell by turnover enzymes of the cell. An example of this could be that when two proteins become cross-linked, they might become resistant to attack by proteolytic enzymes and these new molecules can accumulate progressively in cells just like age pigments which increase with age in the cells of animals.
One of the most common free radicals is oxygen. Ordinary oxygen has two electrons with unpaired spins so sometimes it can be called a biradical. Normally the oxygen radical is quite stable but it can be extremely reactive when other free radicals are present or in collisions with molecules that very reducing. Although oxygen is inert towards most nonradical molecules (it can’t initiate free radical chain reactions) it can react with an organic radical whose unpaired electron is associated with a carbon atom. This results in peroxyl radical molecules which are moderately reactive which will remove hydrogen atoms from many different molecules. This can lead to chain reactions and an accumulation of hydroperoxides. Chemical products of most free radical reactions involving oxygen can also be very reactive and many of them have earned the term “active oxygen” molecules. Several of the most popular species are:
Hydroxyl radical, OH
Superoxide radical, and two non radicals
The most reactive is the hydroxyl radical. No biological molecule is immune to its attack. Thus organisms have evolved to have major defenses to ensure that this radical arises as infrequently as possible. Singlet oxygen which can arise from peroxyl radical reactions is also reactive but not as reactive as the hydroxyl radical. In the presense of freely dissolved or lossely bound iron or copper and mild reducing agents like ascorbic acid
, hydrogen peroxide can be used to form hydroxyl radicals.
Because of the destructiveness of hydroxyl radicals, hydrogen peroxide decomposition could be a major source of biological damage under conditions or iron or copper release from protein binding sites. Hydrogen peroxide can also interact with heme proteins to produce highly oxidizing products (ferryl species) that can initiate free radical reactions (Mehlhorn, 1994).
One of the topics we have not yet discussed is what specific damage does a free radical do to our body. Nagy in 1994 said that the polymerizing effect of OH radicals on organic molecules is strongly dependent on the physical density of the system. If the density of the system is low, a radical will capture an electron from the nearest possible electron shell which can be refilled only rom the orbits of the same molecule (only intramolecular cross-links may from). If the density is high, refilling of the electron shell becomes more possible from other organic molecules (intermolecular cross-link formation becomes higher). These statements have two very important points hidden in them.
First, the density of the cell components is a priori variable, e.g., the cytosol is more diluted than the membranes, ect.; therefore, the highest rate of free radical-induced damage is to be expected in the membranes. Second, the age-dependent loss of intracellular water content may result in a higher rate of damage even from fewer free radicals, because the overall density of the cellular mass increases (Nagy, 1994).
As we near the end of our discussion, I would like to comment on the membrane hypothesis theory of aging. This little known theory is based on many aspects of cellular and membrane interactions. I will talk about one main aspect of this theory. Nagy (1994) said that the cell membrane should be considered the weakest point of the cell. In addition to free radical-induced damage is exposed to another serious damaging factor called residual heat. Residual heat is absent in all other membranous components of the cell. The plasma membrane behaves as a resistance and a capacitance in parallel. The resting membrane potential (reaching values up to –100 mV) is built up on the membrane which is 10 nm thick. To us this polarization is very small but at the molecular level it is very large with voltage reacting up to –100 kV/cm. For people who know about electricity, the discharge of such polarity results in considerable heat production just as we can observe on the condensers used in electronic circuits. Heat productions is due to the free energy changes and a decrease in entropy in the dielectric when the polarized electronic field across it is removed. Polarization of the cell membrane discharges at a rate of 1 to 2 msec during each action potential. This is very frequent when compared to nerve cells which discharge up to 5- to 100 times per second. Thus the membrane is exposed to considerable amount of heating all the time. Studies have shown that 90% of the heat is not dissipated and the residual heat is left in the membrane. Obviously, the energy of the residual heat can remain in the membrane only in the form of chemical bonds which are most likely to be covalent.
The formation of such covalent bonds may be one of the main factors determining the shortest useful life span of the cell plasma membrane among all cellular components. This type of membrane alteration can be considered as a true “wear and tear” phenomenon which certainly contributes to the velocity of plasma membrane deterioration (Nagy, 1994).
Data to support this theory is massive. Shinitzky (1984) wrote a two volume book which gives a extensive survey of some available data. The data indicates:
Microviscosity of the membrane lipid layers is increased.
A serious shift of molecular weight distributions of the synaptosomal membrane proteins toward higher values was observed. This indicates an increased cross-linking of proteins.
Diffusion is lower in most cells.
As you can see, just the heating of the cell membrane can cause major accumulative damage which will hamper cell function which will then correspond to body function. It should be noted that this is only a very small portion of the membrane hypothesis theory. If we were to completely cover this theory we would look at the decrease of passive monovalent ion permeabilities, the changes of intracellular colloids and loss of water
, the effect of increased density enzyme activities, the decrease of rates of RNA and protein synthesis, and the accumulation of waste products. This would take many text books.
Many scientists are looking for the many and exact mechanisms of aging. They will never achieve their goals because if we ever did find out what really causes aging, we could then stop the aging process. As Christians we know that God created science. Without God science will not make sense. The aging process is due to the causes of Sin. All around us there is decay. The very elements are decaying with age. Humans will forever die, age, and grow old until the day when God will restore order, peace, and harmony which will forever reign in his kingdom.
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Kirkwood, T. B. L., and Holliday, R., Aging as a consequence of natural selection
, in The Biology of Human Aging, Bittles, A. H. and Collins, K. J., Eds., Cambridge University Press, Cambridge, 1986, 1.
Kirkwood, T. B. L., 1998, Biological theories of aging: an overview. Aging (Milano). 1998 Apr;10(2):144-6.
Kirkwood, T. B. L. 1986, repair and its evolution: survival versus reproduction, in Physiological Ecology. An Evolutionary Approach to Resources Use
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Mehlhorn, J, Rolf., 1994, Oxidants and antioxidants in aging. CRC Press, London. p61-73.
Nagy, Zs, Imre., 1994. The membrane hypothesis of aging. CRC Press, London.
Niedermuller, H., Hofecker, G., 1992, Is selection for extended life expectancy possible by regularity in aging? Am J Clin Nutr,
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Pine, H, Stanley., 1987, Organic Chemistry. McGraw-Hill, Inc. New York
Tarin, J. J., 1995, Do the fastest concepti have a shorter life span? Z. Gerontol