An attempt at a rational classification of theories of ageing

Aging: phenomena and theories

Aging is the accumulation of diverse adverse changes that increase the risk of death. These changes can be attributed to development, genetic defects, the environment, disease, and the inborn aging process. The chance of death at a given age serves as a measure of the number of accumulated aging changes, that is, of physiologic age, and the rate of change of this measure, as the rate of aging. As living conditions in a population approach optimum, the curve of the logarithm of the chance of death versus age shifts towards a limit determined by the sum of (1) the irreducible contributions to the chance of death by aging changes that can be prevented to varying degrees, and (2) those due to the intrinsic aging process. In the developed countries living conditions are now near optimum, and the ALE-Bs are about 6-9 years less than the potential maximum of around 85 years. The inborn aging process is now the major risk factor for disease and death after about age 28. By age 28 only 1 to 2% of a cohort is dead, the remaining 98 to 99% die at an exponentially increasing rate determined by the aging process. This process ensures that few reach 100 years and none exceed about 122 years. Many theories have been advanced to account for the aging process. No single theory is generally accepted. Theories that can contribute to the important practical goal of increasing the healthy, useful span of humans will endure.

The free radical theory of aging matures

The free radical theory of aging, conceived in 1956, has turned 40 and is rapidly attracting the interest of the mainstream of biological research. From its origins in radiation biology, through a decade or so of dormancy and two decades of steady phenomenological research, it has attracted an increasing number of scientists from an expanding circle of fields. During the past decade, several lines of evidence have convinced a number of scientists that oxidants play an important role in aging. (For the sake of simplicity, we use the term oxidant to refer to all "reactive oxygen species," including O2-., H2O2, and .OH, even though the former often acts as a reductant and produces oxidants indirectly.) The pace and scope of research in the last few years have been particularly impressive and diverse. The only disadvantage of the current intellectual ferment is the difficulty in digesting the literature. Therefore, we have systematically reviewed the status of the free radical theory, by categorizing the literature in terms of the various types of experiments that have been performed. These include phenomenological measurements of age-associated oxidative stress, interspecies comparisons, dietary restriction, the manipulation of metabolic activity and oxygen tension, treatment with dietary and pharmacological antioxidants, in vitro senescence, classical and population genetics, molecular genetics, transgenic organisms, the study of human diseases of aging, epidemiological studies, and the ongoing elucidation of the role of active oxygen in biology.

An update on the oxygen stress-mitochondrial mutation theory of aging: genetic and evolutionary implications

The acceleration of fixed-postmitotic cell aging by a high metabolic rate and the age related loss of mitochondria found in that cell type led us to propose an oxygen stress-mitochondrial mutation theory of aging, according to which senescence may be linked to mutations of the mitochondrial genome (mtDNA) of the irreversibly differentiated cells. This extranuclear somatic gene mutation concept of aging is supported by the fact that mtDNA synthesis takes place at the inner mitochondrial membrane near the sites of formation of highly reactive oxygen species. Mitochondrial DNA may be unable to prevent the intrinsic mutagenesis caused by those byproducts of respiration because, in contrast to the nuclear genome, it lacks excision and recombination repair. The resulting mitochondrial impairment and concomitant cell bioenergetic decline may cause the senescent loss of physiological performance and may play a key role in the pathogenesis of many age-related degenerative diseases. These concepts are integrated with classic and contemporary hypotheses in a unitary theory that reconciles programmed and stochastic concepts of aging. Thus, it is suggested that cells are programmed to differentiate, and then they accumulate mitochondrial-genetic damage because of their high levels of oxyradical stress and the loss of the organelle rejuvenating power of mitosis.

Extending functional life span

Average life expectancy at birth is a rough measure of the span of healthy, productive life--the functional life span. In the developed countries average life expectancies at birth now range from 76-79 years, six to nine years less than the limit of about 85 years imposed by aging. Aging is the accumulation of changes that increase the risk of death. Aging changes can be attributed to development, genetic defects, the environment, disease, and the inborn aging process. The latter is the major risk factor for disease and death after age 28 in the developed countries. The free radical theory of aging arose in 1954; it postulated that aging changes were caused by free radical reactions. There is now a growing consensus, largely based on the results of measures to minimize more-or-less random endogenous free radical reactions, that such reactions are a major cause of aging, possibly the only one. Some of these studies are presented following a brief discussion of free radical reactions.

Glutamine metabolism is changed in lymphocytes from aged rats

Changes in the protein content, maximal activity, and Km of phosphate-dependent glutaminase were measured in the lymphoid organs (thymus, spleen, and mesenteric lymph nodes) from just-weaned, mature (3 months), and aged rats (15 months). Also, [U-14C] glutamine transport and decarboxylation and the production of glutamate and aspartate from 2 and 20 mM glutamine were measured in incubated mesenteric lymph node lymphocytes. The ageing process induced a reduction in the protein content of the thymus and spleen, as well as the phosphate-dependent glutaminase activity in the thymus and isolated lymphocytes. The Km of phosphate-dependent glutaminase, however, was not affected by the process. Ageing reduced [U-14C] glutamine decarboxylation and increased glutamate and aspartate production in incubated lymphocytes. The results indicate that the ageing process does modify several aspects of glutamine metabolism in lymphocytes: reduces maximal glutaminase activity and [U-14C] glutamine decarboxylation and increases the Km for [U-14C] glutamine uptake and the production of glutamate and aspartate.

A network theory of ageing: the interactions of defective mitochondria, aberrant proteins, free radicals and scavengers in the ageing process

Evolution theory indicates that ageing is caused by progressive accumulation of defects, since the evolutionary optimal level of maintenance is always below the minimum required for indefinite survival. Evolutionary theories also suggest that multiple processes are operating in parallel, but unfortunately they make no predictions about specific mechanisms. To understand and evaluate the many different mechanistic theories of ageing which have been proposed, it is therefore important to understand and study the network of maintenance processes which control cellular homeostasis. In this paper we describe a Network Theory of Ageing which integrates the contributions of defective mitochondria, aberrant proteins, and free radicals to the ageing process, and which includes the protective effects of antioxidant enzymes and proteolytic scavengers. The model simulations not only confirm and explain many experimental, age related findings like an increase in the fraction of inactive proteins, a significant rise in protein half-life, an increase in the amount of damaged mitochondria, and a drop in the energy generation per mitochondrion, but they also show interactions between the different theories which could not have been observed without the network approach. In some simulations, for example, the mechanism of the final breakdown seems to be a consequence of the cooperation of mitochondrial and cytoplasmic reactions, the mitochondria being responsible for a long term, gradual change which eventually triggers a short lived cytoplasmic error loop.

Aging and oxidative stress: modulation by dietary restriction

Aging is an inevitable biological process that affects most living organisms. Despite the enormous consequences associated with the aging process, until recently, relatively little systematic effort has been expended on the scientific understanding of this important life process. Society, however, urged by an ever increasing older population, is challenging scientists from many disciplines to explore one of nature's most complex phenomena-biological aging. For the past two decades, research directed toward the basic understanding of biological aging mechanisms and possible aging interventions have given us new insights into the molecular bases and the biological events that contribute to age-related deterioration. To further investigate the aging processes, one probe uniquely suited to exploring the progression of aging in animal models is dietary restriction, currently the only antiaging intervention accepted by gerontologists and nutritionists. Recent research renders a better understanding of how reduced dietary intake extends the life span, supplying evidence that dietary restriction is a diverse and effective modulator of oxidative stress. It has been proposed that this antioxidative mechanism is the underlying anti-aging action of dietary restriction.

Degeneration of human oncogenes and mitochondrial genes occurs in cells that exhibit age-related pathology

The development of a new class of assays to determine in vivo mutation frequencies has provided new perspectives on the timing, location, and distribution of somatic mutagenesis in mitochondrial genes and in oncogenes of the aging human body. This descriptive information has led to the inference of new models for age-related pathophysiology and oncogenesis. Mutations of mitochondrial genes rise rapidly with age to frequencies a thousand-fold higher than those of nuclear genes. Genotypic selection analysis has revealed that mitochondrial mutations accumulate predominantly in nonmitotic cells whose age-dependent loss is associated with pathology. Random mitochondrial mutation is most likely to inactivate Complex I, deficiency of which induces mitochondrial superoxide formation and cell death. Genotypic selection of oncogenic mutations at the BCL2 and p53 loci has revealed that the cell specificity of oncogenic mutations in persons without cancer correlates well with sites of tumor origin, indicating that cells bearing such mutations are the likely precursors of future tumors. Quantitative variation in human BCL2 mutation frequency is extensive, and BCL2 mutation frequency rises with age, concordant with increased risk for lymphoma. The clonality and persistence of BCL2 mutations suggests two specific testable mechanisms of lymphomagenesis. BCL2 mutation frequency rises in persons exposed to cigarette smoke, and more p53 mutations occur in skin exposed to sunlight than in unexposed skin. Thus, in addition to their likely relevance to future cancer risk, the dose-response relationship between exposure and oncogenic mutations indicates promise for their future use as in vivo biodosimeters of human exposure to carcinogens.

Genotypic selection of mitochondrial and oncogenic mutations in human tissue suggests mechanisms of age-related pathophysiology

The invention of the polymerase chain reaction (PCR) has facilitated the development of a new class of assays to quantify human somatic mutations in vivo, based on genotypic selection of mutants at the DNA level rather than phenotypic selection of mutants at the cell level. Use of these assays has provided new perspectives on the timing, location and distribution of somatic mutagenesis in mitochondrial genes and in oncogenes of the aging human body. This descriptive information has led to the inference and development of new models for age-related pathophysiology and oncogenesis. Mutations of mitochondrial genes rise rapidly with age to frequencies a thousand fold higher than those of nuclear genes. Genotypic selection analysis has revealed that mitochondrial mutations accumulate predominantly in non-mitotic cells whose age-dependent loss is associated with pathology. Random mitochondrial mutation is most likely to inactive Complex I, a deficiency of which induces mitochondrial superoxide formation and cell death. Genotypic selection of oncogenic mutations at the BCL2 and p53 loci has revealed that the cell specificity of oncogenic mutations in persons without cancer correlates well with sites of tumor origin, indicating that cells bearing such mutations are the likely precursors of future tumors. Quantitative variation in human BCL2 mutation frequency is extensive, and BCL2 mutation frequency rises with age, concordant with increased risk for lymphoma. The clonality and persistence of BCL2 mutations suggests two specific testable mechanisms of lymphomagenesis. BCL2 mutation frequency rises in persons exposed to cigarette smoke, and more p53 mutations occur in skin exposed to sunlight than in unexposed skin. Thus, in addition to their likely relevance to future cancer risk, the dose-response relationship between exposure and oncogenic mutations indicates promise for their future use as in vivo biodosimeters of human exposure to carcinogens.

Inherited stress resistance and longevity: a stress theory of ageing

Ageing is considered in the context of the abiotic stresses to which free-living organisms are normally exposed. Assuming that the primary target of selection of stress is at the level of energy carriers, trade-offs under the rate-of-living theory of ageing predict increased longevity from selection for stress resistance. Changes in longevity then become incidental to selection for stress resistance. I therefore suggest the reformulation of the rate-of-living theory to become a stress theory of ageing. This directly incorporates the characteristics of habitats in nature. Under this theory, the primary trait inherited is resistance to stress. Consequently, at extreme ages those with inherited resistance to abiotic stress should dominate. Furthermore, the reduction in homeostasis manifested by deteriorating ability to adapt to abiotic stress as ageing proceeds, should be slowest in those surviving longest.

Oxidative damage to mitochondrial DNA and its relationship to ageing

Mitochondria are the most important intracellular source of reactive oxygen species and are protected against them by enzymatic and nonenzymatic antioxidants. Nevertheless, mitochondrial DNA (mtDNA) is subject to severe oxidative damage, and much more so than nuclear DNA (nDNA). Damage is indicated by the detection of various base modifications, particularly 8-hydroxydeoxyguanosine (8OHdG), which can lead to point mutations because of mispairing. MtDNA is also fragmented to some extent. Conceivably, such fragmentation relates to the deletions found in mtDNA. Several hypotheses suggest that defective mitochondria contribute to, or are responsible for, ageing. Recent observations indicate that mitochondria in an old organism differ in many respects from those in a young organism. Thus, with ageing there is an increased production of reactive oxygen species, a decrease in certain antioxidants, a decreased transcription, translation, and cytochrome oxidase content, and an increase in the extent of DNA modifications. Major unresolved questions concerning the role of mtDNA changes in ageing are addressed: is there a causal relationship; what is the true extent of DNA damage; what are significance and functional consequences of mtDNA oxidation; are reactive oxygen species the cause of the DNA modifications found in vivo; what is the relationship between DNA damage and alterations of RNAs and proteins? Future studies promise to clarify the possible causal relationship between mitochondrial dysfunction, reactive oxygen species production, mtDNA modifications, and ageing.

An improved method of mouse liver micronucleus analysis: an application to age-related genetic alteration and polyploidy study

The performance of a micronucleus test in liver cells in vivo requires two laborious procedures: stimulation of hepatocytes to division and dissociation of liver tissue into a single-cell suspension. We propose the method of inhalation treatment of mice with carbon tetrachloride to induce cell proliferation and alkaline dissociation of previously fixed tissue. The micronucleus incidence and ploidy classes in terms of cytophotometric DNA content were determined in liver of mice of three age groups (around 2.5, 5.0 and 7.0 months old) after CCl4 treatment or partial hepatectomy. The data obtained show that both methods give the same results. The fraction of micronucleated hepatocytes was 0.69% at the age of 2.5 months; it increased to 8.5% and then to 13.5% at 5.0 and 7.0 months respectively. Simultaneously, the ploidy classes changed both with the aging of the animal and after induced liver regeneration. The percentage distribution of micronucleated cells by ploidy class showed that cells carrying micronuclei were the higher ploidies rather than the population in general. Since polyploid cells contain multiple molecular targets for genetic damage, the micronucleation index per genome unit was estimated. Then the real rate of accumulation of both intrinsic endogenous (and probably the exogenously induced) preclastogenic genetic alterations in hepatocytes during the adulthood of mice was evaluated to be 0.03% per diploid genome per day. This seems to be the first description of the phenomenon of liver cell aging in terms of micronuclear aberrations.

Human epidermal cells progressively lose their cardiolipins during ageing without change in mitochondrial transmembrane potential

Mitochondria dysfunction is considered to be a major cause of the modifications that occur during cell ageing. For this reason, cardiolipin, a suitable marker of the chondriome, as well as the mitochondrial transmembrane potential were examined in keratinocytes obtained from 9- to 75-year-old women. The study was carried out by flow cytometry using two fluorescent mitochondria probes: nonyl acridine orange, which binds specifically to cardiolipin, and rhodamine 123, which is incorporated mainly in response to transmembrane potential. Cardiolipin levels in cells from elderly donors (75 years old) would be 57% lower (r = 0.540; P = 0.0002) than those in children (9 years old), while the inner transmembrane potential remained unchanged (r = 0.0394; P = 0.8017). The stability of the membrane potential may be explained by either or both of the following hypotheses: (i) the same pool of organelles able to maintain membrane potential is conserved even when cardiolipin levels decrease (ii) mitochondria membrane potential does indeed decrease with age but is compensated by glycolysis energy production. Finally, it may be stated that the fluorescent probes nonyl acridine orange and rhodamine 123 might be of interest in testing the phenotype of senescent cells and would be useful in screening the role of certain specific genes in cell ageing.

Latent capacities for gametogenic cycling in the semelparous invertebrate Nereis

Most nereid polychaetes are strictly semelparous, a single episode of reproduction being invariably followed by death. Endocrine manipulation in Nereis diversicolor by the regular implantation of cerebral ganglia from immature donors unveils characteristics associated with a capacity to engage in repeated gametogenic cycling. Such manipulation permits full maturation of the gametes but blocks spawning. Gamete resorption then leads on to another bout of gametogenesis and a new cohort of gametes is formed. The neurosecretory system adopts a cyclical pattern of activity, which parallels that of gametogenesis. Repair and maintenance of the soma continue throughout sexual maturation, as shown by the persistence of feeding and the capacity for regenerative segment proliferation. In consequence, life is extended apparently indefinitely. These latent capacities are reminiscent of features of iteroparous life histories, characterized by repeated breeding, and are postulated to be vestiges of an iteroparous ancestry. They also constitute a preadaptation for iteroparity and reveal how readily a reversal to this condition could occur. The study suggests that reproductive strategies may be unexpectedly labile in even their most fundamental aspects.

Biological Aging and Longevity: Underlying Mechanisms and Potential Intervention Strategies

Aging is characterized by numerous physical, physiological, biochemical, and molecular changes. The rates at which aging processes occur are highly variable among individuals and are thought to be governed by both environmental and genetic factors. Lifestyle factors such as exercise, dietary, and smoking habits have been demonstrated to alter many of the changes usually associated with human aging. However, at present caloric restriction is the only experimental paradigm that has consistently been demonstrated in animal models to extend not only physiological vigor but also life span. The positive effects of exercise on physiological fitness and the reduction in the risks of certain diseases have been well documented. However, its effects on life span are not as clear. This article explores some of the basic mechanisms thought to be involved causally in the processes of aging, and outlines current and potential interventive strategies to retard or ameliorate the rates of decline in physiological function with advancing age.

Age-associated changes in superoxide dismutase activity, thiobarbituric acid reactivity and reduced glutathione level in the brain and liver in senescence accelerated mice (SAM): a comparison with ddY mice

The antioxidant defense alteration in young and old senescence accelerated mice (SAM) was studied by examining superoxide dismutase (SOD) activity, thiobarbituric acid (TBA) reactivity, and reduced glutathione (GSH) level in brain and liver tissues. The changes were compared with those in age-paired ddY mice, a strain exhibiting normal aging. SAM mice showed an age-dependent increase in SOD activity in liver, and an age-dependent increase in TBA reactivity in both the brain and the liver; they also showed an age-dependent decrease in the GSH level in the brain and the liver. When compared with ddY mice, SAM mice showed a higher SOD activity in the brain (at both 3 and 11 months old), a lower GSH level in the liver (at 3 months old), and a higher TBA reactivity in the liver (at 3 months old). These findings suggest that the mechanism of senescence acceleration in SAM mice is to some extent related to free radical damage.

Prospects for the genetics of human longevity

Longevity varies between and within species. The existence of species-specific limit to human life-span and its partial heritability indicate the existence of genetic factors that influence the ageing process. Insight into the nature of these genetic factors is provided by evolutionary studies, notably the disposable soma theory, which suggests a central role of energy metabolism in determining life-span. Energy is important in two ways. First, the disposable soma theory indicates that the optimum energy investment in cell maintenance and repair processes will be tuned through natural selection to provide adequate, but not excessive, protection against random molecular damages (e.g. to DNA, proteins). All that is required is that the organism remains in a sound condition through its natural expectation of life in the wild environment, where accidents are the predominant cause of mortality. Secondly, energy is implicated because of the intrinsic vulnerability of mitochondria to damage that may interfere with the normal supply of energy to the cell via the oxidative phosphorylation pathways. Oxidative phosphorylation produces ATP, and as a by-product also produces highly reactive oxygen radicals that can damage many cell structures, including the mitochondria themselves. Several lines of evidence link, on the one hand, oxidative damage to cell ageing, and on the other hand, energy-dependent antioxidant defences to the preservation of cellular homeostasis, and hence, longevity. Models of cellular ageing in vitro allow direct investigation of mechanisms, such as oxidative damage, that contribute to limiting human life-span. The genetic substratum of inter-individual differences in longevity may be unraveled by a two-pronged reverse genetics approach: sibling pair analysis applied to nonagenarian and centenarian siblings, combined with association studies of centenarians, may lead to the identification of genetic influences upon human longevity. These studies have become practicable thanks to recent progress in human genome mapping, especially to the development of microsatellite markers and the integration of genetic and physical maps.

The free radical hypothesis of aging: an appraisal of the current status

The objective of this review article is to assess the current status of the predictions of the free radical hypothesis of aging, highlighting some of the controversies surrounding the previous assumptions. Topics for discussion include: metabolic rate and aging, oxidative stress and molecular damage during aging, antioxidants and aging, antioxidant defenses and life spans of different species, and pro-oxidant generation and aging. On the basis of currently available evidence, it is concluded that the free radical hypothesis has neither been proven nor disproven. Some of the earlier assumptions such as that antioxidant intake increases life span, or antioxidant defenses decline with age, or antioxidant defenses are positively correlated with life spans of different species, or that longer life spans are associated with lower autoxidizability, are not clearly supportable. Similarly, the assumption that oxygen free radicals govern the rate of aging via the infliction of molecular damage lacks compelling support. Enough information to lift the free radical hypothesis above the level of speculation has not yet been amassed. Clearly, further studies, some of which specifically focus on disproving this hypothesis, are needed to confirm its veracity.

Gene expression and aging

Considerable amount of data has accumulated during the past few years showing several changes in gene expression as a function of age. However, the basic mechanism of aging still remains poorly understood. In this review, we have mainly analysed the data pertaining to the hypothesis that aging is associated with genetic instability and have attempted further to highlight the gaps that need to be bridged in order to have a clear picture of the aging phenomenon. Extensive investigations employing new and novel approaches are needed in future to elucidate the intricately interwoven patterns of molecular control that underlie the various aspects of gene expression during aging.

Biomarkers of aging in the neuroendocrine-immune domain. Time for a new theory of aging?

A common and generally accepted assumption is that with advancing age, the thymus undergoes progressive and irreversible involution. This is considered the main cause for the age-related deterioration of various immune functions and, ultimately, for the increased incidence of infectious, neoplastic, and automimmune diseases in old age. This assumption is no longer tenable because of several clear-cut demonstrations that age-related thymic involution is not an intrinsic and irreversible phenomenon. Various neuroendocrine or nutritional manipulations can to induce a regrowth of the thymus, even when applied in old age. This thymic reconstitution is followed by a consistent recovery of peripheral immune functions. These data strongly support the idea that thymic involution is a phenomenon secondary to age-related alterations in neuroendocrine-thymus interactions and that it is the disruption of such interactions in old age that is responsible for most of the age-associated dysfunctions. On the basis of this experimental and clinical evidence and as an alternative to purely immune or neuroendocrine theories of aging, a neuroendocrine-immune hypothesis is proposed. Further work is required to determine if the age-related disruption of neuroendocrine-immune interactions occurs because of progressive accumulation of stressor-dependent consequences at the level of one or the other system or if it may depend on a single common cause.

Free radical theory of aging

Free radical reactions are ubiquitous in living things. Studies on the origin and evolution of life provide a reasonable explanation for the prominent presence of this unruly class of chemical reactions. These reactions have been implicated in aging. This phenomenon is the accumulation of changes responsible for the sequential alterations that accompany advancing age and the associated progressive increases in the chance of disease and death. Aging changes are attributed to the environment and disease, and to an inborn process, the aging process. The latter produces aging changes at an exponentially increasing rate with advancing age. Past improvements in general living conditions have decreased the chances for death so that they are now near limiting values in the developed countries. In these countries the intrinsic aging process is the major cause of disease and death after about age 28. The free radical theory of aging postulates that aging changes are caused by free radical reactions. The data supporting this theory indicate that average life expectancy at birth may be increased by 5 or more years, by nutritious low caloric diets supplemented with one or more free radical reaction inhibitors.

LEAFY controls floral meristem identity in Arabidopsis

The first step in flower development is the generation of a floral meristem by the inflorescence meristem. We have analyzed how this process is affected by mutant alleles of the Arabidopsis gene LEAFY. We show that LEAFY interacts with another floral control gene, APETALA1, to promote the transition from inflorescence to floral meristem. We have cloned the LEAFY gene, and, consistent with the mutant phenotype, we find that LEAFY RNA is expressed strongly in young flower primordia. LEAFY expression procedes expression of the homeotic genes AGAMOUS and APETALA3, which specify organ identify within the flower. Furthermore, we demonstrate that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.

Free radical theory of aging: view against the reliability theory

The reliability theory approach to the free radical theory of aging is presented. First, the structural and functional inhomogeneity - hierarchy - of living systems has been taken into account. Hence, the existence of the finite number of critical structures - supervisors - in the system has been postulated. Second, I have postulated the stochastic nature of age-changes in the supervisors, paying particular attention to oxygen free radicals as the by-products of normal oxidative metabolism. Third, according to the cooperativity of main processes in biological systems, the existence of the threshold values of the reliability parameters of the supervisors has been postulated. In the terms of this reliability free radical theory of aging it becomes possible to explain the character of age-changes in mortality data for humans and animals; to explain how the linear kinetics of accumulation of free radical damages in the cellular targets (supervisors) can lead to the exponential kinetics of the mortality rate growth with age, i.e., to deduce the so-called Gompertz law of mortality; to explain the nature of the well-known empiric interspecies correlations between the maximum life span values and metabolic factors; to estimate species-specific life span potentials of humans and animals. The chromosomes of neural postmytotic cells have been suggested to function as the supervisors. In the terms of the theory it has been estimated that the life span of primates could run up 250 years but for the damages due to the oxygen free radicals.

Free radical theory of aging: history

Aging is the accumulation of changes responsible for the sequential alterations that accompany advancing age and the associated progressive increases in the chance of disease and death. These changes can be attributed to disease, environment, and the inborn aging process. The aging process is now the major risk factor for disease and death after about age 28. The free radical theory of aging arose in 1954 from a consideration of aging phenomenon from the premise that a single common process, modifiable by genetic and environmental factors, was responsible for the aging and death of all living things. The theory postulates that aging is caused by free radical reactions, i.e., these reactions may be involved in production of the aging changes associated with the environment, disease and the intrinsic aging process. The origination of the theory and its application to the problem of increasing the functional life span are discussed. Support for the free radical theory of aging has increased progressively and now includes: 1) studies on the origin of life and evolution, 2) studies on the effect of ionizing radiation on living things, 3) dietary manipulations of endogenous free radical reactions, 4) the plausible explanations it provides for aging phenomena, and 5) the growing numbers of studies that implicate free radical reactions in the pathogenesis of specific diseases. The rapidly growing number of scientists involved in studies on the role of free radical reactions in biological systems should assure future significant increases in the healthy, useful, life span of man.

The aging process: major risk factor for disease and death

Aging is the accumulation of changes responsible for the sequential alterations that accompany advancing age and the associated progressive increases in the chance of disease and death. Average life expectancies at birth in the developed countries are now approaching plateau values as the aging changes associated with the environment and disease near irreducible levels. The inborn aging process is now the major risk factor for disease and death after around age 28 in the developed countries and limits average life expectancy at birth to approximately 85 years. Future significant increases in average life expectancy--a rough measure of the healthy, productive life-span, i.e., the functional life-span--in these countries will be achieved only by slowing the rate of production of aging changes by the aging process. Many theories have been advanced to account for the aging process. The free radical theory of aging is discussed briefly. The importance attached to increasing the functional life-span dictates that aging hypotheses be explored for practical means of achieving this goal while work continues toward a consensus on the cause(s) of the aging process. Efforts to further increase the functional life-span by conventional measures are now almost futile, whereas those directed toward slowing the aging process are just beginning. These new efforts show promise.