ANNALS O F CLINICAL AND LABORATORY SCIEN CE, Vol. 25, No. 1 Copyright © 1995, Institute for Clinical Science, Inc.

The Process and Theories of Aging*!

JOSEPH A. KNIGHT, M.D.

Pathology and Laboratory Medicine, Salt Lake VA Medical Center,

andD epartm ent o f Pathology,

University o f Utah School o f Medicine, Salt Lake C ity, UT 84132

ABSTRACT

A wide variety of theories to explain the aging process have been pro­posed including: (1 ) stochastic (somatic mutation, error catastrophe, protein glycosylation); (2) developm ental (immune and neuroendocrine); (3) pro­grammed (genome-based); and (4) free radical. Although no single hypothe­sis fully explains all aging phenomena, the genome-based and free radical theories, in particular, are supported by significant observational and experimental evidence. In addition, these two proposals are related to each other in some respects, and possibly to other theories as well. Continued basic and clinical research in this highly complex and critically important area will add to our understanding and appreciation of the theoretical and practical implications of the aging process.

“The aging process could be extended if it had to make its way through congress.”

President George Bush.

Introduction

Although philosophers and scientists have long been in terested in the aging process, general in terest in this important topic was minimal before about 1960. In recent decades, however, interest in the various aspects of aging has accelerated in great part owing to the realization that not only do the e lderly form an ever

* Send rep rin t requests to: Joseph A. Knight, M.D., D epartm ent of Pathology, University of Utah School of M edicine, Salt Lake City, UT 84132.

t The 1st Kendall Kane M emorial Lecture, pre­sented at the m eeting o f the Association o f Clinical Scientists, Tampa, FL, Novem ber 17, 1994.

increasing percentage of the population, bu t they also utilize a disproportionately high percent of health care costs.

It is estim ated that 5,000 years ago the average life expectancy (birth to death) was about 20 years. From then to the turn of this century, the life expectancy in the United States increased to 47 years; in the subsequent 90 years, it increased an additional 28 years. That is, the National Center for H ealth Statistics reported that the average life expectancy in the U.S. in 1990 was 75.4 years, 78.6 years for women and 71.8 years for men.

It is tru e th a t as p e o p le age th ey become more susceptable to a variety of

0091-7370/95/0100-0001 $01.80 © Institute for C linical Science, Inc. ' j

2 KNIGHT

diseases. N evertheless, it is im portant that it is recognized that aging and dis­ease are distinctly different. Although the term “aging” is understood in general terms, no single formal definition is uni­versally accepted. As a result, there have been many attem pts to define it more p re c is e ly . In th is re g a rd , H a rm a n 1

defined aging as “ . . . the progressive accumulation of changes with time asso­ciated w ith or responsible for the ever increasing susceptibility to disease and death which accompany age.” Rothstein2

stated that “ the changes from maturity through senescence constitutes the aging process,” while Strehler and North3 sug­gested that the aging process was one that m ust be: (1 ) deleterious, (2 ) progres­sive, (3) intrinsic (i.e., not modifiable by environm ental agents), and (4) universal.

Regardless of how aging is defined, it is w idely accepted that its effects include the following:

(1 ) a progressive decrease in efficiency and vigor of essentially all physio­logic functions;

(2 ) atrophy of most, if not all, organs and tissues;

(3) increased vu lnerab ility to traum a, infections, and various immune system

m alfunctions (autoimmune disorders, amyloidosis, lymphoid diseases);

(4) in c re a se d s u sc e p tib ili ty to m ost m alignant processes; and

(5) decreased V 0 2 max (the physiologic m easurem ent of the body’s capacity to extract oxygen from the air and transmit it to the circulatory system).

Life Expectancy versus Maximum Life Span

Do humans die of “old age?” The bio- sc ie n tif ic m ed ica l m odel o f d ise a se assumes that death always results from a single disease process or a combination of diseases. Although this model is com­monly held to be true, it is occasionally difficult to defend. Nevertheless, all e ld ­erly who die and are autopsied have a variety of diseases; however, are they severe enough to cause death, either sin­gly or in combination? In some cases it is difficult to be certain.

As noted previously, the average life expectancy has progressively increased over the past many centuries, bu t par­ticularly so since 1900 (figure 1). The im pressive increase betw een 1900 and

F ig u r e 1. C om pari­son of individuals living at various ages and peri­ods o f t im e w ith th e “ id e a l” m ax im um life span: A (ideal); B (1980); C (1920); D (1900); and E (3000 BC).

PROCESS AND TH EO R IES O F AGING 3

1960 was prim arily due to declining neo­natal, in fan t, and m aterna l m ortality rates, along with the control of various infectious diseases. More recently, there has also been a significant reduction in coronary artery d isease and stroke, as well as im proved m anagem ent and treat­m ent o f a w ide variety of diseases, such as cancer, diabetes, hormone disorders, etc. Nevertheless, there is no evidence that the maximum theoretical life span has increased over the past many centu­ries. In this regard, it was reported in 1993 that the oldest docum ented person in the world, living in France, was doing surprisingly w ell at 118 years of age .4

Furtherm ore, over the past decade, the oldest living person in the world, invari­ably a woman, has been in the 1 1 2 to 116 year age range.

The Aging Theories

Although the specific biologic basis of aging remains obscure, there is general agreem ent that its elucidation will be at the m o lecu la r level. F u rthe rm ore , it should be consistent, not only with the life span differences betw een species, bu t also w ith the fact that non-cycling cells such as neu rons and m yocytes undergo a relatively uniform functional decline w ith age.

There have been numerous theories to explain the aging process. However, cur­ren t th o ugh t g enera lly p roposes tha t senescence results from various extrinsic events tha t progressively lead to cell damage and death or characteristic intrin­sic events such as the genom e-based theory. These general groups have been presented in a variety of ways. Table 1 o u t l in e s th e c la s s if ic a t io n sc h em e referred to in this discussion. It should be em phasized , how ever, th a t no single theory is entirely satisfactory. Rather, it is probable that aging is due to a combina­tion of several theories, in which some are more im portant than others. Further-

TABLEI

Theories of Aging

A. Stochastic (random event; “wear and tear”)1. Somatic mutation/DNA damage2. Error catastrophe3. Protein glycosylation

B. Developmental1. Immune2. Neuroendocrine

C. Programmed (genome-based)D. Free radical

more, it may be difficult in some cases to determ ine which processes are primary and which are secondary to other causes.

A. S t o c h a s t i c T h e o r i e s

1. Som atic m uta tion /D N A damage: This theory is based, in part, on the idea that background radiation and/or various endogenous m utagens produce random chrom osom e dam age. O ver tim e, the genetic loci become sufficiently altered such that various critical functions fail, and the cells die. Certainly, the fact that irradiation of laboratory animals leads to early death lends some credence to this hypothesis. However, since irradiation also produces free radicals, it could be considered as part of that theory.

The ability of a cell to repair itself fol­low ing deoxyribonucle ic acid (DNA) damage is also an integral part of this theory. Indeed, each mammalian cell has an e la b o ra te system of DNA re p a ir enzym es w hich becom e less efficien t with time. Thus, failure to repair dam­aged DNA or to “m isrepair” it could lead to gene inactivation or possibly excision of key genes. Since there is some corre­lation betw een the efficiency of DNA repair and life span , 5 failure to repair completely the damaged DNA is of con­siderable importance for certain types of damage that could further accelerate the aging process .6

4 KNIGHT

A major problem with this theory is that it implies that senescence is based on random events. Aging is, however, clearly a non-random phenomenon.

2. E rror C a tastrophe: T h is theo ry holds that through random errors in trans­lation or transcription, erroneous copies of proteins associated w ith chromosomes lead to genetic abnorm alities .7 These, in turn, result in persistently abnormal pro­tein synthesis, and an eventual “ error catastrophe” destroys the cell. Therefore, the ability of a cell to produce its normal c o m p le m e n t o f fu n c tio n a l p ro te in s depends not only on the correct genetic specification of the various amino acid sequences, bu t also on the competence and fidelity of the protein-synthesizing apparatus. That is, the information m ust be translated correctly.

A lthough it is tru e th a t in c re ase d amounts of abnormal proteins are present in the elderly, most of them are a result of post-translational changes. An example would be the various isoforms of creatine kinase (CK). Here, the major isoenzyme, CK-MM (also designated CK-33), is nor­m ally synthesized . H ow ever, after its release into the circulation, the serum

enzyme carboxypeptidase hydrolyzes off a term inal lysine from one of the M-pep- tides to form CK-32. Subsequent hydroly­sis of the term inal lysine from the second M -peptide produces the th ird CK-MM isoform, CK3! . 8

3. Protein G lycosyla tion: A lthough related to the error catastrophe theory, protein glycosylation’s somewhat unique aspects set it apart. The theory is based on the fact th a t g lucose reacts non- enzymatically w ith num erous proteins, including hem oglobin, enzymes, colla­gen, elastin, and others, to form glycosy­lated end products (figure 2). Glucose also readily reacts w ith nucleic acids. T he chem ical struc tu res of the m ost advanced glycosylation en d products are no t know n. H ow ever, the cross­link shown in figure 2 has been identi­f ie d as 2 - fu ra n y l-4 -(5 )-(2 -fu ra n y l)- lH -im idazole .9

It has been proposed9 that this non- enzymatic reaction triggers a series of ir re v e rs ib le ch em ica l reac tio n s th a t resu lt in the formation and accum ula­tion of various adjacent cross-linked pro­tein molecules. As a result, these abnor­mal proteins are seen to increase pro-

F i g u r e 2. T he reac­tion of glucose w ith a pro­tein amino group to ini­tially form a glycosylated p ro te in (A m adori p ro d ­uct) and subsequent glu- co se -d e riv ed cross-link [2-furanyl-4(5)-(2-furan- y l) - lH im idazo le]. (Sci Amer 1987;256:90-6.)

IH— C — OH

IOH— C — H

IH— C — OH

IH— C — OH

ICH^OH

Glucose Protein

H—CI

H— C — OHI

OH— C — HI

H— C — OHI

H— C — OHI

CH2OH

Schiff Base

Protein

X' 'C\\ IIX—c

\

ProteinI

N— HIH—C—HI

C— oIOH— C— HIH— C— OHIH—-C— OHICH2OH

Amadori Product

H H Lein \\ IIc— cH H

Glucose-derived Cross-link

PROCESS AND THEORIES O F AGING 5

gressively and thereby contribute to a w ide variety of problems, including stif­fening of organs and tissues w ith loss o f e las tic ity so ch arac te ris tic in o ld ­er individuals.

These glycosylated proteins are also postulated to decrease the elasticity and perm eability of the extracellu lar com­partm ent and thereby impair the passage of nutrients and waste products in and out of cells. Both DNA and ribonucleic ac id (RNA) g lycosy la tion cou ld also result in serious cell impairment. It is fur­ther suggested that, in addition to glu­cose, various o ther in trace llu la r su b ­stances, such as aldehydes, free radicals, sulfur cross-linkages, quinones, and poly- basic acids, could participate in protein cross-linkages .9

Although these various glycosylated proteins increase with age, no definite evidence exists that they directly affect the life span. Certainly, further studies are needed in this important and fascinat­ing area.

B. D e v e l o p m e n t a l A g i n g T h e o r i e s

1. Im m u n ity and Aging: It has long been recognized that a decline in imm u­nologic capacity accom panies age and that older people are more prone to vari­ous in fectious d iseases, au to im m une phenom ena, amyloidosis, myelomatosis, chronic lymphocytic leukemia, and vari­ous forms of cancer. T his d ec reased responsiveness of the imm une system is particularly rela ted to thym us-derived ce lls (T -lym phocytes). T h a t is, w ith increasing age there is, in general,

(1 ) decreased response to mitogen stim u­lation;

(2) decreased interleuken-2 (IL-2) pro­duction;

(3) decreased T-cell response to IL-2;(4) decreased antibody production fol­

lowing stimulation; and(5) increased frequency of autoantibod­

ies.

With respect to the thymus, it begins to undergo primarily cortical atrophy dur­ing adolescence and by age 50 years, it is less than 10 to 15% of its m axim um weight. Cell loss, shift in proportion of the subpopulations, and qualitative cel­lular changes have all been described . 10

In terestingly , thym ic transplants have been shown to increase the life span of various experimental anim als . 11

B-lymphocytes are also affected since there is frequently a decreased capacity to produce antibodies w ith advancing age . 12 However, since B-cell function is d e p e n d e n t on T -lym phocytes, it no t certa in w h e th er th is is a p rim ary or secondary p h en o m en o n , or bo th . In addition, the immune system has repeat­edly been shown to be stim ulated by various vitamin (A, E, C) and m ineral (zinc) supplem entation . 13 ,14 ,15

The imm une system is probably also influenced by the endocrine system. For e x a m p le , d e h y d ro e p ia n d r o s te r o n e (D H EA ) p ro d u c tio n d e c re a se s w ith advancing age . 16 Using a m urine model, D aynes and Araneo have show n that m ice rem ain im m unologically norm al during aging if they are regularly supple­m ented with DHEA-sulfate. As indica­tors of im m une sufficiency, an tibody responses to protein antigens w ere m ea­sured, as well as the quantity and pat­terns of various lymphokines.

It is also interesting to note that the his­tocompatibility locus is involved not only in immune regulation, bu t also with the antioxidant enzyme, superoxide dismu- tase. Thus, it appears that th ere may exist a relationship of both the endo­crine and free radical theories with the immune theory.

2. Neuroendocrine Theory: The neuro­endocrine aging theory, having had its proponents since 1928,6 emphasizes the role of the hypothalamic-pituitary system a n d th e e n d o c r in e ta r g e t o rg a n s . Although age-related neuronal cell loss occurs in various areas of the brain, such

6 KNIGHT

as the locus ceruleus, hippocam pus, cau­date nucleus, putam en, substantia nigra, and the cerebral cortex, no similar cell loss has been reported in the hypothala­mus or pituitary. Thus, although a wide variety of neuroendocrine disorders (dia­betes, hyper- and hypothyroidism, m us­cle and organ atrophy owing to decreased growth horm one, gonadal dysfunction, and hypertension) increase with advanc­ing age, at least some of them are most likely secondary phenom ena.

C. P r o g r a m m e d A g i n g (G e n o m e -B a s e d T h e o r y )

T he genom e-based theory suggests that, although everyone is programmed individually, an internal “clock” starts at conception and runs a specific period of tim e. Accordingly, genes carry specific instructions that control not only growth and m aturation, b u t also dec line and death. This theory has considerable sup­p o rt, in c lu d in g th a t o b ta in e d from both observational studies and labora­tory research.

1. General Observations: It is gener­ally accepted that in humans there is a m odest to m oderate correlation in life expectancy be tw een family m em bers. That is, families in which centenarians ex is t a re m ore lik e ly to have o th er equally long-lived m em bers than those families w ithout centenarians. Further­more, the difference in longevity is, on average, less in identical than with frater­nal twins.

2. A c c e le ra te d A g in g S yn d ro m es: There is a considerable body of evidence suggesting that the aging process is, to a significant extent, under genetic control. In this regard, a graphic overview of aging is seen in progeria as well as in both W erner’s and Dow n’s syndromes. T hese d isorders are all characterized by a c ce le ra te d ag ing and d e c re a sed life expectancy.

(a) Progeria (H u tc h in so n -G u illfo rd Syndrom e) is a rare d iso rd e r of unknow n pathogenesis; fam ilial cases have not be reported. Never­theless, their lives are quite charac­teristic and uniform. They appear normal at birth but w ithin a few years somatic grow th slows, and clinically apparent aging features appear: baldness, skin atrophy, ath­erosclerosis, and in tellectual loss may all become apparent. In terest­ingly, the endocrine system appar­ently remains functionally normal. Death, invariably occurring before age 2 0 , is characteristically due to the same diseases affecting un in ­volved elderly individuals-stroke, myocardial infarction, cardiac fail­ure, cancer, and infections.

(b) W erner s Syndrome is a rare auto­somal recessive condition charac­terized by prem ature aging. This disorder is diagnosed in those who have at least three of the following m ajor criteria: (1 ) charac te ris tic habitus and stature; (2 ) prem ature senescence; (3) scleroderm a-like skin manifestations; and (4) endo­crine abnorm alities. T hese in d i­viduals are also often characterized by having hyaluronuria, and their cultured cells usually dem onstrate a dim inished num ber of cell divi­sions. A recent report17 indicated a genetic linkage in this syndrome to five markers on chromosome 8 .

(c) D ow n’s Syndrome is the first con­dition shown to be associated with a chromosomal abnormality; about 85% have 47 chromosomes (three chrom osom e # 2 1 ), w hile in the other 15% the extra chromosome is b o rn e on a n o th e r ch rom osom e (translocation), usually group D. T he d iso rd er is q u ite com m on, occurring once in every 600 to 800 births, but one in about 25 if the

PROCESS AND THEORIES O F AGING 7

m other is over 40 years old. The condi­tion is commonly characterized by a vari­ety o f derm ato log ic fea tu res , m ental retardation, congenital defects, and a dis­tinctly shortened life span.

It is not understood why the life span of these individuals is shortened. How­e v e r, a r e c e n t s tu d y 18 r e p o r te d an increase in their blood plasm a lipoperox- ides in the presence of increased red cell activity of superoxide dism utase and glu­tathione peroxidase. Although these find­ings are somewhat confusing in that the in c re a se d en zy m e v a lu es w o u ld be expected to be protective, it suggests a possible relationship to the free radi­cal theory.

3. Finite Doubling Potential: About 30 years ago Hayflick19 dem onstrated that serially cu ltu red hum an d ip lo id cells have a finite in vitro life span. That is, hum an cells dividing in vitro have a lim­ited num ber of potential cell doublings (50 ± 10), presum ably because of built-in genetic senescence programs. A rough correlation betw een the num ber of fibro­blast doublings and life span has been noted in several animal species. In addi­tion, cells from individuals w ith progeria and W erner’s syndrome have fewer dou­blings than cells from unaffected people. On the other hand, there is a relatively poor correlation betw een donor age and doubling po ten tia l .2 0 Interestingly, fol­lowing exposure to certain viruses and c h e m ic a l a g e n ts , tra n s fo rm e d ce lls become malignant, continue to replicate and, hence, becom e “ im m ortal” w ith respect to their replicative potential .21

In th is regard, the p resence of the enzym e, te lom erase , is ex p ressed in some malignant cells bu t not in normal cells .22 Each time a cell divides, the telo­m ere (chromeric tip of the chromosome arm) cha in sho rtens. O nce c ritica lly shortened telom ere length is reached, the cell stops dividing and presum ably senesces. In some cancer cells, telom ­

erase stimulates continued cell replica­tion by m aintaining the telom ere length.

D . F r e e R a d i c a l s a n d L i p i d P e r o x i d a t i o n

Free radicals were first postulated by Fen ton in 1893. Follow ing this early work, free rad ica l chem istry rap id ly expanded with the recognition that the rancidification of fats and oils was an oxy­gen free radical process. However, prior to these latter events, Lorrain Smith, an Irish physician, dem onstrated that the exposure of experimental animals to ele­vated oxygen levels resulted in serious lung dam age .2 3 Unfortunately, desp ite more than 1 0 0 subsequent publications describing oxygen toxicity, over 60 years from the time of Sm ith’s original work passed before this information becam e widely recognized.

According to this aging theory, the pro­duction of highly reactive oxygen free radicals cause progressive, random dam­age to DNA, RNA, enzymes, and other pro teins, as w ell as unsatu ra ted fatty acids and phospholipids in cell m em ­branes w hich eventually leads to cell d e a th . 2 4 T h e re is now c o n s id e rab le experimental and observational evidence that the aging process is the sum of all free radical reactions throughout all tis­sues and cells, or at least it is a major contributor to it .2 5 ’2 6 ’2 7 Furtherm ore, as noted previously, certain aspects of sev­era l o th e r ag ing th eo rie s a p p e a r to involve free radicals, either directly or indirectly. In addition, a w ide variety of diseases/disorders have been linked to oxygen-derived free radicals and lipid peroxidation .28

A free radical is an atom, m olecule, or compound w ith one or more unpaired electrons. T hese chem ical species are highly electrophilic and, therefore, attack sites of increased electron density (e.g.,

8 KNIGHT

nitrogen atoms in DNA, RNA, and pro­teins, carbon-carbon double bonds in unsaturated fatty acids and phospholip­ids, etc). One of the earliest recognitions of the im portance of free radicals in bio­logical systems was noted following ion­izing irradiation .2 9 Thus, the radiolysis of water in irradiated tissues results in its homolytic dissociation w ith the forma­tion of the highly reactive hydroxyl free radical (H O ), which is the most reactive of all naturally occurring free radical spe­cies (others include RO-, ROO ).

H20 -»• HO- + H-

On the other hand, lipid peroxidation (LP) is an autocatalytic process whereby polyunsaturated lipids undergo degrada­tion by a chain reaction to form lipoper- oxides, as well as a w ide variety of other compounds (figure 3). As a result, cell mem brane lipid bi-layers are highly sus­ceptible to LP. In addition, lipid-lipid, lipid-protein, and protein-protein cross­links are formed, along with damage to DNA and RNA. These events are not too surprising, since it has been estimated that a single free radical results in hun­dreds of lipoperoxides before the reac­tion is term inated. Furtherm ore, Ames et al3 0 calculated that hum an cell nuclei undergo 1 0 , 0 0 0 free radical “hits” each day, w h ile rat DNA undergo 100,000 “hits” each day.

Support for the Free Radical Theory

I . G e n e r a l O b s e r v a t i o n s

In 1928, Pearl31 suggested that aging was dependen t on the “rate of living.” Although this is an oversimplification of the aging process, the metabolic rate is indeed positively correlated with lipid peroxidation, age pigments, and overall life span. It follows that aging might be m odulated by the rate of oxygen con­sumption. In line with this, the metabolic rate is, w ith some variations, inversely

related to body size (large animals have low er m etabolic rates than sm all an i­mals). It follows that, with some excep­tions, large animals live longer than small ones (figure 4 ) . 32

As early as 1935, it was dem onstrated that by restricting food intake of young rats, their longevity could be increased . 33

S ubsequen t studies show ed that food restriction in adult rats is as effective in increasing life span as it is in young rats .34 In fact, the life expectancy of vari­ous laboratory animals can be increased by 30 to 50% by reducing th e ir calo­ric intake to 60% of the am ount nor­m ally consum ed by those given food ad libitum .35

These studies strongly suggest that food restriction reduces the m etabolic rate, and, therefore, the production of oxygen-derived free rad icals. In th is regard, Koizumi et al36 p resen ted evi­dence that long-term food restriction in mice selectively increases the activity of catalase, an antioxidant enzym e and a subsequent decrease in hepatic LP. Food restriction also reportedly delays age- related neoplastic diseases in laboratory anim als .35 Furthermore, Chung and asso­ciates3 7 stud ied the effects of d ietary restriction on DNA damage in both rat ce ll n u c le i and m itochond ria . T h e ir experim en ts show ed ab o u t 15 tim es greater damage to m itochondrial DNA than to nuclear DNA. Importantly, DNA damage in both nuclei and m itochondria w as s ig n if ic a n tly r e d u c e d in d ie t- restricted rats com pared to those who received food ad libitum.

Age pigments, primarily lipofuscin and cero id lipop igm en t, have b e e n com ­monly recognized in various organs of elderly people for well over 1 0 0 years. Since the pigm ent accum ulates p rinci­pally in the lysosomes of non-replicating cells, it is found prim arily in cardiac, smooth, and skeletal muscle, brain, and liver. This yellow -brow n, fluo rescen t pigm ent is formed as a bi-product of oxy-

PROCESS AND THEORIES OF AGING 9

Initiation:

Rv v = v ==w C00H ra /==v=w cooh

r; = \ / = \ A /C O O HW=V=WPropagation:

r;w r v \ CCX)H ♦ 02

R'\\ ^ a ^ a a cooh +

()*Hydroperoxyl Radical (R02*)

r;V \ 0/ V = V \ CCX)H

Hydroperoxide

r R' w y y v

" ' w v ~ v \ .

COOH

COOH

(Conjugated Diene) (R*)

R'\WV~\ACOOHo*

r-/V==Vs=W cooh

Figure 3. Autocata- lytic reaction sequence showing three stages: a) free radical initiation, b) propagation in w hich lipoperoxides are formed and the autocatalytic pro­cess continues, and c) ter­mination.

= x a /C O O H

T erm ination : 2R' — “■ RR

2 R0 2’ -----"-O2 + ROOR

R0 2" + R --------- ROOR

9 0 -

8 0 -

7 0 -

6 0 -

Life 5° - Expectancy

(years)4 0 -

3 0 -

2 0-

10 -

0-

Human (75)

■ 1 Elephant(69)

¡»■■■■■Min Hippopotamus (54)

— — Gorilla (47)

— — Horse (40)

■ Polar Bear (34)

■Dog (19)

- Guinea Pig (6) ■ Rat (3)

Mouse (2-3)

F igure 4. C om pari­son of mean life expectan­c ies o f various an im al s p e c ie s w i th t h a t o f h u m a n s . (A fte r S m ith DWE. Human Longevity. New York: Oxford U ni­versity Press, 1993:9.)

Species (years)

10 KNIGHT

gen-derived free radical reactions and LP. A lthough no d e fin ite functional derangem ents have, as yet, been attrib­uted to it, its presence is suggestive of continuing LP as a result of long-term inadequate defenses against the stresses of activated oxygen.

II. N a t u r a l M e c h a n i s m s t o C o n t r o l F r e e R a d i c a l s

A. A ntioxidant Enzymes: (a) Superox­ide (0 2-_ ), a free radical, is normally gen­erated during oxidative phosphorylation in which electrons are transferred from NADH and FA DH 2 to oxygen by various cytoplasmic reactions (e.g., catalytic con­version of xanthine to uric acid), as well as from the formation of methemoglobin from oxyhem oglobin. This latter non- enzymatic reaction results in the conver­sion of approxim ately 3% of the total h em og lob in lev e l to m ethem og lob in each day.

H bF e2+ -* H bFe3+ + 0a-~

S u p e ro x id e d ism u ta se (SO D ), an enzyme present in all body cells, rapidly converts 0 2'~ to hydrogen perox ide (H2 0 2). Although H 2 0 2 is, by itself, only mildly reactive, it is rapidly converted to HO- by various metal ions. Fortunately, cells have several naturally occurring enzymes which inactivate H 2 0 2.

G lu ta th io n e peroxidase (GPx), also p re se n t in all cells , rap id ly converts h 2 o 2 to water in the presence of reduced glutathione (GSH) as follows:

2GSH + H 2 0 2 -G Px—» GSSG + 2H20

Catalase (Cat), a ubiquitous intracellu­lar enzyme, also inactivates H 2 0 2 by con­verting it to water and oxygen.

2H 2 0 2 —Cat—» 2H20 + 0 2

B. Free R adica l Scaven g ers/A n tio x i­dants: Although the previous enzymatic reactions are very effective inactivators of H 20 2, many free radicals are still pro­

duced. As a result, the body is equipped with a w ide variety of free radical scav­engers, which include water soluble vita­min C and the lipid soluble vitamins, A and E. There are also other im portant naturally occurring antioxidants, such as uric acid , 37 b ilirubin , glutathione, and various sulfhydryl-containing proteins and amino acids, as well as the essential trace elem ent, zinc . 39

C. M etal B inding Proteins: Various transition metal ions, especially iron and copper, are potent catalysts that convert H2 0 2 to the hydroxyl free radical. The F en to n reac tio n was firs t d e sc r ib e d in 1893.

H2 0 2 + Fe2+ -» HO- + O H - + F e3+

In 1937, the two-step Haber-W eiss reac­tion was proposed.

(1) 0 2-" + Fe3+ - ^ 0 2 + F e2+(2) H 2Q2 + Fe2+ -> HO- + OH~ Fe3+[Sum: H 20 2 + 0 2 _ — Fe C a ta l y s t—»

HO- + O H " + 0 2]

Importantly, both H 20 2 and 0 2-- are normally produced in all cells; however, although the various enzymes inactivate m ost of these chem ical species, there remains an excess. Fortunately, iron and copper are primarily protein-bound and therefore non-reactive, thereby greatly reducing the possibility of the Fenton and Haber-W eiss reactions from taking place. These protective protein antioxi­dants include transferrin, ferritin, cerulo­plasm in, lactoferrin, myoglobin, hem o­g lob in , and th e various cy toch rom e oxidases, among others. N evertheless, despite these metal binding proteins and the several other above-described protec­tive mechanisms, abundant free radicals are produced .30

Summary

Although various theories to explain the aging process have been proposed,

PROCESS AND THEORIES O F AGING 11

no single one fully explains all of the phenom ena; rather, this complex process rem ains only partially understood. Nev­ertheless, evidence from a w ide variety of sources suggests that several of the curren t theories are related. Thus, the genetic and free radical theories appear re la ted in tha t studies w ith unusually long-lived fru it flies (D rosophila ) and so il-dw elling worms (C aenorhabditis) bo th have in creased activ ities of the antioxidant enzymes, superoxide dismu- tase and superoxide dism utase and cata- lase respectively .4 0 More recently, simi­lar studies by Orr and Sohal41 showed th a t d ip lo id contro ls, com pared w ith transgenic flies carrying three copies of copper-zinc superoxide dism utase and catalase, exh ib ited an increase in life span by one-third, delayed loss of physi­cal activity, and a lower amount of oxida­tive p ro te in dam age. Furtherm ore, as no ted previously , the im m une theory also appears to be related to these two th eo ries s ince the h istocom patib ility locus is in vo lved no t only w ith the imm une system but to the activities of superox ide d ism utase and the m ixed function oxidase system.

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