Oxidants, antioxidants, and the degenerative diseases of aging

Extensive oxidative DNA damage in hepatocytes of transgenic mice with chronic active hepatitis destined to develop hepatocellular carcinoma

A transgenic mouse strain that expresses the hepatitis B virus (HBV) large envelope protein in the liver was used to determine the extent of oxidative DNA damage that occurs during chronic HBV infection. This mouse strain develops a chronic necroinflammatory liver disease that mimics the inflammation, cellular hyperplasia, and increased risk for cancer that is evident in human chronic active hepatitis. When perfused in situ with nitroblue tetrazolium, an indicator for superoxide formation, the liver of transgenic mice displayed intense formazan deposition in Kupffer cells, indicating oxygen radical production, and S-phase hepatocytes were commonly seen adjacent to the stained Kupffer cells. Similar changes were not observed in nontransgenic control livers. To determine whether these events were associated with oxidative DNA damage, genomic DNA from the livers of transgenic mice and nontransgenic controls was isolated and examined for 8-oxo-2'-deoxyguanosine, an oxidatively modified adduct of deoxyguanosine. Results showed a significant, sustained accumulation in steady-state 8-oxo-2'-deoxyguanosine that started early in life exclusively in the transgenic mice and increased progressively with advancing disease. The most pronounced increase occurred in livers exhibiting microscopic nodular hyperplasia, adenomas, and hepatocellular carcinoma. Thus, HBV transgenic mice with chronic active hepatitis display greatly increased hepatic oxidative DNA damage. Moreover, the DNA damage occurs in the presence of heightened hepatocellular proliferation, increasing the probability of fixation of the attendant genetic and chromosomal abnormalities and the development of hepatocellular carcinoma.

DNA oxidative damage and life expectancy in houseflies

The objective of this study was to explore the relationship between oxidative molecular damage and the aging process by determining whether such damage is associated with the rate of aging, using the adult housefly as the experimental organism. Because the somatic tissues in the housefly consist of long-lived postmitotic cells, it provides an excellent model system for studying cumulative age-related cellular alterations. Rate of aging in the housefly was manipulated by varying the rate of metabolism (physical activity). The concentration of 8-hydroxydeoxyguanosine (80HdG) was used as an indicator of DNA oxidation. Exposure of live flies to x-rays and hyperoxia elevated the level of 8OHdG. The level of 8OHdG in mitochondrial as well as total DNA increased with the age of flies. Mitochondrial DNA was 3 times more susceptible to age-related oxidative damage than nuclear DNA. A decrease in the level of physical activity of the flies was found to prolong the life-span and corresponding reduce the level of 8OHdG in both mitochondrial and total DNA. Under all conditions examined, mitochondrial DNA exhibited a higher level of oxidative damage than total DNA. The 8OHdG levels were found to be inversely associated with the life expectancy of houseflies. The pattern of age-associated accrural of 8OHdG was virtually identical to that of protein carbonyl content. Altoghether, results of this study support the hypothesis that oxidative molecular damage is a causal factor in senescence.

Apoptosis and free radicals

Necrotic cell death is usually a consequence of extensive insult to the cell, leading to release of intracellular contents and an inflammatory response. Apoptosis, however, is a physiological response to damaging influences that requires sufficient maintenance of homeostasis to allow execution of the pathway. Apoptosis circumvents the induction of an inflammatory response, which can be disadvantageous and, therefore, would be more beneficial than necrosis under many circumstances. The apoptotic response appears complicated and involves many factors, including the mitotic rate, the stage of differentiation, the type and strength of the initiating stimulus, and exogenous factors. Recent evidence, however, implicates free radicals as a causal agent in some types of apoptosis, both physiologically and pathologically.

In vivo generation of hydroxyl radicals and MPTP-induced dopaminergic toxicity in the basal ganglia

The in vivo generation of .OH free radicals in specific brain regions can be measured by intracerebral microdialysis perfusion of salicylate, avoiding many of the pitfalls inherent in systemic administration of salicylate. Direct infusion of salicylate into the brain can minimize the hepatic hydroxylation of salicylate and its contribution to brain levels of 2,5-DHBA. Levels of 2,5-DHBA detected in the brain dialysate may reflect the .OH adduct plus some enzymatic hydroxylation of salicylate in the brain. After minimizing the contribution of enzyme and/or blood-borne 2,5-DHBA, the present data demonstrate the validity of the use of 2,3-DHBA and apparently 2,5-DHBA as indices of .OH formation in the brain. Therefore, intracranial microdialysis of salicylic acid and measurement of 2,3-DHBA appears to be a useful .OH trapping procedure for monitoring the time course of .OH generation in the extracellular fluid of the brain. These results indicate that nonenzymatic and/or enzymatic oxidation of the dopamine released by MPTP analogues in the extracellular fluid may play a key role in the generation of .OH free radicals in the iron-rich basal ganglia. Moreover, a site-specific generation of cytotoxic .OH free radicals and quinone/semiquinone radicals in the striatum may cause the observed lipid peroxidation, calcium overload, and retrograde degeneration of nigrostriatal neurons. This free-radical-induced nigral injury can be suppressed by antioxidants (i.e., U-78517F, DMSO, and deprenyl) and possibly hypothermia as well. In the future, this in vivo detection of .OH generation may be useful in answering some of the fundamental questions concerning the relevance of oxidants and antioxidants in neurodegenerative disorders during aging. It could also pave the way for the research and development of novel neuroprotective antioxidants and strategies for the early or preventive treatment of neurodegenerative disorders, such as Parkinson's disease (Wu et al., this issue), amyotrophic lateral sclerosis, head trauma, and possibly Alzheimer's cognitive dysfunction as well. In conclusion, this in vivo free-radical trapping procedure provides evidence to support a current working hypothesis that a site-specific formation of cytotoxic .OH free radicals in the basal ganglia may be one of the neurotoxic mechanisms underlying nigrostriatal degeneration and Parkinsonism caused by the dopaminergic neurotoxin MPTP. Addendum added in proof: The controversy concerning possible neurotoxic and/or neuroprotective roles of NO. in cell cultures was discussed and debated at the symposium (Wink et al., this issue; Dawson et al., this issue; Lipton et al., this issue).(ABSTRACT TRUNCATED AT 400 WORDS)

Oxidative damage and mitochondrial decay in aging

We argue for the critical role of oxidative damage in causing the mitochondrial dysfunction of aging. Oxidants generated by mitochondria appear to be the major source of the oxidative lesions that accumulate with age. Several mitochondrial functions decline with age. The contributing factors include the intrinsic rate of proton leakage across the inner mitochondrial membrane (a correlate of oxidant formation), decreased membrane fluidity, and decreased levels and function of cardiolipin, which supports the function of many of the proteins of the inner mitochondrial membrane. Acetyl-L-carnitine, a high-energy mitochondrial substrate, appears to reverse many age-associated deficits in cellular function, in part by increasing cellular ATP production. Such evidence supports the suggestion that age-associated accumulation of mitochondrial deficits due to oxidative damage is likely to be a major contributor to cellular, tissue, and organismal aging.

Characterization and virulence analysis of catalase mutants of Haemophilus influenzae

In addition to detoxifying peroxides generated by aerobic metabolism, the catalases of pathogenic bacteria have also been hypothesized to serve as virulence factors by enabling microorganisms to resist the oxidative bursts of host inflammatory cells. Using transposon mutagenesis of the hktE gene, encoding the Haemophilus influenzae structural gene for catalase, we constructed defined catalase mutants of H. influenzae strains Rd- and Eagan b+. These mutants show no detectable catalase production during exponential or stationary phases or following induction with hydrogen peroxide or ascorbic acid, indicating that hktE is the only functional hydroperoxidase gene present in these two strains of H. influenzae. Exponential-phase cultures of hktE mutants are 8- to 25-fold more sensitive to hydrogen peroxide than the wild type. Using the infant rat model, hktE mutants of strain Eagan b+ were 2.3-fold less virulent than the wild type following intraperitoneal inoculation (P = 0.07). When administered intranasally, the Eagan b+ hktE mutant produced wild-type levels of bacteremia and nasal colonization. The results of this study show that while the H. influenzae hktE gene is important for survival in the presence of peroxides, deletion of the gene produces only a modest reduction in ability to cause lethal sepsis following parenteral challenge and no change in ability to colonize following intranasal inoculation in the infant rat model of infection.

Oxidative damage to mitochondrial DNA is increased in Alzheimer's disease

Oxidative damage to DNA may play a role in both normal aging and in neurodegenerative diseases. We examined whether Alzheimer's disease (AD) is associated with increased oxidative damage to nDNA and mtDNA in postmortem brain tissue. We measured the oxidized nucleoside, 8-hydroxy-2'-deoxyguanosine (OH8dG), in DNA isolated from three regions of cerebral cortex and cerebellum in 13 AD and 13 age-matched controls. There was a significant threefold increase in the amount of OH8dG in mtDNA in parietal cortex of AD patients compared with controls. In the entire group of samples there was a small significant increase in oxidative damage to nDNA and a highly significant threefold increase in oxidative damage to mtDNA in AD compared with age-matched controls. These results confirm that mitochondrial DNA is particularly sensitive to oxidative damage, and they show that there is increased oxidative damage to DNA in AD, which may contribute to the neurodegenerative process.

The development of mitochondrial medicine

Primary defects in mitochondrial function are implicated in over 100 diseases, and the list continues to grow. Yet the first mitochondrial defect--a myopathy--was demonstrated only 35 years ago. The field's dramatic expansion reflects growth of knowledge in three areas: (i) characterization of mitochondrial structure and function, (ii) elucidation of the steps involved in mitochondrial biosynthesis, and (iii) discovery of specific mitochondrial DNA. Many mitochondrial diseases are accompanied by mutations in this DNA. Inheritance is by maternal transmission. The metabolic defects encompass the electron transport complexes, intermediates of the tricarboxylic acid cycle, and substrate transport. The clinical manifestations are protean, most often involving skeletal muscle and the central nervous system. In addition to being a primary cause of disease, mitochondrial DNA mutations and impaired oxidation have now been found to occur as secondary phenomena in aging as well as in age-related degenerative diseases such as Parkinson, Alzheimer, and Huntington diseases, amyotrophic lateral sclerosis and cardiomyopathies, atherosclerosis, and diabetes mellitus. Manifestations of both the primary and secondary mitochondrial diseases are thought to result from the production of oxygen free radicals. With increased understanding of the mechanisms underlying the mitochondrial dysfunctions has come the beginnings of therapeutic strategies, based mostly on the administration of antioxidants, replacement of cofactors, and provision of nutrients. At the present accelerating pace of development of what may be called mitochondrial medicine, much more is likely to be achieved within the next few years.

Formamidopyrimidine DNA glycosylase in the yeast Saccharomyces cerevisiae

A DNA glycosylase that excises, 2,6-diamino-4-hydroxy-5N-methylformamidopyrimidine (Fapy) from double stranded DNA has been purified 28,570-fold from the yeast Saccharomyces cerevisiae. Gel filtration chromatography shows that yeast Fapy DNA glycosylase has a molecular weight of about 40 kDa. The Fapy DNA glycosylase is active in the presence of EDTA, but is completely inhibited by 0.2 M KCl. Yeast Fapy DNA glycosylase does not excise N7-methylguanine, N3-methyladenine or uracil. A repair enzyme for 7,8-dihydro-8-oxoguanine (8-OxoG) co-purifies with the Fapy DNA glycosylase. This repair activity causes strand cleavage at the site of 8-OxoG in DNA duplexes. The highest rate of incision of the 8-OxoG-containing strand was observed for duplexes where 8-OxoG was opposite guanine. The mode of incision at 8-OxoG was not established yet. The results however suggest that the Fapy- and 8-OxoG-repair activities are associated with a single protein.

The role of ascorbate in antioxidant protection of biomembranes: interaction with vitamin E and coenzyme Q

One of the vital roles of ascorbic acid (vitamin C) is to act as an antioxidant to protect cellular components from free radical damage. Ascorbic acid has been shown to scavenge free radicals directly in the aqueous phases of cells and the circulatory system. Ascorbic acid has also been proven to protect membrane and other hydrophobic compartments from such damage by regenerating the antioxidant form of vitamin E. In addition, reduced coenzyme Q, also a resident of hydrophobic compartments, interacts with vitamin E to regenerate its antioxidant form. The mechanism of vitamin C antioxidant function, the myriad of pathologies resulting from its clinical deficiency, and the many health benefits it provides, are reviewed.

Effects of n-3 and n-6 fatty acids on the activities and expression of hepatic antioxidant enzymes in autoimmune-prone NZBxNZW F1 mice

Menhaden fish oil (FO) containing n-3 fatty acids dramatically extends the life span and delays the onset and progression of autoimmune disease in (NZBxNZW)F1 (B/W) female mice as compared to those fed corn oil (CO) rich in n-6 lipids. As an inefficient antioxidant defense system has been linked to autoimmune diseases, the present study was undertaken to determine whether the protective action of n-3 lipids is mediated through their antioxidant defense system. Weanling B/W mice were fed a nutritionally adequate, semipurified diet containing CO or krill oil (KO) or FO at 10% level (w/w) ad libitum until the mice were 6.5 months old. All diets contained the same level of vitamin E (21.5 mg/100 g diet). We compared the effects of feeding n-6 and n-3 lipids on survival, kidney disease, hepatic microsomal lipid composition, peroxidation, and on the activity and mRNA expression of the antioxidant enzymes catalase, glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) in 6.5-month-old B/W mice. The results showed that when compared to livers from CO-fed mice, livers from KO- and FO-fed mice showed: (i) significantly higher (P < 0.001) activities and expression of CAT, GSH-Px and SOD; (ii) significantly lower (P < 0.001) arachidonic acid (20:4n-6) and linoleic acid (18:2n-6) and higher (P < 0.001) eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) levels in hepatic microsomes; and (iii) significantly lower (P < 0.001) estimated peroxidation indices and thiobarbituric acid reactive substances generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity of the yeast rev3 delta antimutator and REV3 dependency of the mutator resulting from a defect (rad1 delta) in nucleotide excision repair

The yeast REV3 gene has been predicted to encode a DNA polymerase specializing in translesion synthesis. This polymerase likely participates in spontaneous mutagenesis, as rev3 mutants have an antimutator phenotype. Translesion synthesis also may be necessary for the mutator caused by a RAD1 (nucleotide excision repair) deletion (rad1 delta). To further examine the role of REV3 in spontaneous mutagenesis, we characterized SUP4-o mutations that arose spontaneously in strains having combinations of normal or mutant REV3 and RAD1 alleles. The largest fraction of the rev3 delta-dependent mutation rate decrease was observed for single base-pair substitutions and deletions, although the rates of all mutational classes detected in the RAD1 background were reduced by at least 30%. Interestingly, inactivation of REV3 was associated with a doubling of the number of sites at which the retrotransposon Ty inserted. rev3 delta also greatly diminished the magnitude of the rad1 delta mutator, but not to the rev3 delta antimutator level, implicating REV3-dependent and independent processes in the rad1 delta mutator effect. However, the specificity of the rev3 delta antimutator suggested that the same REV3-dependent processes gave rise to the majority of spontaneous mutations in the RAD1 and rad1 delta strains.

Vitamin E: molecular and biological function

There is a growing body of evidence implicating free radicals in a wide variety of medical diseases and conditions, especially the diseases of ageing, such as cancer and cardiovascular disease, which appear to be ultimate expressions of long-term, cumulative and sustained cellular damage. Vitamin E is an excellent lipid-soluble, chain-breaking antioxidant in the presence of other co-operative antioxidants such as vitamin C or ubiquinol, but it can act as a pro-oxidant in their absence. Epidemiological findings and animal studies support the belief that vitamin E is protective against cardiovascular disease and possibly cancer. The wide range of symptoms associated with vitamin E deficiency is consistent with a loss of antioxidant protection in those long-lived cells in which there is sufficient opportunity for accumulation of free radical damage. The cellular damage is proposed to arise from the generation of free radicals during normal aerobic metabolism. Some susceptible tissues may have enhanced levels of radicals that are produced, for example, by the action of cytochrome P-450 enzymes in steroidogenic tissues, or by the generation of NO in neural tissues.

Membrane ionic current photomodification by rose bengal and menadione: role of singlet oxygen

Photosensitized modification of ionic leak current and potassium current was studied in frog cardiac atrial cells using whole cell patch clamp techniques. Rose bengal (RB) and menadione (MQ) were used as photosensitizers. Separate photophysical studies of the photosensitizers in deuterium oxide solution demonstrated that MQ did not produce singlet oxygen as evidenced by the lack of luminescence at 1270 nm, whereas RB was an efficient singlet oxygen generator. Both photosensitizers sensitized block of potassium current in atrial cells, and both sensitized an increase of ionic leak current. However, when photosensitizer concentrations and illumination intensities were adjusted to match the rate of block of potassium current by the two photosensitizers, there were dramatic differences in leak current increase, both quantitatively and qualitatively. Menadione sensitized a much slower increase in leak current than did RB. Further, the leak current sensitized by MQ had a more positive reversal potential than that sensitized by RB, suggesting a less potassium-selective leak current pathway. The results suggest that, while the effects of singlet oxygen and non-singlet oxygen modification of cell membranes may be similar, there may also be significant differences in the resulting membrane permeabilities. The results also demonstrate that MQ and RB may be useful agents to study the role of singlet oxygen versus non-singlet oxygen modification of biological systems.

DNA alkylation repair limits spontaneous base substitution mutations in Escherichia coli

The Escherichia coli Ada and Ogt DNA methyltransferases (MTases) are known to transfer simple alkyl groups from O6-alkylguanine and O4-alkylthymine, directly restoring these alkylated DNA lesions to guanine and thymine. In addition to being exquisitely sensitive to the mutagenic effects of methylating agents, E. coli ada ogt null mutants display a higher spontaneous mutation rate than the wild type. Here, we determined which base substitution mutations are elevated in the MTase-deficient cells by monitoring the reversion of six mutated lacZ alleles that revert via each of the six possible base substitution mutations. During exponential growth, the spontaneous rate of G:C to A:T transitions and G:C to C:G transversions was elevated about fourfold in ada ogt double mutant versus wild-type E. coli. Furthermore, compared with the wild type, stationary populations of the MTase-deficient E. coli (under lactose selection) displayed increased G:C to A:T and A:T to G:C transitions (10- and 3-fold, respectively) and increased G:C to C:G, A:T to C:G, and A:T to T:A transversions (10-, 2.5-, and 1.7-fold, respectively). ada and ogt single mutants did not suffer elevated spontaneous mutation rates for any base substitution event, and the cloned ada and ogt genes each restored wild-type spontaneous mutation rates to the ada ogt MTase-deficient strains. We infer that both the Ada MTase and the Ogt MTase can repair the endogenously produced DNA lesions responsible for each of the five base substitution events that are elevated in MTase-deficient cells. Simple methylating and ethylating agents induced G:C to A:T and A:T to G:C transitions in these strains but did not significantly induce G:C to C:G, A:T to C:G, and A:T to T:A transversions. We deduce that S-adenosylmethionine (known to e a weak methylating agent) is not the only metabolite responsible for endogenous DNA alkylation and that at least some of the endogenous metabolites that cause O-alkyl DNA damage in E. coli are not simple methylating or ethylating agents.

Senescence-like growth arrest induced by hydrogen peroxide in human diploid fibroblast F65 cells

Human diploid fibroblast cells lose replicative potential after a certain number of population doublings. We use this experimental system to investigate the role of oxidative damage in cellular aging. Treating cells with H2O2 at < 300 microM did not affect the viability of the majority of cells when judged by morphology, trypan blue exclusion, and protein synthesis. However, the treatment caused a dose-dependent inhibition of DNA synthesis. After a 2-hr treatment with 200 microM H2O2, the cells failed to respond to a stimulus of serum, platelet-derived growth factor, basic fibroblast growth factor, or epidermal growth factor by synthesizing DNA, and the loss of response could not be recovered by 4 days. Subcultivation showed that, as in senescent cells, division of the treated cells was inhibited. The life-time cumulative growth curve showed that the loss of replication due to H2O2 treatment was cumulative and irreversible. The H2O2 treatment decreased the number of the population doublings in the rest of the life span by 35.3 +/- 10.3%. Enzymatic assays indicated that, like the cells in their senescent state, the treated cells were less able to activate ornithine decarboxylase and thymidine kinase. Furthermore, subcultivation after the H2O2 treatment showed that the cells developed the morphology of senescent cells. In conclusion, sublethal treatment of H2O2 "stunned" F65 cells and caused the cells to enter a state resembling senescence.

A peroxide/ascorbate-inducible catalase from Haemophilus influenzae is homologous to the Escherichia coli katE gene product

Bacterial catalases are induced by exposure to peroxide (e.g., Escherichia coli katG) or entry into stationary phase (e.g., E. coli katE). To study regulatory systems in Haemophilus influenzae, we complemented an E. coli rpoS mutant, which is unable to induce katE in stationary phase, with a plasmid library of H. influenzae Rd- chromosomal DNA. Nineteen complementing clones with a catalase-positive phenotype were obtained and characterized after screening about 10(5) transformants. All carried the same structural gene for an H. influenzae catalase. The DNA sequence of this gene, called hktE, encodes a 508-amino-acid polypeptide with strong homology to eukaryotic catalases and E. coli katE. However, hktE is regulated like E. coli katG, with catalase activity increasing 10-fold and hktE mRNA levels increasing 4-fold upon exposure to ascorbic acid, which serves to generate hydrogen peroxide. Mutations in the known global regulatory genes of H. influenzae--crp, cya, and sxy--do not affect the inducibility of hktE. The hktE gene maps to a 225-kb segment of the H. influenzae chromosome in a region encoding resistance to spectinomycin.

Free radicals and ocular disease

Ames, Shigenaga, and Hagen recently published a thorough review of the relationship between oxidants, antioxidants, and degenerative diseases of ageing. They point out that only 9% of Americans daily consume the two fruits and three vegetables recommended by the National Cancer Institute and the National Research Council/National Academy of Science. In addition to antioxidants, these foodstuffs contain many essential micronutrients. To date, specific recommendations for antioxidant supplementation have not been made by any governmental agency or professional association. A number of clinical, basic, and epidemiological studies have implicated free radical induced lipid peroxidation in various ocular disorders. It would seem prudent that those persons at greatest risk for these disorders take some precautions, which could include sunglasses that filter ultraviolet light; hats that shield the eyes from direct sunlight; and the ingestion of fruits, vegetables, and antioxidants.

Does cancer kill the individual and save the species?

I have argued that most germline mutations are due to endogenous process. I speculate that endogenous control of germline mutation serves an important biological function. A certain rate of mutation is required to generate sufficient variation for adaptation during evolutionary time. Sexual reproduction and recombination may serve to enhance variation, but ultimately new germline mutation is required to replenish variant alleles lost secondary to negative selection, genetic drift, and population bottlenecks. Unfortunately, the requisite mutation rates carry a terrible price: for each advantageous mutation, there are many disadvantageous ones. Thus, all mammals are plagued with substantial Mendelian and multifactorial disease. Consequently, the optimal mutation rate should be at a level just sufficient to maintain the requisite variation needed for adaptation. In this view, mechanisms for negative selection are necessary to keep the mutation rate in check. If a high germline mutation rate produces a high mutation rate in somatic tissues, cancer may be an important mediator of negative selection. The multiple mutations necessary to produce cancer serve to amplify relatively small differences in the mutation rate, thereby providing an efficient selection against individuals with germline mutations that result in a high mutation rate. This hypothesis can account for the general similarity of the longevity-corrected cancer incidence profile and the small but significant prevalence of cancer before and during the reproductive period. While this hypothesis must presently be viewed as speculative, it integrates certain previously disjointed observations and suggests an alternative to the general assumption that cancer represents a breakdown in normal physiology.

Protein restriction (PR) and caloric restriction (CR) compared: effects on DNA damage, carcinogenesis, and oxidative damage

Protein restriction (PR) and caloric restriction (CR) similarly impinge upon various physiological factors that can significantly inhibit the growth of DNA-damaged tissue and, therefore, carcinogenesis. Whether this effect is largely, or only in part, due to simple inhibition of body weight gain is examined. Among their many other health-improving effects, PR and CR delay the onset of puberty. It has been suggested that animals have developed mechanisms to cope with lean periods and that, when food is limited, resources are diverted from those physiological functions that offer no benefit for immediate survival (e.g., reproductive capacity) to thereby support an increase in the maintenance functions that prolong life. PR has also been shown to affect numerous other varied mechanisms that can affect carcinogenesis, including gene expression and metabolism of xenobiotics. The effects of PR on initiational and promotional growth of DNA-damaged tissue is also discussed. PR also seems to boost antioxidant defenses and inhibit the accumulation of oxidative damage (as does CR). Protein restricted animals have been shown to accumulate more calories, but develop fewer preneoplastic lesions and tumors than their high-protein counterparts. This observation seems quite counter to most ideas about dietary restrictions and CR. Despite the fact that both PR and CR induce many beneficial physiological effects in common, it is possible that PR is the more feasible option for human consideration. The levels of PR likely to improve health without negative side effects are discussed.