Alexander G. Trubitsyn

*Institute of Biology and Soil Sciences, Far East Division Russian Academy of Sciences, Vladivostok, Russia* 

#### **1. Introduction**

62 Bioenergetics

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> The average lifespan of people in developed countries has tripled since ancient times while its maximum longevity (about 120 years) has remained invariable. The strategic goal of gerontology is to exceed this limit, i.e. to develop remedies which would allow the living of an indefinitely long life. However there have not been any significant advances in solving this problem so far. There is still no answer to even the fundamental question: what is the primary cause of degradation for all of an organism's functions (otherwise known as aging)? Actually, there are too many answers to this question: over 300 aging theories have been developed, and each of them provides a different response (Medvedev, 1990), although the majority of these theories now have only historical importance. Theories of aging are traditionally divided into two alternative groups. First, stochastic theories claim that there are no specific aging genes and that an organism's deterioration is the result of damaging factors. Second, and by way of contrast, programmed-aging theories assert that longevity is predetermined by a genetic program. Stochastic theories have dominated the discussion since gerontology became a branch of science, and the idea that aging is programmed has not yet received wide recognition, even though there is a lot of empirical evidence supporting it. There are several factors which impede the wide recognition of this idea. First, there is no evidence that longevity is under the control of natural selection; and second, there is no convincing mechanism the programmed of aging. Adherents of this view currently search for longevity genes in a practically blind or ad hoc fashion (Holzenberger et al., 2003; Kenyon, 2010). Many such genes have been found for various organisms, ranging from unicellular creatures to mammals, but it is still unclear what processes they control (Anisimov, 2003).

> There are several different theories which are currently under consideration and which are based on reliable, proven evidence: i) the free radical theory which claims that aging is caused by an increased damage rate in cell structures due to an increased generation-rate of reactive oxygen species (ROS) by their own mitochondria; ii) the protein error theory which states that the primary cause is the age-dependent retardation of the protein synthesis rate; iii) the replicative senescence theory which argues that an age-dependent organism's senility is caused by the limitation of cell proliferation. There is also reliable evidence in support of other theories which are not as popular, for instance the immunological theory and several versions of neuroendocrinal theories.

Bioenergetics Theory of Aging 65

When the free radical theory of aging appeared, it stirred up a brisk discussion which continues to this day. Empirical data has shown that there is no appreciable loss in the respiratory chain's functions during aging (Barrientos et al., 1996; Rasmussen et al., 2003). This is also supported by experiments indirectly related to the electron transport chain. For example, research into intra-cellular organelle transfers has shown that the mitochondria of old donors recover their functional activity completely when transferred to ρ0 HeLa cells (HeLa cells free of mtDNA) (Hayashi et al., 1994; Isobe et al., 1998). Mitochondrial dysfunctions are also eliminated when HeLa cell nuclei are transferred into the cells of old donors (Isobe et al., 1997). The authors concluded that nuclear factors are responsible for age-related mitochondrial deficiency. In addition, the conclusion that the age-dependent accumulation of mtDNA mutations is modulated by the nuclear genome was also made by Yao et al. (2007). The discussion has become especially vigorous over the last decade. On the one hand, based on this theory, it was claimed that "aging is no longer an unsolved problem in biology" (Hayflick, 2007; Holliday, 2006). On the other hand, R.M. Howes (2006) has declared that the "overly exuberant and exaggerated past expectations and claims of the free-radical theory have been quieted by extensive randomised, double-blind, controlled human studies. A half century of data demonstrates its lack of predictability and it has not been validated by the scientific method. Widespread use of antioxidants has failed to quell the current pandemic of cancer, diabetes, and cardiovascular disease or [even] to stop or reverse the aging process." His position is also supported by G. Bjelakovic et al. (2008) who have collected a great deal of data to show that antioxidants neither result in the beneficial effects expected nor do they increase life expectancy (in the best-case scenario). Gems & Doonan (2009) asked a question in a recent review: "Is the theory really dead, or does it just need to be modified?" Actually, there is more than ample evidence against the aging

mechanism postulated by this theory than there is evidence in support of it.

Mitochondria generate about 90 percent of the energy in any eukaryotic cell. Therefore, only the mitochondrial bioenergetics machine will be considered here. Any energy system can be quantitatively described by its propellant power (F) and by its effect (A): F = kA: this is the force and the work in mechanical engineering (k is the friction) and the electromotive force and current in electrical engineering (k is the resistance). It is the free-energy change (Gibbs energy, ΔG) and current in chemical thermodynamics (in bioenergetics in particular). Such terms as the bioenergetics level and the level of energy production are used in bioenergetics to express the propellant power. To make it clear, let us recall that the ΔG of macroergic (high-energy) coenzymes that function in the bioenergetics machine (ATP, NAD, NADP, GSH, etc.) is determined by the value of their concentrations ratio of the reduced form to the oxidised one and by the temperature. For ATP, for instance, ΔG = ΔG0 - RT ln[ATP]/[ADP][Pi], where ΔG0 is the standard Gibbs energy that is measured with everything at 1 molar concentration: [ATP] = [ADP] = [Pi] = 1M; R is the gas constant; T is the absolute temperature. The more negative Gibbs energy there is, the higher the energy potential the bioenergetics machine generates. As follows from the above expression for ΔG, the concentrations ratio of the reduced to oxidised forms of macroergic coenzymes ([ATP]/[ADP], [NADH]/[NAD+], etc.) is the only variable which determines the energy

**2.2 Schema of the mitochondrial bioenergetics machine** 

potential for warm-blooded animals.

The goal of this report is: (1) to show that despite the beliefs of the supporters of the stochastic theories, longevity is controlled by natural selection, i.e. specific aging genes exist; (2) such genes program a lowering of the bioenergetics level (degradation of Gibbs energy, ΔG). In turn, such degradation results in an age-dependent increase in the ROS generation rate, a decrease in the protein synthesis rate, and a limitation of cell division. These three phenomena form the basis for a large number of secondary destructive processes which result in the degradation of all physiological organisms' functions, i.e. the causes of aging. The very idea that bioenergetics exerts an impact upon aging is not a novelty. Hasty and Vijg (2002) have recently stated in theory that proper energy-saving could support a living system indefinitely. B.N. Ames (2004) has remarked that mitochondrial bioenergetics supports the metabolism's cell processes and that its attenuation can result in the agedependent degradation of all of an organism's physiological functions. And indeed, life as a phenomenon is characterised by a number of physical and chemical processes driven by the power of the bioenergetics machine. A gradual decrease in bioenergetics level can cause the degradation of all vital processes. However they also believe that the cause of agedependent bioenergetics attenuation is to be identified with the mechanism postulated by the free-radical theory. The following fact seems to reject the assumption of the direct programming of bioenergetics attenuation: one of main bioenergetics parameters is the mitochondrial membrane potential ∆ψ. In vitro tests have shown that the superoxide (O2 •−) generation rate in the electron transport chain decreases as ∆ψ decreases. Consequently, in the process of bioenergetics attenuation the ROS level should decrease, but the tests show its increase in all tissues. And only that version of the vicious cycle brought forward by the free-radical theory can explain this paradox. Another mechanism which we have already suggested explains the increase in the number of reactive oxygen species during programmed bioenergetics attenuation (Trubitsyn, 2006). The bioenergetics mechanism of aging under consideration represents the integration of several of the author's articles published earlier (Trubitsyn, 2006, 2006a, 2009, 2010, 2011).
