Search

The overarching role of coenzyme Q(10) in gene regulation, bioenergy formation, cellular redox poise regulation, and hydrogen peroxide formation is presented. Coenzyme Q(10) has a central role acting as a prooxidant in the generation of H(2)O(2). Contrary to the dogma that superoxide and H(2)O(2) formation are highly deleterious to cell survival this premise is rejected. Data are discussed that continuous superoxide and hydrogen peroxide formation are essential for normal cell function and that they play a major role in subcellular redox state modulation. It is the prooxidant activity of the so-called antioxidants that may be responsible for previously claimed benefits for high doses of oxido-reduction nutritional supplements such as alpha lipoic acid and coenzyme Q(10). Oxygen-free radical formation is essential for the biological function and is not a direct causation of the mammalian aging process; aging is a multisystem stochastic process.

In this paper, we review two parts of our recent work on human skeletal muscle. The first part mainly describes changes occurring during aging, whereas the second part discusses the functions of coenzyme Q10 (CoQ10), particularly in relation to the aging process. During the lifetime of an individual, mtDNA undergoes a variety of mutation events and rearrangements. These mutations and their consequent bioenergenic decline, together with nuclear DNA damage, contribute to the reduced function of cells and organs, especially in postmitotic tissues. In skeletal muscle, this functional decline can be observed by means of changes with age in fiber type profile and the reduction in the number and size of the muscle fibers. In addition to the functions of coenzyme Q10 as an electron carrier in the respiratory chain and as an antioxidant, CoQ10 has been shown to regulate global gene expression in skeletal muscle. We hypothesize that this regulation is achieved via superoxide formation with H2O2 as a second messenger to the nucleus.

Several lines of evidence support the view that the bioenergetic function of the mitochondria in postmitotic tissue deteriorates during normal aging. Skeletal muscle is one such tissue that undergoes age-related fiber loss and atrophy and an age-associated rise in the number of cytochrome c oxidase (COX) deficient fibers. With such metabolic pressure placed on skeletal muscle it would be an obvious advantage to supplement the cellular requirement for energy by up-regulating glycolysis, and alternative pathway for energy synthesis. Analysis of rat skeletal muscle utilizing antibodies directed against key enzymes involved in glycolysis has provided evidence of an age-associated increase in the enzymes involved in glycolysis. Fructose-6-phosphate kinase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase protein levels appeared to increase in the soleus, gracilis, and quadriceps muscle from aged rats. The increase in the level of these proteins appeared to correlate to a corresponding decrease in the amount of cytochrome c oxidase protein measured in the same tissue. Together these results are interpreted to represent a general upregulation of glycolysis that occurs in response to the age-associated decrease in mitochondrial energy capacity. Mitochondrial DNA (mtDNA) damage and mutations may accumulate with advancing age until they reach a threshold level were they impinge on the bioenergy capacity of the cell or tissue. Evidence indicates that mtDNA from the skeletal muscle of both aged rats and humans not only undergoes changes at the nucleotide sequence level (mutations and DNA damage), but also undergoes modifications at the tertiary level to generate unique age-related conformational mtDNA species. One particular age-related conformational form was only detected in aged rat tissues with high demands on respiration, specifically in heart, kidney, soleus muscle, and, to a lesser extent, the quadriceps muscle. The age-related form was not detected in gracilis muscle which is predominantly dependent upon glycolysis with regard to its energy requirements. Finally, a comprehensive hypothesis is presented that features the stochastic nature of the mitochondrial system. The basis of the hypothesis is that a dynamic relationship exists between endogenous mutagen production, DNA repair, mtDNA turnover, and nuclear control of mtDNA copy number and that age-associated changes in the dynamics of this relationship lead to a loss of functional full-length mtDNA that eventually leads to bioenergy decline.

Mitochondria, according to the free radical theory of aging, are the major source of reactive oxygen species (ROS). The results, presented in this paper, question the role of reactive oxygen species in contributing significantly to the extent of mitochondrial bioenergy degradation of the tissues, which can be correlated with mtDNA rearrangements. We report here that mtDNA rearrangements, including deletions and duplications, in tissues from human aged subjects, occur in levels ranging from very low in liver, to considerable in cardiac muscle, to almost total in skeletal muscle. The extent of mtDNA rearrangements is correlated at both the individual tissue and cell level with cytochrome oxidase (COX) activity as the exemplifier of cellular bioenergy capacity. Thus, the ROS proposal in its simplest form as it affects mtDNA and mitochondrial electron transport system is not supported by the available data.

During the present century there has been a dramatic change in life expectancy in advanced societies, now exceeding 80 years. As distinct from life expectancy, life potential is said to be at least 120 years, so that the continuing increase in knowledge has the potential for further major changes in the survival of humans conceivably in the near future. This presentation will be concerned with one aspect of the development of biomedical advances related in part to a concept of an "age-related universality of bioenergetic disease," and its potential amelioration and proposed impact on age-related disease and lifestyle. Aging is a complex biological process associated with a progressive decline in the physiological and biochemical performance of individual tissues and organs, leading to age-associated disease and senescence. Consideration of the progressive accumulation of mitochondrial DNA mutation with age and the tissue/cellular bioenergy decline associated with the aging process has led us to the proposal of a "universality of bioenergetic disease" and the potential for a redox therapy for the condition. This concept envisages that a tissue-bioenergetic decline will be intrinsic to various diseases of the aged and thereby contribute to their pathology, in particular, heart failure, degenerative brain disease, muscle and vascular diseases, as well as other syndromes. The information and concepts embodied in this proposal will be reviewed under the following headings: (1) mitochondrial DNA deletion mutation in some tissue is very extensive and shows mosaicism; (2) age-associated tissue/cellular bioenergy mosaic closely corresponds to the mtDNA profile; (3) cellular bioenergy as a function of mitochondrial bioenergy, glycolysis, and plasma membrane oxidoreductase; (4) redox therapy for the reenergization of cells, tissues, and whole organs. A redox therapy based on coenzyme Q10 has demonstrated profound alteration in heart function of old rats; no significant effect was observed with young rats.

Previous theories of aging based on somatic mutation neglected mtDNA, which has a high propensity for mutational error. Knowledge of yeast mtDNA mutations and their functional effects, and of human mtDNA mutations identified in the mitochondrial cytopathies, provides for a concept of aging based on the cumulative effect of mutations affecting human mtDNA. An essential feature of this concept is heteroplasmy, representing mixtures of normal and mutant mtDNA at the cellular and mitochondrial level, resulting in a "tissue mosaic" of focal bioenergetic deficits. Direct evidence for the concept is provided by (i) focal loss of staining for mitochondrially encoded enzymes, such as cytochrome c oxidase, in tissues of aged individuals (humans and rats) and (ii) an age-related increase in deletional mutations in mtDNA demonstrable by application of the polymerase chain reaction to DNA templates from individuals of different ages.

Subunit 8 of yeast mitochondrial ATP synthase is a small hydrophobic component of the membrane-associated F0 sector. Structure/function relations in subunit 8 were studied by focusing on three structural domains: a highly conserved NH2-terminal region, a central hydrophobic region (previously suggested to be a transmembrane stem), and a COOH-terminal region bearing a conserved array of three positively charged residues. A combined approach was used, which encompasses site-directed mutagenesis, in vitro import and assembly tests, and an in vivo allotopic expression system (using host cells unable to synthesise subunit 8 in mitochondria). The results indicate that the NH2-terminal region of subunit 8 is involved functionally in the F0 sector. As the central hydrophobic region can functionally tolerate the introduction of multiple, positively charged residues (which abolishes the proteolipid solubility characteristics of the entire subunit), the role of this hydrophobic region as a transmembrane stem is brought into question. Each of the three positively charged residues toward the COOH-terminus of subunit 8 is required for the efficient assembly of this subunit into the F0 sector. Removal of the more proximal charged residues Arg37 or Arg42 has a more severe impact on subunit 8 assembly than does removal of the most distal residue Lys47 in terms of both in vitro import and assembly as well as the ability of the subunit 8 variant to function in mitochondrial ATP synthase in vivo.