The role of mitochondrial function and cellular bioenergetics in ageing and disease

Abstract

Mitochondria constitute an important topic of biomedical enquiry (one paper in every 154 indexed in PubMed since 1998 is retrieved by the keyword 'mitochondria') because of widespread recognition of their importance in cell physiology and pathology. Mitochondrial dysfunction is widely implicated in ageing and in the diseases of ageing, through dysfunction in adenosine triphosphate (ATP) synthesis, Ca(2+) homeostasis, central metabolic pathways or radical production. Nonetheless, the mechanisms and regulation of superoxide and hydrogen peroxide formation by mitochondria remain poorly described. Measurement of the capacities of different sites of superoxide and hydrogen peroxide production in isolated skeletal muscle mitochondria show that the maximum capacities of sites in complexes I, II and III and in several associated redox enzymes greatly exceed the native rates observed in the absence of respiratory chain inhibitors. In vitro, the native rates and the relative importance of different sites both depend on the substrate being oxidized, with sites IQ, IIF, GPDH, IF and IIIQo each being important with particular substrates. The techniques involved in measuring rates from each site should become applicable to cell cultures and in vivo in the future.

Fig 1

Papers in biomedicine and papers on ‘mitochondria’ 1950–2011. (a) All papers in PubMed each year. (b) Papers in PubMed each year retrieved using the keyword ‘mitochondria’. (c) Mitochondrial papers as a percentage of all papers in PubMed each year. Search date October 2012.

Fig 2

Chemiosmotic coupling of oxidative phosphorylation in mitochondria. Electrons harvested from oxidizable substrates are passed through the respiratory chain in an exergonic process that drives proton pumping by respiratory complexes I, III and IV. The resulting electrochemical proton gradient across the mitochondrial inner membrane can be dissipated in two ways: (i) through the F_OF₁–ATP synthase, where relieving the proton-motive force drives ADP phosphorylation, and (ii) via proton leak pathways that do not generate ATP, but regulate physiological processes including nonshivering thermogenesis and perhaps glucose-stimulated insulin secretion and protection from oxidative damage. Proton leak pathways are structurally represented by ANT, which can mediate both basal and inducible proton conductance. The structures depicted are: complex I from Thermus thermophilus (PDB ID: 3M9S); complex II from porcine heart (PDB ID: 1ZOY); dimeric complex III from bovine heart (PDB ID: 1BGY); dimeric complex IV from bovine heart (PDB ID: 2OCC); F1c10 ATP synthase complex from Saccharomyces cerevisiae (PDB ID: 2XOK) and carboxyatractyloside-inhibited ANT from bovine heart (PDB ID: 1OKC). Reproduced from Divakaruni and Brand. ADP, adenosine diphosphate; ANT, adenine nucleotide translocase; ATP, adenosine triphosphate.

Fig 3

Hydrogen peroxide generation at different sites in isolated muscle mitochondria. Maximum capacities of sites defined using different combinations of substrates and inhibitors are in blue, actual overall rates during oxidation of glutamate plus malate in the absence of respiratory inhibitors are in red. All rates were either measured in the presence of 1-chloro-2,4-dinitrobenzene to greatly attenuate losses of hydrogen peroxide in the matrix by endogenous glutathione-linked peroxidases, or corrected to such measurements using the equations in Quinlan et al. and Treberg et al. St 4, state 4 (no ATP synthesis); st 3, state 3 (maximum ATP synthesis); OGDH, 2-oxoglutarate dehydrogenase; ETF, electron-transferring flavoprotein and ETF:Q oxidoreductase; DHODH, dihydroorotate dehydrogenase; GPDH, glycerol 3-phosphate dehydrogenase; glut + mal, glutamate plus malate; IF, IQ, IIF, IIIQo, see Table 2. Data are from unpublished observations and references , and . Values are means ± SEM (n ≥ 3).

Fig 4

The contributions of different sites to native rates of hydrogen peroxide production in mitochondria isolated from muscle during oxidation of different substrates in state 4. All rates were either measured in the presence of 1-chloro-2,4-dinitrobenzene to greatly attenuate losses of hydrogen peroxide in the matrix by endogenous glutathione-linked peroxidases, or corrected to such measurements using the equations in Quinlan et al. and Treberg et al. Open bars, observed total rates; coloured stacks of bars, calculated contributions of different sites to the observed rates, as indicated. Values are means ± SEM (n ≥ 3). Data are from unpublished observations and references and .