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PubMed2002

Mitochondrial function and dysfunction in the cell: its relevance to aging and aging-related disease.

Mitochondria plays a complex multi-factorial role in the cell. In addition to their primary role in ATP generation, the organelles sequester calcium and both generate and detoxify reactive oxygen species. All these functions are intimately inter-linked through the central bioenergetic parameter of the proton electrochemical gradient across the inner mitochondrial membrane. Subtle changes in respiratory chain capacity, substrate supply, glutathione levels, cytoplasmic calcium and membrane potential occur in aging and in conditions predisposing towards neurodegenerative disease. These interactions are incompletely understood and in this review I present an overview of some of the current research in this area, and its possible relevance to aging and aging-related disease.

Aging↗
PubMed1985

Effect of alkyl side chain variation on the electron-transfer activity of ubiquinone derivatives.

The effect of the alkyl side chain of the ubiquinone molecule on the electron-transfer activity of ubiquinone in mitochondrial succinate-cytochrome c reductase is studied by using synthetic ubiquinone derivatives that possess the basic ubiquinone structure of 2,3-dimethoxy-5-methyl-1,4-benzoquinone with different alkyl side chains at the 6-position. The alkyl side chains vary in chain length, degree of saturation, and location of double bonds. When a ubiquinone derivative is used as an electron acceptor for succinate-ubiquinone reductase, an alkyl side chain of six carbons is needed to obtain the maximum activity. However, when it serves as an electron donor for ubiquinol-cytochrome c reductase or as a mediator in succinate-cytochrome c reductase, an alkyl side chain of 10 carbons gives maximal efficiency. Introduction of one or two isolated double bonds into the alkyl side chain of the ubiquinone molecule has little effect on electron-transfer activity. However, a conjugated double bond system in the alkyl side chain drastically reduces electron-transfer efficiency. The effect of the conjugated double bond system on the electron-transferring efficiency of ubiquinone depends on its location in the alkyl side chain. When location is far from the benzoquinone ring, the effect is minimal. These observations together with the results obtained from photoaffinity-labeling studies lead us to conclude that flexibility in the portion of the alkyl side chain immediately adjacent to the benzoquinone ring is required for the electron-transfer activity of ubiquinone.

Alkylation↗
PubMed1986

Spin-label electron paramagnetic resonance and differential scanning calorimetry studies of the interaction between mitochondrial succinate-ubiquinone and ubiquinol-cytochrome c reductases.

The interaction between succinate-ubiquinone and ubiquinol-cytochrome c reductases in the purified, dispersed state and in embedded phospholipid vesicles was studied by differential scanning calorimetry and by electron paramagnetic resonance (EPR). When the purified, detergent-dispersed succinate-ubiquinone reductase, ubiquinol-cytochrome c reductase, and cytochrome c oxidase undergo thermodenaturation, they show an endothermic transition. However, when these isolated electron-transfer complexes are embedded in phospholipid vesicles, they undergo exothermodenaturation. The energy released could result from the collapse of the strained interaction between unsaturated fatty acyl groups of phospholipids and an exposed area of the complex formed by removal of interacting proteins. The exothermic enthalpy change of thermodenaturation of a protein-phospholipid vesicle containing both succinate-ubiquinone and ubiquinol-cytochrome c reductases was smaller than that of a mixture of protein-phospholipid vesicles formed from the individual electron-transfer complexes. This suggests specific interaction between succinate-ubiquinone reductase and ubiquinol-cytochrome c reductase in the membrane. This idea is supported by saturation transfer EPR studies showing that the rotational correlation time of spin-labeled ubiquinol-cytochrome c reductase is increased when mixed with succinate-ubiquinone reductase prior to embedding in phospholipid vesicles. These results indicate that succinate-ubiquinone reductase and ubiquinol-cytochrome c reductase are indeed present in the membrane as a supermacromolecular complex. No such supermacromolecular complex is detected between NADH-ubiquinone and ubiquinol-cytochrome c reductases or between succinate-ubiquinone and NADH-uniquinone reductases.

Animals↗
PubMed2001

Dramatic improvement in mitochondrial cardiomyopathy following treatment with idebenone.

Idebenone, a synthetic analogue of coenzyme Q10, has been shown to improve cardiac function in patients with Friedreich ataxia and a deficiency of respiratory chain complexes I-III. We describe a woman with severe combined right and left heart failure due to a mitochondrial cardiomyopathy. The patient underwent an endomyocardial biopsy as part of an evaluation for cardiac transplantation. It showed severely decreased respiratory complex activities dependent on CoQ, pointing to CoQ depletion. Following idebenone treatment there was a dramatic improvement in her clinical status with resolution of the heart failure.

Adult↗
PubMed1992

Global ischaemia induces a biphasic response of the mitochondrial respiratory chain. Anoxic pre-perfusion protects against ischaemic damage.

Studies of Langendorff-perfused rat hearts have revealed a biphasic response of the mitochondrial respiratory chain to global ischaemia. The initial effect is a 30-40% increase in the rate of glutamate/malate oxidation after 10 min of ischaemia, owing to an increase in the capacity for NADH oxidation. This effect is followed by a progressive decrease in these oxidative activities as the ischaemia is prolonged, apparently owing to damage to Complex I at a site subsequent to the NADH dehydrogenase component. This damage is exacerbated by reperfusion, which causes a further decrease in Complex I activity and also decreases the activities of the other complexes, most notably of Complex III. Perfusion for up to 1 h with anoxic buffer produced only the increase in NADH oxidase activity, and neither anoxia alone, nor anoxia and reperfusion, caused loss of Complex I activity. Perfusing for 3-10 min with anoxic buffer before 1 h of global ischaemia had a significant protective effect against the ischaemia-induced damage to Complex I.

Animals↗
PubMed1996

Altered mitochondrial function in fibroblasts containing MELAS or MERRF mitochondrial DNA mutations.

A number of human diseases are caused by inherited mitochondrial DNA mutations. Two of these diseases, MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and MERRF (myoclonic epilepsy and ragged-red fibres), are commonly caused by point mutations to tRNA genes encoded by mitochondrial DNA. Here we report on how these mutations affect mitochondrial function in primary fibroblast cultures established from a MELAS patient containing an A to G mutation at nucleotide 3243 in the tRNA(Leu(UUR) gene and a MERRF patient containing an A to G mutation at nucleotide 8344 in the tRNA(Lys) gene. Both mitochondrial membrane potential and respiration rate were significantly decreased in digitonin-permeabilized MELAS and MERRF fibroblasts respiring on glutamate/malate. A similar decrease in mitochondrial membrane potential was found in intact MELAS and MERRF fibroblasts. The mitochondrial content of these cells, estimated by stereological analysis of electron micrographs and from measurement of mitochondrial marker enzymes, was similar in control, MELAS and MERRF cells. Therefore, in cultured fibroblasts, mutation of mitochondrial tRNA genes leads to assembly of bioenergetically incompetent mitochondria, not to an alteration in their amount. However, the cell volume occupied by secondary lysosomes and residual bodies in the MELAS and MERRF cells was greater than in control cells, suggesting increased mitochondrial degradation in these cells. In addition, fibroblasts containing mitochondrial DNA mutations were 3-4-fold larger than control fibroblasts. The implications of these findings for the pathology of mitochondrial diseases are discussed.

Cells, Cultured↗
PubMed1999

Glutamate neurotoxicity in rat cerebellar granule cells involves cytochrome c release from mitochondria and mitochondrial shuttle impairment.

To gain some insight into the mechanism by which glutamate neurotoxicity takes place in cerebellar granule cells, two steps of glucose oxidation were investigated: the electron flow via respiratory chain from certain substrates to oxygen and the transfer of extramitochondrial reducing equivalents via the mitochondrial shuttles. However, cytochrome c release from intact mitochondria was found to occur in glutamate-treated cells as detected photometrically in the supernatant of the cell homogenate suspension. As a result of cytochrome c release, an increase of the oxidation of externally added NADH was found, probably occurring via the NADH-b5 oxidoreductase of the outer mitochondrial membrane. When the two mitochondrial shuttles glycerol 3-phosphate/dihydroxyacetone phosphate and malate/oxaloacetate, devoted to oxidizing externally added NADH, were reconstructed, both were found to be impaired under glutamate neurotoxicity. Consistent early activation in two NADH oxidizing mechanisms, i.e., lactate production and plasma membrane NADH oxidoreductase activity, was found in glutamate-treated cells. In spite of this, the increase in the cell NADH fluorescence was found to be time-dependent, an index of the progressive damage of the cell.

Animals↗
PubMed2001

Thyroid hormone regulates oxidative phosphorylation in the cerebral cortex and striatum of neonatal rats.

We have previously shown that thyroid hormone (T(3)) regulates mitochondrial gene expression, morphology and transmembrane potential in the developing brain. Here, we have analysed the effect of thyroid hormone on mitochondrial function in different brain regions. For this purpose we have determined, in control, hypothyroid and T(3)-treated hypothyroid neonatal rats, the rate of oxidative phosphorylation in isolated mitochondria and the activity of the respiratory complexes in tissue homogenates. Our results showed a decrease in oxidative phosphorylation rate (only in the presence of NADH-generating substrates) and mitochondrial complexes I and III activity in the cerebral cortex and striatum of hypothyroid neonates, but not in the other areas analysed (hippocampus, cerebellum, thalamus, mid brain and brain stem). In parallel with mitochondrial activity, the levels of mitochondrially encoded transcripts were decreased only in the cerebral cortex and striatum of hypothyroid rats. The administration of T(3) corrected all these parameters. In summary, this study showed a down-regulation of mitochondrial gene expression accompanied by a decrease in mitochondrial activity in the cerebral cortex and striatum of developing hypothyroid neonatal rats.

Adenosine Triphosphatases↗