Julie St-Pierre

Superoxide activates mitochondrial uncoupling proteins.

Uncoupling protein 1 (UCP1) diverts energy from ATP synthesis to thermogenesis in the mitochondria of brown adipose tissue byxa0catalysing a regulated leak of protons across the inner membrane. The functions of its homologues, UCP2 and UCP3, in other tissues are debated. UCP2 and UCP3 are present at much lower abundance than UCP1, and the uncoupling with which they are associated is not significantly thermogenic. Mild uncoupling would, however, decrease the mitochondrial production of reactive oxygen species, which are important mediators of oxidative damage. Here we show that superoxide increases mitochondrial proton conductance through effects on UCP1, UCP2 and UCP3. Superoxide-induced uncoupling requires fatty acids and is inhibited by purine nucleotides. It correlates with the tissue expression of UCPs, appears in mitochondria from yeast expressing UCP1, and is absent in skeletal muscle mitochondria from UCP3 knockout mice. Our findings indicate that the interaction of superoxide with UCPs may be a mechanism for decreasing the concentrations of reactive oxygen species inside mitochondria.

AMP decreases the efficiency of skeletal-muscle mitochondria.

Mitochondrial proton leak in rat muscle is responsible for approx. 15% of the standard metabolic rate, so its modulation could be important in regulating metabolic efficiency. We report in the present paper that physiological concentrations of AMP (K(0.5)=80 microM) increase the resting respiration rate and double the proton conductance of rat skeletal-muscle mitochondria. This effect is specific for AMP. AMP also doubles proton conductance in skeletal-muscle mitochondria from an ectotherm (the frog Rana temporaria), suggesting that AMP activation is not primarily for thermogenesis. AMP activation in rat muscle mitochondria is unchanged when uncoupling protein-3 is doubled by starvation, indicating that this protein is not involved in the AMP effect. AMP activation is, however, abolished by inhibitors and substrates of the adenine nucleotide translocase (ANT), suggesting that this carrier (possibly the ANT1 isoform) mediates AMP activation. AMP activation of ANT could be important for physiological regulation of metabolic rate.

Mitochondrial Proton Conductance, Standard Metabolic Rate and Metabolic Depression

Proton cycling across the mitochondrial inner membrane makes up a significant proportion (20–30%) of Standard Metabolic Rate (SMR) in rats. If proton cycling is equally important in other animals, those that metabolically depress to 25% or less of SMR have a problem: either their entire energy budget will be wasted by proton cycling, or they have to suppress the leak of protons across the mitochondrial membrane. Muscle mitochondria from metabolically depressed, hypoxic overwintering frogs (Rana temporaria) do have decreased proton leak rate. This is achieved not by decreasing the proton conductance of the membrane, but by lowering the protonmotive force (the driving force for the leak). Protonmotive force is lowered aerobically by restricting electron supply, and in anoxia by restricting mitochondrial ATPase activity. There is also a temperature component to the physiological depression of overwintering frogs. The proton conductance of frog muscle mitochondria decreases steeply with temperature. Frog hepatocytes also respond strongly to temperature, and decrease their proton cycling in parallel to other reactions, so preserving metabolic efficiency at different temperatures. Hepatopancreas cells from the land snail (Helix aspersa) provide a good new model system to study biochemical mechanisms of depression without the complications of temperature change. Cells from aestivating animals show a persistent metabolic depression to 30% of controls, partly through intrinsic effects and partly through the extrinsic effects of pH and pO2. In depressed cells, proton cycling decreases at least as much as cellular respiration rate. These results using frogs and snails show that mitochondrial proton cycling is strongly suppressed in metabolic depression, so that metabolic efficiency is maintained or even enhanced.

NO PAIN, NO GAIN?

![Figure][1] nnWouldnt it be nice to enjoy the benefits of exercise without the effort? For example, endurance physical training results in elevated muscle mitochondrial content for increased aerobic respiration and the transition of fast-twitch fibers into slower oxidative fibers. One step