The yeast sir2 gene and its orthologues in Drosophila and C. elegans have well-established roles in lifespan determination and response to caloric restriction. We have studied mice carrying two null alleles for SirT1, the mammalian orthologue of sir2, and found that these animals inefficiently utilize ingested food. These mice are hypermetabolic, contain inefficient liver mitochondria, and have elevated rates of lipid oxidation. When challenged with a 40% reduction in caloric intake, normal mice maintained their metabolic rate and increased their physical activity while the metabolic rate of SirT1-null mice dropped and their activity did not increase. Moreover, CR did not extend lifespan of SirT1-null mice. Thus, SirT1 is an important regulator of energy metabolism and, like its orthologues from simpler eukaryotes, the SirT1 protein appears to be required for a normal response to caloric restriction.

Oxidative stress in skeletal muscle is a hallmark of various pathophysiologic states that also feature increased reliance on long-chain fatty acid (LCFA) substrate, such as insulin resistance and exercise. However, little is known about the mechanistic basis of the LCFA-induced reactive oxygen species (ROS) burden in intact mitochondria, and elucidation of this mechanistic basis was the goal of this study. Specific aims were to determine the extent to which LCFA catabolism is associated with ROS production and to gain mechanistic insights into the associated ROS production. Because intermediates and byproducts of LCFA catabolism may interfere with antioxidant mechanisms, we predicted that ROS formation during LCFA catabolism reflects a complex process involving multiple sites of ROS production as well as modified mitochondrial function. Thus, we utilized several complementary approaches to probe the underlying mechanism(s). Using skeletal muscle mitochondria, our findings indicate that even a low supply of LCFA is associated with ROS formation in excess of that generated by NADH-linked substrates. Moreover, ROS production was evident across the physiologic range of membrane potential and was relatively insensitive to membrane potential changes. Determinations of topology and membrane potential as well as use of inhibitors revealed complex III and the electron transfer flavoprotein (ETF) and ETF-oxidoreductase, as likely sites of ROS production. Finally, ROS production was sensitive to matrix levels of LCFA catabolic intermediates, indicating that mitochondrial export of LCFA catabolic intermediates can play a role in determining ROS levels. Reactive oxygen species (ROS)2 are generated in mitochondria as a normal byproduct of aerobic metabolism; 0.2-2% of O 2 consumption is estimated to be lost as superoxide under normal conditions (1, 2). Within the mitochondrial matrix, a suite of enzymes manages the ROS load by converting ROS to less toxic species or by mitigating their formation. Nevertheless, mitochondria are significant sources of cellular ROS, and oxidative stress is a hallmark of various physiological and pathological states, including exercise, insulin resistance, and atherosclerosis (3-10).Inhibitor studies in mitochondria utilizing NADH-or FADH 2 -linked substrates suggest that complexes I and III of the electron transport chain (ETC) are predominant sites of superoxide production (11-13). Whether this is the case under physiologic conditions is not well appreciated. ROS generation by the ETC is critically dependent upon ETC redox state, such that ROS production is low until the inner membrane is significantly polarized and then rises steeply with small increments in membrane potential (14). However, a membrane potential-independent component of ROS production has been observed in brain mitochondria (15)(16)(17). Indeed, electrons may escape from sites other than the respiratory complexes, such as from the electron transfer flavoprotein (ETF) and ETF-ubiquinone oxidoreductase (ETF-QO) (18) or the ␣-ket...

Background: Mitochondrial proteins are controlled by glutaredoxin-2 (Grx2)-mediated deglutathionylation reactions. Results: Grx2 deficiency compromises cardiac mitochondrial functions leading to hypertrophy and fibrosis in male mice. This is associated with deregulated glutathionylation reactions and mitochondrial dysfunction. Conclusion: Through deglutathionylation, Grx2 controls mitochondrial oxidative phosphorylation in cardiac muscle. Significance: Deregulated glutathionylation in heart can have pathological consequences.

Background: Glutaredoxin-2 (Grx2) modulates reversible glutathionylation of mitochondrial proteins. Results: Grx2 glutathionylates and inhibits UCP3-mediated proton leak. Conclusion: Grx2 modulates UCP3 activity. Significance: Grx2 is required to control proton leak through UCP3 and mitochondrial metabolism.

Reduced glutathione (GSH) is the major determinant of redox balance in mitochondria and as such is fundamental in the control of cellular bioenergetics. GSH is also the most important nonprotein antioxidant molecule in cells. Surprisingly, the effect of redox environment has never been examined in skeletal muscle and brown adipose tissue (BAT), two tissues that have exceptional dynamic range and that are relevant to the development of obesity and related diseases. Here, we show that the redox environment plays crucial, yet divergent, roles in modulating mitochondrial bioenergetics in skeletal muscle and BAT. Skeletal muscle mitochondria were found to naturally have a highly reduced environment (GSH/GSSG≈46), and this was associated with fairly high (∼40%) rates of state 4 (nonphosphorylating) respiration and decreased reactive oxygen species (ROS) emission. The deglutathionylation of uncoupling protein 3 (UCP3) following an increase in the reductive potential of mitochondria results in a further increase in nonphosphorylating respiration (∼20% in situ). BAT mitochondria were found to have a much more oxidized status (GSH/GSSG≈13) and had basal reactive oxygen species emission that was higher (∼250% increase in ROS generation) than that in skeletal muscle mitochondria. When redox status was subsequently increased (i.e., more reduced), UCP1-mediated uncoupling was more sensitive to GDP inhibition. Surprisingly, BAT was found to be devoid of glutaredoxin-2 (Grx2) expression, while there was abundant expression in skeletal muscle. Taken together, these findings reveal the importance of redox environment in controlling bioenergetic functions in both tissues, and the highly unique characteristics of BAT in this regard.