Biomedical subjects

M D Brand

Publications and source records attributed to M D Brand.

At least 19 recordsLinked to original sources

PubMed2005

The efficiency and plasticity of mitochondrial energy transduction.

Since it was first realized that biological energy transduction involves oxygen and ATP, opinions about the amount of ATP made per oxygen consumed have continually evolved. The coupling efficiency is crucial because it constrains mechanistic models of the electron-transport chain and ATP synthase, and underpins the physiology and ecology of how organisms prosper in a thermodynamically hostile environment. Mechanistically, we have a good model of proton pumping by complex III of the electron-transport chain and a reasonable understanding of complex IV and the ATP synthase, but remain ignorant about complex I. Energy transduction is plastic: coupling efficiency can vary. Whether this occurs physiologically by molecular slipping in the proton pumps remains controversial. However, the membrane clearly leaks protons, decreasing the energy funnelled into ATP synthesis. Up to 20% of the basal metabolic rate may be used to drive this basal leak. In addition, UCP1 (uncoupling protein 1) is used in specialized tissues to uncouple oxidative phosphorylation, causing adaptive thermogenesis. Other UCPs can also uncouple, but are tightly regulated; they may function to decrease coupling efficiency and so attenuate mitochondrial radical production. UCPs may also integrate inputs from different fuels in pancreatic beta-cells and modulate insulin secretion. They are exciting potential targets for treatment of obesity, cachexia, aging and diabetes.

Adenosine Triphosphate↗

PubMed2004

Respiration rate of hepatocytes varies with body mass in birds.

Hepatocytes were isolated from eight species of birds ranging from 13 g zebra finches to 35 kg emus. This represents a 2800-fold range of body mass (Mb). Liver mass (g) was allometrically related to species body mass by the equation: liver mass=19.6 x Mb(0.91). There was a significant allometric decline in hepatocyte respiration rate (HRR; nmol O2 mg(-1) dry mass min(-1)) with species body mass (kg) described by the relationship: HRR=5.27 x Mb(-0.10). The proportions of hepatocyte oxygen consumption devoted to (i) mitochondrial ATP production, (ii) mitochondrial proton leak and (iii) non-mitochondrial processes were estimated by using excess amounts of appropriate inhibitors. It was found that although hepatocyte respiration rate varied with body mass in birds, these processes constitute a relatively constant proportion of hepatocyte metabolic rate irrespective of the size of the bird species. The respective percentages were 54%, 21% and 25%. The portion of hepatocyte respiration devoted to ATP production for use by the sodium pump was estimated and found to be a relatively constant 24% of hepatocyte respiration and 45% of mitochondrial ATP production in different-sized bird species. These results are discussed in the context of competing theories to explain the metabolism-body size allometry, and are found to support the 'allometric cascade' model.

Adenosine Triphosphate↗

PubMed2003

Mitochondrial matrix reactive oxygen species production is very sensitive to mild uncoupling.

Mitochondria produce ROS (reactive oxygen species) as a by-product of aerobic respiration. Several studies in mammals and birds suggest that the most physiologically relevant ROS production is from complex I following reverse electron flow, and is highly sensitive to membrane potential. A study of Drosophila mitochondria respiring glycerol 3-phosphate revealed that membrane potential-sensitive ROS production from complex I following reverse electron flow was on the matrix side of the inner membrane. A 10 mV decrease in membrane potential was enough to abolish around 70% of the ROS produced by complex I under these conditions. Another important ROS generator in this model, glycerol-3-phosphate dehydrogenase, produced ROS mostly to the cytosolic side; this ROS production was totally insensitive to a small decrease in membrane potential (10 mV). Thus mild uncoupling may be particularly significant for ROS production from complex I on the matrix side of the mitochondrial inner membrane.

Animals↗

PubMed2003

Extracting meaning from microarray data.

Gene expression is complex: many mRNAs change in abundance in response to a new condition. But while some of these expression changes may be direct, many may be downstream, indirect effects. One of the major problems of microarray data analysis is distinguishing between these changes. Some of the most common methods of analysis are discussed, in the context of their ability to distinguish between direct and indirect expression changes. The application of modular control analysis to microarray data in order to partition and quantify the importance of mRNA clusters in mediating responses is described.

Cluster Analysis↗

PubMed2002

Proton leak in hepatocytes and liver mitochondria from archosaurs (crocodiles) and allometric relationships for ectotherms.

It has previously been shown that mitochondrial proton conductance decreases with increasing body mass in mammals and is lower in a 250-g lizard than the laboratory rat. To examine whether mitochondrial proton conductance is extremely low in very large reptiles, hepatocytes and mitochondria were prepared from saltwater crocodiles ( Crocodylus porosus) and freshwater crocodiles ( Crocodylus johnstoni). Respiration rates of hepatocytes and liver mitochondria were measured at 37 degrees C and compared with values obtained for rat or previously measured for other species. Respiration rates of hepatocytes from either species of crocodile were similar to those reported for lizards and approximately one fifth of the rates measured using cells from mammals (rat and sheep). Ten-to-thirty percent of crocodile hepatocyte respiration was used to drive mitochondrial proton leak, similar to the proportion in other species. Respiration rates of crocodile liver mitochondria were similar to those of mammalian species. Proton leak rate in isolated liver mitochondria was measured as a function of membrane potential. Contrary to our prediction, the mitochondrial proton conductance of liver mitochondria from crocodiles was greater than that of liver mitochondria from lizards and was similar to that of rats. The acyl composition of liver mitochondrial phospholipids from the crocodiles was more similar to that in mitochondria from rats than in mitochondria from lizards. The relatively high mitochondrial proton conductance was associated with a relatively small liver, which seems to be characteristic of crocodilians. Comparison of data from a number of diverse ectothermic species suggested that hepatocyte respiration rate may decrease with body mass, with an allometric exponent of about -0.2, similar to the exponent in mammalian hepatocytes. However, unlike mammals, liver mitochondrial proton conductance in ectotherms showed no allometric relationship with body size.

Alligators and Crocodiles↗

PubMed2001

A mitochondrial uncoupling artifact can be caused by expression of uncoupling protein 1 in yeast.

Uncoupling protein 1 (UCP1) from mouse was expressed in yeast and the specific (GDP-inhibitable) and artifactual (GDP-insensitive) effects on mitochondrial uncoupling were assessed. UCP1 provides a GDP-inhibitable model system to help interpret the uncoupling effects of high expression in yeast of other members of the mitochondrial carrier protein family, such as the UCP1 homologues UCP2 and UCP3. Yeast expressing UCP1 at modest levels (approx. 1 microg/mg of mitochondrial protein) showed no growth defect, normal rates of chemically uncoupled respiration and an increased non-phosphorylating proton conductance that was completely GDP-sensitive. The catalytic-centre activity of UCP1 in these yeast mitochondria was similar to that in mammalian brown-adipose-tissue mitochondria. However, yeast expressing UCP1 at higher levels (approx. 11 microg/mg of mitochondrial protein) showed a growth defect. Their mitochondria had depressed chemically uncoupled respiration rates and an increased proton conductance that was partly GDP-insensitive. Thus, although UCP1 shows native behaviour at modest levels of expression in yeast, higher levels (or rates) of expression can lead to an uncoupling that is not a physiological property of the native protein and is therefore artifactual. This observation might be important in the interpretation of results from experiments in which the functions of UCP1 homologues are verified by their ability to uncouple yeast mitochondria.

Animals↗

PubMed2001

Mitochondrial proton leak and the uncoupling protein 1 homologues.

Mitochondrial proton leak is the largest single contributor to the standard metabolic rate (SMR) of a rat, accounting for about 20% of SMR. Yet the mechanisms by which proton leak occurs are incompletely understood. The available evidence suggests that both phospholipids and proteins in the mitochondrial inner membrane are important determinants of proton conductance. The uncoupling protein 1 homologues (e.g. UCP2, UCP3) may play a role in mediating proton leak, but it is unlikely they account for all of the observed proton conductance. Experimental data regarding the functions of these proteins include important ambiguities and contradictions which must be addressed before their function can be confirmed. The physiological role of the proton leak, and of the uncoupling protein 1 homologues, remains similarly unclear.

Amino Acid Sequence↗

PubMed2001

Physiological levels of mammalian uncoupling protein 2 do not uncouple yeast mitochondria.

We assessed the ability of human uncoupling protein 2 (UCP2) to uncouple mitochondrial oxidative phosphorylation when expressed in yeast at physiological and supraphysiological levels. We used three different inducible UCP2 expression constructs to achieve mitochondrial UCP2 expression levels in yeast of 33, 283, and 4100 ng of UCP2/mg of mitochondrial protein. Yeast mitochondria expressing UCP2 at 33 or 283 ng/mg showed no increase in proton conductance, even in the presence of various putative effectors, including palmitate and all-trans-retinoic acid. Only when UCP2 expression in yeast mitochondria was increased to 4 microg/mg, more than an order of magnitude greater than the highest known physiological concentration, was proton conductance increased. This increased proton conductance was not abolished by GDP. At this high level of UCP2 expression, an inhibition of substrate oxidation was observed, which cannot be readily explained by an uncoupling activity of UCP2. Quantitatively, even the uncoupling seen at 4 microgram/mg was insufficient to account for the basal proton conductance of mammalian mitochondria. These observations suggest that uncoupling of yeast mitochondria by UCP2 is an overexpression artifact leading to compromised mitochondrial integrity.

Gene Expression Regulation↗

PubMed2001

Signaling takes a breath--new quantitative perspectives on bioenergetics and signal transduction.

Immune cells are in constant need of energy for both basic housekeeping and specific immune functions. Increased energy demand during lymphocyte stimulation is coordinated by signal transduction pathways. This review explores the interface between lymphocyte signaling and energy metabolism. In particular, it discusses recent work that allows weighing signaling routes with respect to their role in the regulation of energy metabolism during lymphocyte activation.

Adenosine Triphosphate↗

PubMed2001

Coenzyme Q induces GDP-sensitive proton conductance in kidney mitochondria.

Addition of coenzyme Q(10) (CoQ) at low concentration (29 nmol/mg of protein) to kidney but not liver mitochondria resulted in an increase in proton conductance. This uncoupling activity required fatty acid and was completely inhibited by GDP. CoQ activated when it was likely to be reduced but not when it was likely to become oxidized. However, the redox state of endogenous CoQ did not affect mitochondrial proton conductance. Stimulation by CoQ was not inhibited by cyclosporin A, carboxyatractylate, bongkrekate and catalase but could be reversed by superoxide dismutase. We conclude that CoQ acted in mitochondria through production of superoxide, which mediated uncoupling, probably by acting through uncoupling protein 2.

Animals↗

PubMed2001

Mitochondrial uncoupling as a target for drug development for the treatment of obesity.

Mitochondrial proton cycling is responsible for a significant proportion of basal or standard metabolic rate, so further uncoupling of mitochondria may be a good way to increase energy expenditure and represents a good pharmacological target for the treatment of obesity. Uncoupling by 2,4-dinitrophenol has been used in this way in the past with notable success, and some of the effects of thyroid hormone treatment to induce weight loss may also be due to uncoupling. Diet can alter the pattern of phospholipid fatty acyl groups in the mitochondrial membrane, and this may be a route to uncoupling in vivo. Energy expenditure can be increased by stimulating the activity of uncoupling protein 1 (UCP1) in brown adipocytes either directly or through beta 3-adrenoceptor agonists. UCP2 in a number of tissues, UCP3 in skeletal muscle and the adenine nucleotide translocase have also been proposed as possible drug targets. Specific uncoupling of muscle or brown adipocyte mitochondria remains an attractive target for the development of antiobesity drugs.

Anti-Obesity Agents↗

PubMed2001

Radiation-associated valvular heart disease in Hodgkin's disease is associated with characteristic thickening and fibrosis of the aortic-mitral curtain.

Radiation-associated valvular dysfunction is characterized by variable aortic and mitral valve thickening. A review of three patients assessed echocardiographically revealed that radiation-associated valvular dysfunction after radiation treatment for Hodgkin's disease may be characterized by a unique and consistent pattern of thickening of the aortic and mitral valves involving the aortic-mitral curtain.

Aortic Valve↗

PubMed2000

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.

Adenosine Monophosphate↗

PubMed2000

Mice overexpressing human uncoupling protein-3 in skeletal muscle are hyperphagic and lean.

Uncoupling protein-3 (UCP-3) is a recently identified member of the mitochondrial transporter superfamily that is expressed predominantly in skeletal muscle. However, its close relative UCP-1 is expressed exclusively in brown adipose tissue, a tissue whose main function is fat combustion and thermogenesis. Studies on the expression of UCP-3 in animals and humans in different physiological situations support a role for UCP-3 in energy balance and lipid metabolism. However, direct evidence for these roles is lacking. Here we describe the creation of transgenic mice that overexpress human UCP-3 in skeletal muscle. These mice are hyperphagic but weigh less than their wild-type littermates. Magnetic resonance imaging shows a striking reduction in adipose tissue mass. The mice also exhibit lower fasting plasma glucose and insulin levels and an increased glucose clearance rate. This provides evidence that skeletal muscle UCP-3 has the potential to influence metabolic rate and glucose homeostasis in the whole animal.

Adipose Tissue↗

PubMed2000

Mitochondria as ATP consumers: cellular treason in anoxia.

In anoxia, mitochondria change from being ATP producers to potentially powerful ATP consumers. This change occurs, because the mitochondrial F(1)F(0)-ATPase begins to hydrolyze ATP to avoid the collapse of the proton motive force. Species that can survive prolonged periods of O(2) lack must limit such ATP use; otherwise, this process would dominate glycolytic metabolism and threaten ATP delivery to essential ATP-consuming processes of the cell (e.g., ion-motive ATPases). There are two ways to limit ATP hydrolysis by the F(1)F(0)-ATPase, namely (i) reduction of the proton conductance of the mitochondrial inner membrane and (ii) inhibition of the enzyme. We assessed these two possibilities by using intact mitochondria isolated from the skeletal muscle of anoxia-tolerant frogs. Our results show that proton conductance is unaltered between normoxia and anoxia. However, ATP use by the F(1)F(0)-ATPase is limited in anoxia by a profound inhibition of the enzyme. Even so, ATP use by the F(1)F(0)-ATPase might account for approximately 9% of the ATP turnover in anoxic frog skeletal muscle.

Adenosine Triphosphate↗

PubMed2000

Effects of magnesium and nucleotides on the proton conductance of rat skeletal-muscle mitochondria.

During oxidative phosphorylation most of the protons pumped out to the cytosol across the mitochondrial inner membrane return to the matrix through the ATP synthase, driving ATP synthesis. However, some of them leak back to the matrix through a proton-conductance pathway in the membrane. When the ATP synthase is inhibited with oligomycin and ATP is not being synthesized, all of the respiration is used to drive the proton leak. We report here that Mg(2+) inhibits the proton conductance in rat skeletal-muscle mitochondria. Addition of Mg(2+) inhibited both oligomycin-inhibited respiration and the proton conductance, while removal of Mg(2+) using EDTA activated these processes. The proton conductance was inhibited by more than 80% as free Mg(2+) was raised from 25 nM to 220 microM. Half-maximal inhibition occurred at about 1 microM free Mg(2+), which is close to the contaminating free Mg(2+) concentration in our incubations in the absence of added magnesium chelators. ATP, GTP, CTP, TTP or UTP at a concentration of 1 mM increased the oligomycin-inhibited respiration rate by about 50%. However, these NTP effects were abolished by addition of 2 mM Mg(2+) and any NTP-stimulated proton conductance was explained completely by chelation of endogenous free Mg(2+). The corresponding nucleoside diphosphates (ADP, GDP, CDP, TDP or UDP) at 1 mM had no effect on oligomycin-inhibited respiration. We conclude that proton conductance in rat skeletal-muscle mitochondria is very sensitive to free Mg(2+) concentration but is insensitive to NTPs or NDPs at 1 mM.

Adenosine Triphosphate↗

PubMed2000

Therapeutically targeting lymphocyte energy metabolism by high-dose glucocorticoids.

Lymphocytes use a considerable amount of energy, mainly in the form of ATP, especially when they become stimulated following activation by antibodies or mitogens. Cellular respiration is the major energy source, and in quiescent cells the ATP produced is used to drive protein synthesis and sodium transport. In stimulated cells there is significantly higher ATP production to balance the higher ATP demand of specific processes resulting from activation. The major ATP-consuming processes under these conditions are protein synthesis and Na(+),K(+)-ATPase (about 20% each), while Ca(2+)-ATPase and RNA/DNA syntheses contribute about 10% each. There is a wealth of available information about glucocorticoid effects on lymphocytes, but here we focus on the extent to which this lymphocyte bioenergetic machinery is targeted by glucocorticoids when they are used therapeutically at high doses. High-dose glucocorticoids have been shown recently to interfere with processes that are essential for the activation and maintenance of lymphocytes, such as sodium and potassium transport. Therefore, in this article we describe the bioenergetics of lymphocytes in resting, activated, and glucocorticoid-treated states and present a concept for discussion to describe the relationship among these states in fundamental and clinical terms.

Adenosine Triphosphate↗

PubMed2000

Calcium regulation of oxidative phosphorylation in rat skeletal muscle mitochondria.

Activation of oxidative phosphorylation by physiological levels of calcium in mitochondria from rat skeletal muscle was analysed using top-down elasticity and regulation analysis. Oxidative phosphorylation was conceptually divided into three subsystems (substrate oxidation, proton leak and phosphorylation) connected by the membrane potential or the protonmotive force. Calcium directly activated the phosphorylation subsystem and (with sub-saturating 2-oxoglutarate) the substrate oxidation subsystem but had no effect on the proton leak kinetics. The response of mitochondria respiring on 2-oxoglutarate at two physiological concentrations of free calcium was quantified using control and regulation analysis. The partial integrated response coefficients showed that direct stimulation of substrate oxidation contributed 86% of the effect of calcium on state 3 oxygen consumption, and direct activation of the phosphorylation reactions caused 37% of the increase in phosphorylation flux. Calcium directly activated phosphorylation more strongly than substrate oxidation (78% compared to 45%) to achieve homeostasis of mitochondrial membrane potential during large increases in flux.

Animals↗