layered envelope, the cristae and the crista junctions that link thecristae to the IBM. Correct architecture is prerequisite for mitochon-drial function, in particular for OXPHOS and inheritance of themitochondrial DNA. Whereas there is a plethora of information onthe mitochondrial OXPHOS complexes only little is known about themolecules that determine mitochondrial architecture. We havestudied several aspects of the complexity of mitochondrial architec-ture. One aspect relates to the structure and function of the variousmolecular machines that mediate the topogenesis of newly synthe-sized, nuclear-encoded proteins that are imported into the mitochon-dria. For instance, the TOM translocase in the outer membrane andthe TIM23 translocase in the inner membrane work in physicalconjunction to transport proteins, at the same time, across bothmembranes. Thus, import of these proteins is confined to the IBM.This raises the important question as to whether there is a permanentor dynamic subcompartmentation of proteins in the various parts ofthe inner membrane. A largely open question in this context relates tothe kinds of interactions of OM and IBM in various other transportprocesses, one of the most important being the translocation of lipidsinto the mitochondria. Another aspect regards the nature, functionand molecular structure of the crista junctions and crista tips andrims. In particular the proteins that are shaping these structures arelargely unknown. A number of experiments and results will bepresented that provide some answers to some of these questions.

doi:10.1016/j.bbabio.2010.04.026

PL. 7

Proton circuits and mitochondrial dysfunctionDavid G. NichollsBuck Institute for Age Research, California, USAE-mail: dnicholls@buckinstitute.org

The terms mitochondrial ‘function and dysfunction’ are usedwidely in the cell biology field, generally without a precise definitionof their meaning. Mitchell's chemiosmotic proton circuit, firstpublished in 1966, provides a precise quantitative framework withinwhich to quantify these critical parameters for the life and death ofthe cell. The proton circuit has units of potential (the protonmotiveforce, Δp) and flux (the proton current, JH+), and these additionallyallow calculation of inner membrane leak conductance, CmH+ (JH+

per unit Δp) and power (JH+×Δp). The analogy with an equivalentelectrical circuit has considerable utility for visualizing and manip-ulating the proton circuit, and is equally applicable to isolatedmitochondria and intact cells. An early application of this quantitativeapproach was the elucidation of the regulatable proton conductancepathway in brown adipose tissue, leading to the identification ofUCP1. One observation that emerged from these studies is that‘uncoupling’ is not an all-or-nothing process. Thus while a largeexcess of a protonophore can almost totally collapse Δp, at the criticalconcentration at which respiratory control is just lost Δp may be only10–20% below its maximal State 4 value, and thermodynamicallycompetent to maintain ATP synthesis. Until this threshold is reachedΔp changes modestly as CmH+ is increased. In intact cells titration tothis threshold can help to define a critical parameter of mitochondrial‘function’ — the spare respiratory capacity, defined as the capacityover basal of the electron transport chain in concert with theinputting metabolic pathways to support an increase in flux inresponse to this imposed increase in proton conductance. With theproviso that this proton current could all be utilized by the ATPsynthase in the absence of protonophore, the spare respiratorycapacity provides a safety margin preventing an ‘ATP crisis’ duringperiods of maximal ATP demand, for example in neurons during

potentially excitotoxic stimulation. Mitochondrial ‘dysfunction’ de-fined as a decrease in this spare respiratory capacity has been shownin various neural preparations to greatly potentiate cell death underconditions of high energy demand.

doi:10.1016/j.bbabio.2010.04.027

PL. 8

Redox-optimized mitochondrial ROS balanceBrian O'Rourke, Sonia Cortassa, Miguel A. AonJohns Hopkins University School of Medicine, Institute of MolecularCardiobiology, Baltimore, Maryland, USAE-mail: bor@jhmi.edu

While it is generally accepted that mitochondrial reactive oxygenspecies (ROS) balance depends on the both rate of single electronreduction of O2 to superoxide (O2

•−) by the electron transport chain andthe rate of scavenging by intracellular antioxidant pathways, consider-able controversy exists regarding the conditions leading to oxidativestress in intact cells versus isolated mitochondria. Here, we postulatethat mitochondria have been evolutionarily optimized to maximizeenergy output while keeping ROS overflow to a minimum by operatingin an intermediate redox state. We show that at the extremes ofreduction or oxidation of the redox couples involved in electrontransport (NADH/NAD+) or ROS scavenging (NADPH/NADP+, GSH/GSSG), respectively, ROS balance is lost. This results in a net overflow ofROS that increases as one moves farther away from the optimal redoxpotential. At more reduced mitochondrial redox potentials, ROSproduction exceeds scavenging, while under more oxidizing conditions(e.g., at higherworkloads) antioxidant defense canbe compromised andeventually overwhelmed. Experimental support for this hypothesis isprovided in both cardiomyocytes and in isolated mitochondria fromguinea pig hearts. The model reconciles, within a single framework,observations that isolated mitochondria tend to display increasedoxidative stress at high reduction potentials (and high mitochondrialmembrane potential), whereas intact cardiac cells can display oxidativestress either when mitochondria become more uncoupled (i.e., lowmitochondrial membrane potential) or when mitochondria are maxi-mally reduced (as in ischemia or hypoxia). The continuum described bythemodel has the potential to account formany disparate experimentalobservations and also provides a rationale for graded physiological ROSsignaling at redox potentials near the minimum.

doi:10.1016/j.bbabio.2010.04.028

PL. 9

The pivotal roles of mitochondria in cancer: Warburg and beyondand encouraging prospects for effective therapiesPeter L. Pedersen*Johns Hopkins University School of Medicine, Departments of BiologicalChemistry and Center for Cell Metabolism, Baltimore, Maryland, USAE-mail: ppederse@jhmi.edu

Tumors usurp established metabolic steps used by normal tissuesfor glucose utilization and ATP production that rely heavily onmitochondria and employ a route that, although involving mitochon-dria, includes a much greater dependency on glycolysis. Firstdescribed by Otto Warburg, this aberrant phenotype becomes morepronounced with increased tumor malignancy. Thus, while main-taining their capacity for respiration, tumors “turn more parasitic” byenhancing their ability to scavenge glucose. Relying significantly on

Abstracts 3