MitoPedia: Respiratory states

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MitoPedia: Respiratory states

MitoPedia - high-resolution terminology - matching measurements at high-resolution.
The MitoPedia terminology is developed continuously in the spirit of Gentle Science.

Coupling control states
Free and excess capacities of respiration
Free OXPHOS capacity Free ROUTINE activity Free ETS capacity Excess E-P capacity Excess E-R capacity
See also
» MitoPedia: Respiratory control ratios
» Respirometry
» Cell ergometry

Respiratory states: recommended terms

Background stateYThe background state, Y, is the non-activated or inhibited respiratory state at background flux, which is low in relation to the higher flux in the reference state, Z. The transition from the background state to the reference state is a step change. A metabolic control variable, X (substrate, activator), is added to the background state to stimulate flux to the level of the reference state. Alternatively, the metabolic control variable, X, is an inhibitor, which is present in the background state, Y, but absent in the reference state, Z. The background state is the baseline of a single step in the definition of the flux control factor. In a sequence of step changes, the common baseline state is the state of lowest flux in relation to all steps, which can be used as a baseline correction.
Basal respirationBMRBasal respiration or basal metabolic rate (BMR) is the minimal rate of metabolism required to support basic body functions, essential for maintenance only. BMR (in humans) is measured at rest 12 to 14 hours after eating in a physically and mentally relaxed state at thermally neutral room temperature. Maintenance energy requirements include mainly the metabolic costs of protein turnover and ion homeostasis. In many aerobic organisms, and particularly well studied in mammals, BMR is fully aerobic, i.e. direct calorimetry (measurement of heat dissipation) and indirect calorimetry (measurement of oxygen consumption multiplied by the oxycaloric equivalent) agree within errors of measurement (Blaxter KL 1962. The energy metabolism of ruminants. Hutchinson, London: 332 pp [1]). In many cultured mammalian cells, aerobic glycolysis contributes to total ATP turnover (Gnaiger and Kemp 1990 [2]), and under these conditions, 'respiration' is not equivalent to 'metabolic rate'. Basal respiration in humans and skeletal muscle mitochondrial function (oxygen kinetics) are correlated (Larsen et al 2011 [3]). » MiPNet article
Baseline stateThe baseline state in a sequence of step changes is the state of lowest flux in relation to all steps, which can be used as a baseline correction. Correction for residual oxygen consumption, ROX, is an example where ROX is the baseline state. In a single step, the baseline state is equivalent to the background state.
Complex IV single stepCIVCIV: Electron flow through Complex IV (cytochrome c oxidase) is measured in intact mitochondria after inhibiton of CIII by antimycin A, and addition of ascorbate (As) and the artificial substrate TMPD (Tm). Ascorbate has to be titrated first. It reduces TMPD, which further reduces cytochrome c, which is the substrate of CIV. Since CIV is a proton pump of the electron transfer system, the single step of CIV-linked respiration can be measured in different coupling states: L, P, and E. Measurement of CIV activity requires uncoupler titrations to eliminate any potential control by the phosphorylation system, and a cytochrome c test to avoid any limitation by cytochrome c release. Total oxygen uptake in the ascorbate&TMPD(&c) stimulated state (Tm) has to be corrected for chemical background oxygen consumption.
Coupling control stateCCSCoupling control states are defined in mitochondrial preparations (isolated mitochondria, permeabilized cells, permeabilized tissues, homogenates) as LEAK, OXPHOS, and ETS states of respiration (L, P, E) in any pathway control state which is competent for electron transfer. These coupling states are induced by application of specific inhibitors of the phosphorylation system, titration of ADP and uncouplers. In intact cells, the coupling control states are LEAK, ROUTINE, and ETS states of respiration (L, R, E). Coupling control protocols induce these coupling control states sequentially at a constant pathway control state.
Dyscoupled respirationDyscoupled respiration is LEAK respiration distinguished from intrinsically (physiologically) uncoupled and from extrinsic experimentally uncoupled respiration as an indication of extrinsic uncoupling (pathological, toxicological, pharmacological by agents that are not specifically applied to induce uncoupling, but are tested for their potential dyscoupling effect). Dyscoupling indicates a mitochondrial dysfunction. In addition to intrinsic uncoupling, dyscoupling occurs under pathological and toxicological conditions. Thus a distinction is made between physiological uncoupling and pathologically defective dyscoupling in mitochondrial respiration.
ETS capacityEE.jpg ETS capacity is the respiratory electron transfer system capacity, E, of mitochondria in the experimentally induced noncoupled state. In intact mitochondria, the ETS capacity depends not only on the inner membrane-bound ETS (mETS, with respiratory Complexes CI to CIV, electron-transferring flavoprotein complex ETF, and glycerophosphate dehydrogenase complex) but also integrates transporters across the inner mt-membrane, the TCA cycle and other matrix dehydrogenases. Its experimental determination in mitochondrial preparations or intact cells requires the measurement of oxygen consumption in the presence of defined substrates and of an established uncoupler at optimum concentration. This optimum concentration is determined by stepwise titration of the uncoupler up to the concentration inducing maximum flux as the determinant of ETS capacity. » MiPNet article
Electron transfer systemETSE.jpg The mitochondrial electron transfer system (ETS; synonymous with 'electron transport system') transfers electrons from externally supplied reduced substrates to oxygen. It consists of the membrane-bound ETS (mETS) with enzyme complexes located in the inner mt-membrane, mt-matrix dehydrogenases generating NADH, and the transport systems involved in metabolite exchange across the mt-membranes (see ETS capacity).
Excess E-P capacityExPExcess E-P capacity The excess E-P capacity, ExP, is the difference of the ETS capacity and OXPHOS capacity, ExP = E-P. At ExP > 0, the capacity of the phosphorylation system exerts a limiting effect on OXPHOS capacity. In addition, ExP depends on coupling efficiency, since P approaches E at increasing uncoupling.
Excess E-R capacityExRExcess E-R capacity The Excess E-R capacity, ExR, is the difference of ETS capacity and ROUTINE respiration, ExR = E-R. For further information, see Cell ergometry.
Fatty acid oxidation pathway control stateF, FAO
In the fatty acid oxidation pathway control state (F, FAO), one or several fatty acids are supplied to feed electrons into the F-junction through fatty acyl CoA dehydrogenase (reduced form FADH2), to electron transferring flavoprotein (CETF), and further through the Q-junction to Complex III (CIII). FAO not only depends on electron transfer through the F-junction (which is typically rate-limiting) but simultaneously generates NADH and thus depends on N-junction throughput. Hence FAO can be inhibited completely by inhibition of Complex I (CI). In addition and independent of this source of NADH, the type N substrate malate is required as a co-substrate for FAO in mt-preparations, since accumulation of AcetylCo inhibits FAO in the absence of malate. Malate is oxidized in a reaction catalyzed by malate dehydrogenase to oxaloacetate (yielding NADH), which then stimulates the entry of AcetylCo into the TCA cycle catalyzed by citrate synthase.
Free ETS capacity≈EFree ETS capacity The free ETS capacity, ≈E, is the ETS capacity corrected for LEAK respiration, ≈E = E-L. ≈E is the respiratory capacity potentially available for ion transport and phosphorylation of ADP to ATP. Oxygen consumption in the ETS state, therefore, is partitioned into the free ETS capacity, ≈E, and LEAK respiration, LP, compensating for proton leaks, slip and cation cycling: E = ≈E+LP (see free OXPHOS capacity).
Free OXPHOS capacity≈PFree OXPHOS capacity The free OXPHOS capacity, ≈P, is the OXPHOS capacity corrected for LEAK respiration, ≈P = P-L. ≈P is the scope for ADP stimulation, the respiratory capacity potentially available for phosphorylation of ADP to ATP. Oxygen consumption in the OXPHOS state, therefore, is partitioned into the free OXPHOS capacity, ≈P, strictly coupled to phosphorylation, ~P, and nonphosphorylating LEAK respiration, LP, compensating for proton leaks, slip and cation cycling: P = ≈P+LP. It is frequently assumed that LEAK respiration, L, as measured in the LEAK state, overestimates the LEAK component of respiration, LP, as measured in the OXPHOS state, particularly if the protonmotive force is not adjusted to equivalent levels in L and LP. However, if the LEAK component increases with enzyme turnover during P, the low enzyme turnover during L may counteract the effect of the higher Δpmt.
Free ROUTINE activity≈RFree ROUTINE activity The free ROUTINE activity, ≈R, is ROUTINE respiration corrected for LEAK respiration, ≈R = R-L. ≈R is the respiratory activity available for phosphorylation of ADP to ATP. Oxygen consumption in the ROUTINE state of respiration measured in intact cells, therefore, is partitioned into the free ROUTINE activity, ≈R, strictly coupled to phosphorylation, ~P, and nonphosphorylating LEAK respiration, LR, compensating for proton leaks, slip and cation cycling: R = ≈R+LR. It is frequently assumed that LEAK respiration, L, as measured in the LEAK state, overestimates the LEAK component of respiration, LR, as measured in the ROUTINE state, particularly if the protonmotive force is not adjusted to equivalent levels in L and LR. However, if the LEAK component increases with enzyme turnover during R, the low enzyme turnover during L may counteract the effect of the higher Δpmt.
Glycerophosphate pathway control stateGp
The glycerophosphate pathway control state (Gp) is an ETS pathway level 3 control state, supported by the fuel substrate glycerophosphate and electron transfer through glycerophosphate dehydrogenase complex into the Q-junction. The glycerolphosphate shuttle represents an important pathway, particularly in liver and blood cells, of making cytoplasmic NADH available for mitochondrial oxidative phosphorylation. Cytoplasmic NADH reacts with dihydroxyacetone phosphate catalyzed by cytoplasmic glycerophos-phate dehydrogenase. On the outer face of the inner mitochondrial membrane, mitochondrial glycerophosphate dehydrogenase oxidises glycerophosphate back to dihydroxyacetone phosphate, a reaction not generating NADH but reducing a flavin prosthesic group. The reduced flavoprotein donates its reducing equivalents to the electron transfer system at the level of CoQ.
LEAK respirationLL.jpg LEAK respiration or LEAK oxygen flux, L, compensating for proton leak, proton slip, cation cycling and electron leak, is a dissipative component of respiration which is not available for performing biochemical work and thus related to heat production. LEAK respiration is measured in the LEAK state, in the presence of reducing substrate(s), but absence of ADP (theoretically, absence of inorganic phosphate presents an alternative), or after enzymatic inhibition of the phosphorylation system. The LEAK state is the non-phosphorylating resting state of intrinsic uncoupled or dyscoupled respiration when oxygen flux is maintained mainly to compensate for the proton leak at a high chemiosmotic potential, when ATP synthase is not active. In this non-phosphorylating resting state, the electrochemical proton gradient is increased to a maximum, exerting feedback control by depressing oxygen flux to a level determined mainly by the proton leak and the H+/O2 ratio. In this state of maximum protonmotive force, LEAK respiration is higher than the LEAK component in state P (OXPHOS capacity). The conditions for measurement and expression of respiration vary (oxygen flux in state L, JO2L or oxygen flow in state L, IO2L). If these conditions are defined and remain consistent within a given context, then the simple symbol L for respiratory state can be used as a substitute for the more explicit expression for respiratory activity. » MiPNet article
LEAK state with ATPLTL.jpg LEAK state with ATP, LT, obtained in mt-preparations without ATPase activity after ADP is maximally phosphorylated to ATP (State 4; Chance and Williams 1955) or after addition of high ATP in the absence of ADP (Gnaiger et al 2000).
LEAK state with oligomycinLOmyL.jpg The LEAK state with Omy is a LEAK state induced by inhibition of ATP synthase by oligomycin (LOmy). ADP and ATP may or may not be present.
LEAK state without adenylatesLNL.jpg In the LEAK state without adenylates, LN (N for no adenylates), mitochondrial respiration is measured after addition of substrates, which decreases slowly to the LEAK state after oxidation of endogenous substrates with no adenylates.
Mitochondrial membrane potentialmtMP, ΔψmtThe mitochondrial membrane potential, mtMP, is the electric part of the protonmotive force, Δpmt.

Δψmt = Δpmt - ΔµH+ / F

mtMP or Δψmt is the potential difference across the inner mitochondrial (mt) membrane, expressed in the electric unit of volt [V]. Electric force of the mitochondrial membrane potential is the electric energy change per ‘motive’ electron or per electron moved across the transmembrane potential difference, with the number of ‘motive’ electrons expressed in the unit coulomb [C].

The chemical part of the protonmotive force, µH+ / F stems from the difference of pH across the mt-membrane. It contains a factor that bridges the gap between the electric force [J/C] and the chemical force [J/mol]. This factor is the Faraday constant, F, for conversion between electric force expressed in joules per coulomb or Volt [V=J/C] and chemical force with the unit joules per mole or Jol [Jol=J/mol],

F = 96.4853 kJol/V = 96,485.3 C/mol
NADH pathway control stateN
The NADH pathway control state (N) is obtained by addition of NADH-linked substrates (CI-linked), feeding electrons into the N-junction catalyzed by various mt-dehydrogenases. N-supported flux is induced in mt-preparations by addition of NADH-generating substrate combinations of pyruvate (P), glutamate (G), malate (M), oxaloacetate (Oa), oxoglutarate (Og), citrate, hydroxybutyrate. These N-junction substrates are (indirectly) linked to Complex I by the corresponding dehydrogenase-catalyzed reactions reducing NAD+ to NADH+H+. The most commonly applied N-junction substrate combinations are: PM, GM, PGM. The malate anaplerotic pathway control state (M alone) is a special case related to malic enzyme (mtME). The glutamate anaplerotic pathway control state (aN; G alone) supports respiration through glutamate dehydrogenase (mtGDH). In mt-preparations, succinate dehydrogenase (SDH; CII) is largely substrate-limited in N-linked respiration, due to metabolite depletion into the incubation medium. The residual involvement of S-linked respiration in the N-pathway control state can be further suppressed by the CII-inhibitor malonic acid). In the N-pathway control state ETS pathway level 4 is active.
NS-pathway control stateNS, CI&II
NS-pathway control
NS-pathway control is exerted in the NS-linked substrate state (flux in the NS-linked substrate state, NS; or Complex I&II, CI&II-linked substrate state). NS is induced in mt-preparations by addition of NADH-generating substrates (N-pathway control state, or CI-linked pathway control) in combination with succinate (S-pathway control state; S- or CII-linked). Whereas NS expresses substrate control in terms of substrate types (N and S), CI&II defines the same concept in terms of the convergent pathway to the Q-junction (pathway control). NS is the abbreviation for the combination of N- or NADH-linked substrates (CI-linked) and S- or succinate-linked substrates (CII-linked). This physiological substrate combination is required for partial reconstitution of TCA cycle function and convergent electron-input into the Q-junction, to compensate for metabolite depletion into the incubation medium. NS in combination exerts an additive effect of convergent electron flow in most types of mitochondria.
Noncoupled respirationEE.jpg Noncoupled respiration is distinguished from general (pharmacological or mechanical) uncoupled respiration, to give a label to an effort to reach the state of maximum uncoupler-activated respiration without inhibiting respiration. Noncoupled respiration, therefore, yields an estimate of ETS capacity. Experimentally uncoupled respiration may fail to yield an estimate of ETS capacity, due to inhibition of respiration above optimum uncoupler concentrations or insufficient stimulation by sub-optimal uncoupler concentrations. Optimum uncoupler concentrations for evaluation of (noncoupled) ETS capacity require inhibitor titrations (Steinlechner-Maran 1996 Am J Physiol Cell Physiol; Huetter 2004 Biochem J; Gnaiger 2008 POS). Noncoupled respiration is maximum electron flow in an open-transmembrane proton circuit mode of operation (see ETS capacity).
OXPHOS capacityPP.jpg OXPHOS capacity (P) is the respiratory capacity of mitochondria in the ADP-activated state of oxidative phosphorylation, at saturating concentrations of ADP (possibly in contrast to State 3), inorganic phosphate, oxygen, and defined reduced substrates. » MiPNet article
Oxidative phosphorylationOXPHOSP.jpg Oxidative phosphorylation (OXPHOS) is the oxidation of reduced fuel substrates by electron transfer to oxygen, chemiosmotically coupled to the phosphorylation of ADP to ATP and accompanied by an intrinsically uncoupled component of respiration. The OXPHOS state (P) of respiration provides a measure of OXPHOS capacity, which is frequently corrected for residual oxygen consumption (ROX).
Pathway control statePCS, mtPCS
SUIT-catg FNSGpCIV.jpg

Pathway control states (synonymous with ETS substrate control states) are obtained in mitochondrial preparations (isolated mitochondria, permeabilized cells, permeabilized tissues, tissue homogenate) by depletion of endogenous substrates and addition to the mitochondrial respiration medium of fuel substrates (CHNO) activating specific mitochondrial pathways. Mitochondrial pathway control states, mtPCS, have to be defined complementary to mitochondrial coupling control states. Coupling states (LEAK, OXPHOS, ETS) require electron transfer system competent substrate control states, including oxygen supply. Categories of SUIT protocols are defined according to mitochondrial pathway control states.

» MiPNet article
ROUTINE respirationRR.jpg In the intact cell, ROUTINE respiration or ROUTINE activity in the physiological coupling state R, is controlled by cellular energy demand, energy turnover and the degree of coupling to phosphorylation (intrinsic uncoupling and pathological dyscoupling). The conditions for measurement and expression of respiration vary (oxygen flux in state R, JO2R or oxygen flow in state R, IO2R). If these conditions are defined and remain consistent within a given context, then the simple symbol R for respiratory state can be used to substitute the more explicit expression for respiratory activity. R and growth of cells is supported by exogenous substrates in culture media. In media without energy substrates, R depends on endogenous substrates. R cannot be measured in permeabilized cells or isolated mitochondria. R is corrected for residual oxygen consumption (ROX), whereas R´ is the uncorrected apparent ROUTINE respiration or total cellular oxygen consumption of cells including ROX.
Reference stateZThe reference state, Z, is the respiratory state with high flux in relation to the background state, Y. The transition between the background state and the reference state is a step brought about by a metabolic control variable, X. If X stimulates flux (ADP, fuel substrate), it is present in the reference state but absent in the background state. If X is an inhibitor of flux, it is absent in the reference state but present in the background state. The reference state is specific for a single step to define the flux control factor. In contrast, in a sequence of multiple steps, the common reference state is frequently taken as the state with the highest flux in the entire sequence, as used in the definition of the flux control ratio.
Residual oxygen consumptionROXROX.jpg Residual oxygen consumption, ROX, is respiration due to oxidative side reactions remaining after inhibition of the electron transfer system (ETS) in mitochondrial preparations or cells, or in mt-preparations incubated without addition of fuel substrates (in the presence of ADP following a stimulation of the consumption of endogenous fuel substrates: State 2). Different conditions designated as ROX states (different combinations of inhibitors of CI, CII, CIII and CIV; or respiration of mt-preparations without addition of fuel substrates) may result in consistent or significantly different levels of oxygen consumption. Hence the best quantitative estimate of ROX has to be carefully evaluated. Mitochondrial respiration is frequently corrected for ROX as the baseline state. Then total ROUTINE, LEAK, OXPHOS or ETS (R, L, P and E) respiration is distinguished from the corresponding ROX-corrected, mitochondrial (ETS-linked) fluxes: R(mt), L(mt), P(mt) and E(mt). When expressing ROX as a fraction of ETS capacity (flux control ratio), total flux, E (not corrected for ROX), should be taken as the reference. ROX may be related to, but is of course different from ROS production. » MiPNet article
Respiratory stateRespiratory states of mitochondrial preparations and intact cells are defined in the current literature in many ways and with a diversity of terms. Mitochondrial respiratory states must be defined in terms of both, the coupling control state and the pathway control state.
Resting metabolic rateRMRResting respiration or resting metabolic rate (RMR) is measured under standard conditions of an 8–12-h fast and a 12-h abstinence from exercise. In an exemplary study (Haugen 2003 Am J Clin Nutr), "subjects rested quietly in the supine position in an isolated room with the temperature controlled to 21–24° C. RMR was measured for 15–20 min. Criteria for a valid RMR was a minimum of 15 min of steady state, determined as a <10% fluctuation in oxygen consumption and <5% fluctuation in respiratory quotient". The main difference between RMR and BMR (basal metabolic rate) is the position of the subject during measurement. Resting metabolic rate is the largest component of the daily energy budget in most human societies and increases with physical training state (Speakman 2003 Proc Nutr Soc).
SGp-pathway control stateSGpSGp: Succinate & Glycerophosphate.

MitoPathway control state: SGp; obtained with OctPGMSGp(Rot)


This pathway control state is obtained in the presence of CI-linked and FAO-linked substrates after inhibition of CI by rotenone, which simultaneously inhibits FAO.
Succinate pathway control stateS

The Succinate pathway control state (S) is achieved with succinate as the single substrate, at ETS-level pathway level 3: succinate-induced respiratory state; CII-linked; SRot. S supports electron flux through Complex II to CII-bound flavin adenine dinucleotide (FADH2) to Q. Inhibition of Complex I by rotenone (Rot; or amytal, piericidine) prevents accumulation of oxaloacetate which is a potent inhibitor of succinate dehydrogenase. After inhibition of CI by rotenone, the NADH-linked dehydrogenases become inhibited by the redox shift from NAD+ to NADH. Succinate dehydrogenase is activated by succinate and ATP, which explains in part the time-dependent increase of respiration in isolated mitochondria after addition of rotenone (first), succinate and ADP.

The Complex II-linked substrate state is induced in mt-preparations by addition of succinate&rotenone (Complex I inhibitor). Succinate is the direct substrate of Complex II (succinate dehydrogenase). In CII-linked respiration, only Complex III and Complex IV are involved in pumping protons from the matrix (P-phase) to the N-phase with a ~P/O ratio of 1.75 (P/O2 = 3.5).

Respiratory states: find synonyms, historically used and controversial terms

TermAbbreviationRedirected to
Complex I&II-linked substrate stateCI&II-linked, NSSee NS-pathway control state
Complex I-linked substrate stateCI-linked, NSee N-pathway control state
Complex II-linked substrate stateCII-linked, SRot, SSee S-pathway control state
ETS-competent pathway control stateETS-competent substrate control state, see Pathway control state.
Level flowEE.jpg Level flow is a steady state of a system with an input process coupled to an output process (coupled system), in which the output force is zero. Clearly, energy must be expended to maintain level flow, even though output is zero (Caplan and Essig 1983; referring to zero output force, while output flow may be maximum).
State 1State 1 is the first respiratory state in an oxygraphic protocol described by Chance and Williams (1955), when isolated mitochondria are added to mitochondrial respiration medium containing oxygen and inorganic phosphate, but no ADP and no reduced respiratory substrates. In State 1, LEAK respiration may be supported to some extent by undefined endogenous substrates, which are oxidized and slowly exhausted. After oxidation of endogenous substrates, only residual oxygen consumption remains (ROX).
State 2ROXDROX.jpg Substrate limited state of residual oxygen consumption, after addition of ADP to isolated mitochondria suspended in mitochondrial respiration medium in the absence of reduced substrates (ROXD). Residual endogenous substrates are oxidized during a transient stimulation of oxygen flux by ADP. The peak – supported by endogenous substrates – is, therefore, a pre-steady state phenomenon preceding State 2. Subsequently oxygen flux declines to a low level (or zero) at the steady State 2 (Chance and Williams 1955). ADP concentration (D) remains high during ROXD.
State 3PP.jpg State 3 respiration is the ADP stimulated respiration of isolated coupled mitochondria in the presence of high ADP and Pi concentrations, supported by a defined substrate or substrate combination at saturating oxygen levels (Chance and Williams, 1955). State 3 respiration can also be induced in permeabilized cells, including permeabilized tissue preparations and tissue homogenates. ADP concentrations applied in State 3 are not necessarily saturating, whereas OXPHOS capacity is measured at saturating concentrations of ADP and Pi (state P). For instance, non-saturating ADP concentrations are applied in State 3 in pulse titrations to determine the P/O ratio in State 3→4 (D→T) transitions, when saturating ADP concentrations would deplete the oxygen concentration in the closed oxygraph chamber before State 4 is obtained (Gnaiger et al 2000; Puchowicz et al 2004). Respiration in the OXPHOS state or in State 3 is partially coupled, and partially uncoupled (physiological) or partially dyscoupled (pathological). A high mt-membrane potential provides the driving force for oxidative phosphorylation, to phosphorylate ADP to ATP and to transport ADP and ATP across the inner mt-membrane through the adenine nucleotide translocase (ANT). The mt-membrane potential is reduced, however, in comparison to the LEAK state of respiration, whereas the cytochromes are in a more oxidized redox state.
State 3uEE.jpg Noncoupled state of ETS capacity. State 3u (u for uncoupled) has been used frequently in bioenergetics, without sufficient emphasis (e.g. Villani et al 1998) on the fundamental difference between OXPHOS capacity (P, coupled with an uncoupled contribution; State 3) and noncoupled ETS capacity (E; State 3u) (Gnaiger 2009; Rasmussen and Rasmussen 2000).
State 4LTL.jpg State 4 is the respiratory state obtained in isolated mitochondria after State 3, when added ADP is phosphorylated maximally to ATP driven by electron transfer from defined respiratory substrates to O2 (Chance and Williams, 1955). State 4 represents LEAK respiration, LT (L for LEAK; T for ATP), or an overestimation of LEAK respiration if ATPase activity prevents final accumulation of ATP and maintains a continuous stimulation of respiration by recycled ADP. This can be tested by inhibition of ATP synthase by oligomycin; LOmy). In the LEAK state (state of non-phosphorylating resting respiration; static head), oxygen flux is decreased to a minimum (corrected for ROX), and the mt-membrane potential is increased to a maximum for a specific substrate or substrate combination.
State 5State 5 is the respiratory state obtained in a protocol with isolated mitochondria after a sequence of State 1 to State 4, when the concentration of O2 is depleted in the closed oxygraph chamber and zero oxygen (the anaerobic state) is reached (Chance and Williams, 1955; Table I). State 5 is defined in the original publication in two ways: State 5 may be obtained by antimycin A treatment or by anaerobiosis (Chance and Williams, 1955; page 414). Antimycin A treatment yields a State 5 equivalent to a state for measurement of residual oxygen consumption, ROX (which may also be induced by rotenone+myxothiazol; Gnaiger 2009). Setting State 5 equivalent to ROX or anoxia (Chance and Williams 1955) can be rationalized only in the context of measurement of cytochrome redox states, whereas in the context of respiration State 5 is usually referred to as anoxic.
Static headLL.jpg Static head is a steady state of a system with an input process coupled to an output process (coupled system), in which the output force is maximized at constant input or driving force up to a level at which the conjugated output flow is reduced to zero. In an incompletely coupled system, energy must be expended to maintain static head, even though the output is zero (Caplan and Essig 1983; referring to output flow at maximum output force). LEAK respiration is a measure of input flow at static head, when the output flow of phosphorylation (ADP->ATP) is zero at maximum phosphorylation potential (Gibbs force of phosphorylation; Gnaiger 1993a). In a completely coupled system, not only the output flux but also the input flux are zero at static head, which then is a state of ergodynamic equilibrium (Gnaiger 1993b). Whereas the output force is maximum at ergodynamic equilibrium compensating for any given input force, all forces are zero at thermodynamic equilibrium. Flows are zero at both types of equilibria, hence entropy production or power (power = flow x force) are zero in both cases, i.e. at thermodynamic equilibrium in general, and at ergodynamic equilibrium of a completely coupled system at static head.
Substrate control stateSee Pathway control state
Succinate control stateS
S: When succinate is added without rotenone, oxaloacetate is formed from malate by the action of malate dehydrogenase. Oxaloacetate accumulates and is a more potent competitive inhibitor of succinate dehydrogenase than malonate even at small concentration. Reverse electron flow from CII to CI is known to stimulate production of reactive oxygen species under these conditions to extremely high, pathological levels. Addition of malate reduces superoxide production with succinate, probably due to a shift in the redox state and oxaloacetate inhibition of CII. Compare: S-pathway control state.