MitoPedia: Respiratory states

high-resolution terminology - matching measurements at high-resolution

MitoPedia: Respiratory states

The MitoPedia terminology is developed continuously in the spirit of Gentle Science.

MitoPathways 2020: Fig. 2.4.

Coupling-control states - symbols for rates

Gnaiger Erich et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1. doi:10.26124/bec:2020-0001.v1. - »Bioblast link«

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Net and excess capacities of respiration

See also

» MitoPedia: Respiratory control ratios

» Respirometry

» Cell ergometry

Respiratory states: recommended terms

Term	Abbreviation	Description
Background state	Y	The background state Y (background rate Y_X) is the non-activated or inhibited respiratory state at background rate, which is low in relation to the higher rate Z_X 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 efficiency. 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 respiration	BMR	Basal 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 state		The 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.
Chemical background	CHB, Chb	Chemical background Chb is due to autooxidation of the reagents. During CIV assays, ascorbate and TMPD are added to maintain cytochrome c in a reduced state. External cytochrome c may be included in the CIV assay. The autooxidation of these compounds is linearly oxygen-dependent down to approximately 50 µM oxygen and responsible for the chemical background oxygen flux after the inhibition of CIV. Oxygen flux due to the chemical reaction of autooxidation must be corrected for the instrumental O2 background. The correction for chemical background is necessary to determine CIV activity, in which case the instrumental O2 background and chemical background may be combined in an overall correction term.
Coupling-control state	CCS	Coupling-control states are defined in mitochondrial preparations (isolated mitochondria, permeabilized cells, permeabilized tissues, homogenates) as LEAK respiration, OXPHOS, and ET states, with corresponding respiration rates (L, P, E) in any electron-transfer-pathway state which is competent for electron transfer. These coupling states are induced by titration of ADP and uncouplers, and application of specific inhibitors of the phosphorylation pathway. In living cells, the coupling-control states are LEAK respiration, ROUTINE, and ET states of respiration with corresponding rates L, R, E, using membrane-permeable inhibitors of the phosphorylation system (e.g. oligomycin) and uncouplers (e.g. CCCP). Coupling-control protocols induce these coupling-control states sequentially at a constant electron-transfer-pathway state.
Dyscoupled respiration		Dyscoupled 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.
E-P excess capacity	E-P	The E-P excess capacity is the difference of the ET capacity and OXPHOS capacity. At E-P > 0, the capacity of the phosphorylation system exerts a limiting effect on OXPHOS capacity. In addition, E-P depends on coupling efficiency, since P aproaches E at increasing uncoupling.
E-R reserve capacity	E-R	The E-R reserve capacity is the difference of ET capacity and ROUTINE respiration. For further information, see Cell ergometry.
ET capacity	E	T capacity is the respiratory electron-transfer-pathway capacity E of mitochondria measured as oxygen consumption in the noncoupled state at optimum uncoupler concentration. This optimum concentration is obtained by stepwise titration of an established protonophore to induce maximum oxygen flux as the determinant of ET capacity. The experimentally induced noncoupled state at optimum uncoupler concentration is thus distinguished from (1) a wide range of uncoupled states at any experimental uncoupler concentration, (2) physiological uncoupled states controlled by intrinsic uncoupling (e.g. UCP1 in brown fat), and (3) pathological dyscoupled states indicative of mitochondrial injuries or toxic effects of pharmacological or environmental substances. ET capacity in mitochondrial preparations requires the addition of defined fuel substrates to establish an ET-pathway competent state. » MiPNet article
Electron-transfer-pathway state	ET-pathway state	Electron-transfer-pathway 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, and possibly inhibitors of specific pathways. Mitochondrial electron-transfer-pathway states have to be defined complementary to mitochondrial coupling-control states. Coupling-control states require ET-pathway competent states, including oxygen supply. Categories of SUIT protocols are defined according to mitochondrial ET-pathway states. » MiPNet article
FN	FN	FN is induced in mt-preparations by addition of NADH-generating substrates (N-pathway control state, or CI-linked pathway control) in combination with one or several fatty acids, which 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 FADH₂ and NADH and thus depends on N-junction throughput. Hence FAO can be inhibited completely by inhibition of Complex I (CI). 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. FS in combination exerts an additive effect of convergent electron flow in most types of mitochondria.
FNS	FNS	FNS 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) and one or several fatty acids, which 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 FADH₂ and NADH and thus depends on N-junction throughput. Hence FAO can be inhibited completely by inhibition of Complex I (CI). 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. FNS in combination exerts an additive effect of convergent electron flow in most types of mitochondria.
Fatty acid oxidation pathway control state	F, FAO	In the fatty acid oxidation pathway control state (F- or FAO-pathway), 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 relative to the N-pathway branch), but simultaneously generates FADH₂ and 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 at low concentration (0.1 mM) as a co-substrate for FAO in mt-preparations, since accumulation of Acetyl-CoA 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 Acetyl-CoA into the TCA cycle catalyzed by citrate synthase. Peroxysomal β-oxidation carries out few β-oxidation cycles, thus shortening very-long-chain fatty acids (>C₂₀) for entry into mitochondrial β-oxidation. Oxygen consumption by peroxisomal acyl-CoA oxidase is considered as residual oxygen consumption rather than cell respiration.
Glycerophosphate pathway control state	Gp	The glycerophosphate pathway control state (Gp) is an ET-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-pathway at the level of CoQ.
LEAK respiration	L	EAK 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 - abbreviated as L(n) (theoretically, absence of inorganic phosphate presents an alternative), or after enzymatic inhibition of the phosphorylation system, which can be reached with the use of oligomycin - abbreviated as L(Omy). 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, L, is higher than the LEAK component of OXPHOS capacity, P. The conditions for measurement and expression of respiration vary (oxygen flux in the LEAK state, J_O₂L, or oxygen flow, I_O₂L). If these conditions are defined and remain consistent within a given context, then the simple symbol L for respiratory rate can be used as a substitute for the more explicit expression for respiratory activity. » MiPNet article
LEAK state with ATP	L(T)	The LEAK state with ATP is 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). Respiration in the LEAK state with ATP, L(T), is distinguished from L(n) and L(Omy).
LEAK state with oligomycin	L(Omy)	The LEAK state with oligomycin is a LEAK state induced by inhibition of ATP synthase by oligomycin. ADP and ATP may or may not be present. LEAK respiration with oligomycin, L(Omy), is distinguished from L(n) and L(T).
LEAK state without adenylates	L(n)	In the LEAK state without adenylates mitochondrial LEAK respiration, L(n) (n for no adenylates), is measured after addition of substrates, which decreases slowly to the LEAK state after oxidation of endogenous substrates with no adenylates. L(n) is distinguished from L(T) and L(Omy).
Mitochondrial membrane potential	mtMP, ΔΨ_p⁺, Δ_elF_ep⁺ [V]	The mitochondrial membrane potential difference, mtMP or ΔΨ_p⁺ = Δ_elF_ep⁺, is the electric part of the protonmotive force, Δp = Δ_mF_eH⁺. Δ_elF_ep⁺ = Δ_mF_eH⁺ - Δ_dF_eH⁺ ΔΨ_p⁺ = Δp - Δµ_H+·(z_H⁺·F)^-1 ΔΨ_p⁺ is the potential difference across the mitochondrial inner membrane (mtIM), expressed in the electric unit of volt [V]. Electric force of the mitochondrial membrane potential is the electric energy change per ‘motive’ charge or per charge moved across the transmembrane potential difference, with the number of ‘motive’ charges expressed in the unit coulomb [C].
NADH electron transfer-pathway state	N	The NADH electron transfer-pathway 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 the 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⁺ + 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 (G alone) supports respiration through glutamate dehydrogenase (mtGDH). Oxidation of tetrahydrofolate is a NAD(P)H linked pathway with formation of formate. 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 ET pathway level 4 is active.
NS-pathway control state	NS, CI&II	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-OXPHOS capacity provides an estimate of physiologically relevant maximum mitochondrial respiratory capacity. NS is induced in mt-preparations by addition of NADH-generating substrates (N-pathway control state in combination with succinate (Succinate pathway; S). Whereas NS expresses substrate control in terms of substrate types (N and S), CI&II defines the same concept in terms of convergent electron transfer to the Q-junction (pathway control). NS is the abbreviation for the combination of NADH-linked substrates (N) and succinate (S). 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 respiration	E	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 ET capacity. Experimentally uncoupled respiration may fail to yield an estimate of ET 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) ET 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 ET capacity). » MiPNet article
OXPHOS capacity	P	OXPHOS capacity P is the respiratory capacity of mitochondria in the ADP-activated state of oxidative phosphorylation, at saturating concentrations of ADP and inorganic phosphate (which may not be the case in State 3), oxygen, and defined reduced CHNO-fuel substrates.
Oxidative phosphorylation	OXPHOS	Oxidative phosphorylation (OXPHOS) is the oxidation of reduced fuel substrates by electron transfer to oxygen, chemiosmotically coupled to the phosphorylation of ADP to ATP (P») and accompanied by an intrinsically uncoupled component of respiration. The OXPHOS state of respiration provides a measure of OXPHOS capacity (P), which is frequently corrected for residual oxygen consumption (ROX).
R-L net ROUTINE capacity	R-L	The R-L net ROUTINE capacity is ROUTINE respiration corrected for LEAK respiration. R-L is the respiratory capacity available for phosphorylation of ADP to ATP. Oxygen consumption in the ROUTINE state of respiration measured in living cells, therefore, is partitioned into the R-L net ROUTINE capacity, strictly coupled to phosphorylation P», and nonphosphorylating LEAK respiration, L_R, compensating for proton leaks, slip and cation cycling: R = R-L+L_R. It is frequently assumed that LEAK respiration L, as measured in the LEAK state, overestimates the LEAK component of respiration, L_R, as measured in the ROUTINE state, particularly if the protonmotive force is not adjusted to equivalent levels in L and L_R. However, if the LEAK component increases with enzyme turnover during R, the low enzyme turnover during L may counteract the effect of the higher pmF.
ROUTINE respiration	R	In the living cell, ROUTINE respiration (R) or ROUTINE activity in the physiological coupling state 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, J_O₂R or oxygen flow in state R, I_O₂R). 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 state	Z	The reference state Z (reference rate Z_X) is the respiratory state with high flux in relation to the background state Y with low background flux Y_X. 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 efficiency. 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 endogenous substrates	REN, Ren	Oxygen consumption due to residual endogenous substrates. Ren is the respiration in the REN state. It is due to oxidative reactions in mt-preparations incubated without addition of fuel substrates in the absence or presence of ADP (in the presence of ADP to stimulate the consumption of endogenous fuel substrates: State 2). Ren values may be used as technical replicates when obtained from the same mt-preparation in different protocols. Ren may be higher than Rox. Correspondingly, Q and NAD are not fully oxidized in the REN state compared to the ROX state. In previous editions (including Gnaiger 2020 BEC MitoPathways), the REN state was not distinguished from the ROX state. However, in novel applications (Q-Module and NADH-Module), a distinction of these states is necessary. Care must be taken when assuming Ren as a substitute of Rox correction of mitochondrial respiration.
Residual oxygen consumption	ROX, Rox	Residual oxygen consumption Rox — respiration in the ROX state — is due to oxidative side reactions remaining after inhibition of the electron transfer pathway (ET pathway) in mitochondrial preparations or living cells. Different conditions designated as ROX states (different combinations of inhibitors of CI, CII, CIII and CIV) 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 respiration, OXPHOS or ET (R, L, P and E) respiration are distinguished from the corresponding Rox-corrected, mitochondrial (ET-pathway linked) fluxes: R(mt), L(mt), P(mt) and E(mt). Alternatively, R, L, P and E are defined as Rox-corrected rates, in contrast to total rates R´, L´, P´ and E´. When expressing Rox as a fraction of ET 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. In previous editions, (including Gnaiger 2020 BEC MitoPathways), the REN state was not distinguished from the ROX state. However, in novel applications (Q-Module and NADH-Module), a distinction of these states is necessary. Care must be taken when assuming Ren as a substitute of Rox correction of mitochondrial respiration.
Respiratory state		Respiratory states of mitochondrial preparations and living 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 electron-transfer-pathway state.
Resting metabolic rate	RMR	Resting 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 state	SGp	SGp: Succinate & Glycerophosphate. MitoPathway control state: SGp; obtained with OctPGMSGp(Rot) SUIT protocol: SUIT-001 and ((SUIT-002
Succinate pathway	S, SRot	The Succinate pathway (S-pathway; S) is the electron transfer pathway that supports succinate-linked respiration (succinate-induced respiratory state; previously used nomenclature: CII-linked respiration; SRot; see Gnaiger 2009 Int J Biochem Cell Biol). The S-pathway describes the electron flux through Complex II (CII; see succinate dehydrogenase, SDH) from succinate and FAD to fumarate and CII-bound flavin adenine dinucleotide (FADH₂) to the Q-junction. The S-pathway control state is usually induced in mt-preparations by addition of succinate&rotenone. In this case, only Complex III and Complex IV are involved in pumping protons from the matrix (positive phase, P-phase) to the negative phase (N-phase) with a P»/O₂ of 3.5 (P»/O ratio = 1.75).

Respiratory states: find synonyms, historically used and ambiguous terms

Term	Abbreviation	Redirected to
Complex I&II-linked substrate state	NS	See NS-pathway control state (previous: CI&II-linked)
Complex I-linked substrate state	N	See N-pathway control state (previous: CI-linked) versus Complex I
Complex II-linked substrate state	SRot, S	See S-pathway control state (previous: CII-linked)
ET-pathway competent state		Electron transfer pathway competent state, see Electron-transfer-pathway state.
Level flow	E	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 1		State 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 2	ROX_D	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 (ROX_D). 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 ROX_D.
State 3	P	State 3 respiration is the ADP stimulated respiration of isolated coupled mitochondria in the presence of high ADP and P_i 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 P_i (OXPHOS state). 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 well 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 mitochondrial inner membrane (mtIM) 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 3u	E	Noncoupled state of ET 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 ET capacity (E; State 3u) (Gnaiger 2009; Rasmussen and Rasmussen 2000).
State 4	L_T	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, L_T (L for LEAK respiration; 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; L_Omy). 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 5		State 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 head	L	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 state		See Electron-transfer-pathway state

References

Bioblast link	Reference	Year
Chance 1955 J Biol Chem-III	Chance B, Williams GR (1955) Respiratory enzymes in oxidative phosphorylation: III. The steady state. J Biol Chem 217:409-27.	1955
Chance 1955 J Biol Chem-I	Chance B, Williams GR (1955) Respiratory enzymes in oxidative phosphorylation. I. Kinetics of oxygen utilization. J Biol Chem 217:383-93.	1955
Chance 1956 Adv Enzymol Relat Subj Biochem	Chance B, Williams GR (1956) The respiratory chain and oxidative phosphorylation. Adv Enzymol Relat Subj Biochem 17:65-134.	1956
Gnaiger 2009 Int J Biochem Cell Biol	Gnaiger E (2009) Capacity of oxidative phosphorylation in human skeletal muscle. New perspectives of mitochondrial physiology. https://doi.org/10.1016/j.biocel.2009.03.013	2009
Gnaiger 2020 BEC MitoPathways	Gnaiger E (2020) Mitochondrial pathways and respiratory control. An introduction to OXPHOS analysis. 5^th ed. https://doi.org/10.26124/bec:2020-0002	2020
BEC 2020.1 doi10.26124bec2020-0001.v1	Gnaiger E et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. https://doi.org/10.26124/bec:2020-0001.v1	2020