ET capacity

high-resolution terminology - matching measurements at high-resolution

ET capacity

Description

ETS capacity is the respiratory electron transfer system capacity, E, of mitochondria in the experimentally induced non-coupled (fully uncoupled) state. In this condition E, the mt-membrane potential is almost fully collapsed and provides a reference state for flux control ratios. 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 ETF, and glycerophosphate dehydrogenase) 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 titration of the uncoupler to the concentration inducing maximum flux. > MiPNet article

Abbreviation: E

Reference: Gnaiger 2012 MitoPathways, Gnaiger 2009 Int J Biochem Cell Biol

MitoPedia methods: Respirometry 

MitoPedia topics: "Respiratory state" is not in the list (Enzyme, Medium, Inhibitor, Substrate and metabolite, Uncoupler, Sample preparation, Permeabilization agent, EAGLE, MitoGlobal Organizations, MitoGlobal Centres, ...) of allowed values for the "MitoPedia topic" property. Respiratory state"Respiratory state" is not in the list (Enzyme, Medium, Inhibitor, Substrate and metabolite, Uncoupler, Sample preparation, Permeabilization agent, EAGLE, MitoGlobal Organizations, MitoGlobal Centres, ...) of allowed values for the "MitoPedia topic" property. 

Gnaiger E (2014) Why not State 3u? Mitochondr Physiol Network 2014-04-18.

OROBOROS (2014) Mitochondr Physiol Network

Abstract: Measurement of ETS capacity in the noncoupled state at optimum uncoupler concentration represents no general substitute for determination of OXPHOS capacity or State 3.

• O2k-Network Lab: AT Innsbruck Gnaiger E

Labels:

Coupling state: ETS"ETS" is not in the list (LEAK, ROUTINE, OXPHOS, ET) of allowed values for the "Coupling states" property. 

HRR: Theory 

Why not State 3u?

The important difference between states P and E

The abbreviation State 3u is used frequently in bioenergetics, to indicate the non-coupled state of maximum respiration, E, without sufficient emphasis on the fundamental difference between state P (OXPHOS capacity; coupled, with an uncoupled component; State 3) and state E (ETS capacity, non-coupled).

• P=E: The specific case of equal OXPHOS and ETS capacity (P/E=1) yields the important information that the capacity of the phosphorylation system matches or is in excess of the ETS capacity, such that OXPHOS capacity is not limited by the phosphorylation system in the specific mitochondria. This varies with species and tissues, and changes as a result of pathologies due to defects in the phosphorylation system. An example for P/E=1 is mouse skeletal muscle mitochondria (Aragones_2008_NatureGen).
• P<E: When OXPHOS is less than ETS capacity, the phosphorylation system limits OXPHOS capacity, and there is an apparent ETS excess capacity. For example, this is the case in healthy human skeletal muscle mitochondria (Pesta_2011).
• P>E: If ETS is less than OXPHOS capacity in intact cells, or in mitochondrial preparations with defined substrate(s), then you have encountered an experimental artefact, and the apparent ETS capacity is too low. Artificially low ETS capacity may be obtained due to overtitration of uncoupler. Inhibitors of ATP synthase may suppress ETS capacity in intact cells, particularly in stressed cells.

Consequences for evaluation of coupling

In some textbooks on Bioenergetics, the RCR is defined as either the State 3/State 4 ratio or the State 3u/State 4 ratio. This reflects lack of conceptual and terminological distinction between State 3 (or P) and 3u (E): ETS capacity but not OXPHOS capacity provides a valid reference for an index of uncoupling. This confusion requires clarification, which is best achieved by replacing not only the ambiguous term '3u', but also by restricting RCR to the original definition as 'acceptor control ratio' or 'adenylate control ratio' (see LEAK control ratio, L/E).

References

• Gnaiger 2009 Int J Biochem Cell Biol
• Gnaiger 2012 MitoPathways
• Bioblast alert 2013(02): CCCP versus FCCP for high-resultion respirometry (HRR).
• Gnaiger E. Biochemical coupling efficiency: from 0 to <1. Mitochondr Physiol Network. >> Biochemical coupling efficiency
• Gnaiger E. Electron transfer system versus electron transport chain. Mitochondr Physiol Network. >> Electron transfer system