Talk:ET capacity
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 [ Boushel_2007_Diabetologia ].
- 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 3 and 3u,: 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 the RCR (see LEAK control ratio, L/E).
--Erich 17:00, 18 September 2010 (CEST)
