Table 1.

Definitions of Substrate States and Coupling States used to Characterize Mitochondrial Energetics.

Abbreviation	Definition
CI-linked	The Complex I-linked substrate state is induced in mt-preparations by addition of NADH-generating substrates.
CII-linked	The Complex II-linked substrate state is induced in mt-preparations by addition of succinate and rotenone (Complex I inhibitor).
CI&II-linked	The Complex I&II-linked substrate state is induced in mt-preparations by addition of NADH-generating substrates (CI-linked) in combination with succinate (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. An additive effect of convergent CI&II-linked electron flow is observed in most types of mitochondria.
R	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 of ADP (intrinsic uncoupling and pathological dyscoupling; [16]).
L	LEAK respiration or LEAK oxygen flux 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 state L, 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. 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 [17].
P	OXPHOS capacity is the respiratory capacity of mitochondria in the ADP-activated state of oxidative phosphorylation, at saturating concentrations of ADP, inorganic phosphate, oxygen, and defined reduced substrates [17]. It thus differs from State 3 respiration which is respiration of isolated coupled mitochondria in the presence of high ADP and Pi concentrations [18]. ADP concentrations applied in State 3 are not necessarily saturating, whereas OXPHOS capacity is measured at saturating concentrations of ADP and Pi.
E	The mitochondrial electron transfer system (ETS) 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 [19]. ETS capacity is max. O2 flux at optimum uncoupler concentration.