Ubisemiquinone in the NADH-ubiquinone reductase region of the mitochondrial respiratory chain

Coupled bovine heart submitochondrial particles exhibit a rotenone-sensitive g = 2.00 low-temperature EPR signal attributable to ubisemiquinone which is observed during steady-state electron transfer from NADH to oxygen or from succinate to NAD⁺ in Δ \ ̃gmH⁺-dependent reverse electron transfer. Quantitation of the signal under optimal conditions gives a value for maximal semiquinone of approx. 0.5 spins per spin of the fully reduced NADH dehydrogenase iron-sulfur center N-2. The intensity of the signal is drastically reduced when electron transfer from NADH to oxygen is blocked by cyanide or in the case of the reverse electron transfer from succinate to NAD⁺ being prevented by anaerobiosis. Only those particles which contain 'turnover-preconditioned" NADH-ubiquinone reductase demonstrate the ubisemiquinone signal together with N-1, N-2, N-3 and N-4 iron-sulfur centers. The spin relaxation characteristics of the rotenone-sensitive ubisemiquinone signal point to its interaction with one of the rapidly relaxing (4Fe-4S) centers, most probably N-2.