Slow active/inactive transition of the mitochondrial NADH-ubiquinone reductase

NADH-ubiquinone reductase of bovine heart submitochondrial particles as prepared is unable to catalyze either the direct or reverse electron transfer from NADH to ubiquinone. The deactivated state of the enzyme in coupled particles was revealed as: (i) the absence of the rotenone-sensitive, Δ \ ̃gm_H⁺-dependent succinate-ferricyanide reductase activity; (ii) a prominent lag in the aerobic succinate-supported, Δ \ ̃gm_H⁺-dependent NAD⁺ reduction; and (iii) a lag in the rotenone-sensitive NADH-ubiquinone reductase or NADH oxidase activities. Being inactive as NADH-ubiquinone reductase (direct or reverse), the enzyme is fully active as rotenone-insensitive NADH-ferricyanide reductase. The enzyme can be activated by preincubation with substrates (NADH or NADPH) only under the conditions where the turnover of the NADH-ubiquinone reductase reaction (but not in the NADH-ferricyanide reductase) occurs. Partial activation of the enzyme was observed when the particles were preincubated with rotenone. Neither NADH under the conditions when the ubiquinone pool was reduced nor succinate plus Δ \ ̃gm_H⁺ or dithionite were able to activate the enzyme. Once activated, the enzyme remains in the active state for quite a long time (more than 5 h at 0 °C). The deactivation rate is extremely temperature-dependent, being insensitive to NAD⁺, ferricyanide or succinate. A comparison of the enzyme activation/deactivation kinetics showed that the same mechanism is involved in the slow activation of the direct and reverse electron transfer from NADH to ubiquinone. Activated particles catalyze the aerobic Δ \ ̃gm_H⁺-dependent succinate-supported reverse electron transfer in the absence of ATP at a rate comparable with that of NADH-ubiquinone reductase.