A competitive inhibition of the mitochondrial NADH-ubiquinone oxidoreductase (Complex I) by ADP-ribose

AbstractConsiderable quantitative variations in the competitive inhibition of NADH oxidase activity of bovine heart submitochondrial particles (SMP) by different samples of NAD+ were observed. ADP-ribose (ADPR) was identified as the inhibitory contaminating substance responsible for variations in the inhibition observed. ADPR competitively inhibits NADH oxidation with Ki values (25°C, pH 8.0) of 26 μM, 30 μM, and 180 μM for SMP, purified Complex I and three-subunit NADH dehydrogenase (FP), respectively. ADPR decreases NADH-induced flavin reduction and prolongs the cyclic bleaching of FP during aerobic oxidation of NADH. Ki for inhibition of the rotenone-sensitive NADH oxidase in SMP by ADPR does not depend on Δμ̄H+. The initial rate of the energy-dependent NAD+ reduction by succinate is insensitive to ADPR. The inhibitor increases the steady-state level of NAD+ reduction reached during aerobic succinate-supported reverse electron transfer catalyzed by tightly coupled SMP. The results obtained are consistent with the proposal on different nucleotide-binding sites operating in the direct and reverse reactions catalyzed by the mitochondrial NADH-ubiquinone reductase