A COMPETITIVE-INHIBITION OF THE MITOCHONDRIAL NADH-UBIQUINONE OXIDOREDUCTASE (COMPLEX-I) BY ADP-RIBOSE

Considerable 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 id entified as the inhibitor?, contaminating substance responsible for va riations in the inhibition observed. ADPR competitively inhibits NADH oxidation with K-i values (25 degrees C, pH 8.0) of 26 mu M, 30 mu M, and 180 mu M for SMP, purified Complex I and three-subunit NADH dehydr ogenase (FP), respectively. ADPR decreases NADH-induced flavin reducti on and prolongs the cyclic bleaching of FP during aerobic oxidation of NADH, K-i for inhibition of the rotenone-sensitive NADH oxidase in SM P by ADPR does not depend on <Delta(mu)over bar (H-)>. The initial rat e of the energy-dependent NAD(+) reduction by succinate is insensitive to ADPR. Tile inhibitor increases the steady-state level of NAD(+) re duction reached during aerobic succinate-supported reverse electron tr ansfer catalyzed by tightly coupled SMP. The results obtained are cons istent with the proposal on different nucleotide-binding sites operati ng in the direct and reverse fractions catalyzed by the mitochondrial NADH-ubiquinone reductase.