The NADH oxidation domain of Complex I: do bacterial and mitochondrial enzymes catalyze ferricyanide reduction similarly?

The hexammineruthenium (HAR) and ferricyanide reductase activities of Compl ex I (H+-translocating NADH:ubiquinone reductase) from Paracoccus denitrifi cans and bovine heart mitochondria were studied. The rates of HAR reduction are high, and its steady-state kinetics is similar in both P. denitrifican s and bovine Complex I. The deamino-NADH:HAR reductase activity of Complex I from both sources is significantly higher than the respective activity in the presence of NADH. The HAR reductase activity of the bacterial and mito chondrial Complex I is similarly and strongly pH dependent. The pK(a) of th is activity could not be determined, however, due to low stability of the e nzymes at pH values above 8.0. In contrast to the high similarity between b ovine and P, denitrificans Complex I as far as HAR reduction is concerned, the ferricyanide reductase activity of the bacterial enzyme is much lower t han in mitochondria. Moreover, ferricyanide reduction in P, denitrificans, but not bovine mitochondria, is partially sensitive to dicyclohexylcarbodii mide (T. Yagi, Biochemistry 26 (1987) 2822-2838), On the other hand, the in hibition of ferricyanide reduction by high concentration of NADH, a typical phenomenon in bovine Complex I, is much weaker in the bacterial enzyme. Th e functional differences between the two enzymes might be linked to the pro perties of their binuclear Fe-S clusters. (C) 2000 Elsevier Science B.V. Al l rights reserved.