Inhibition of mitochondrial and Paracoccus denitrificans NADH-ubiquinone reductase by oxacarbocyanine dyes : A structure-activity study

Abstract

In this study, we determined that three structurally related oxacarbocyanine dyes, 3,3'-diethyloxacarbocyanine (DiOC2(3)), 3,3'-dipentyloxacarbocyanine (DiOC5(3)), and 3,3'-dihexyl-oxacarbocyanine (DiOC6(3)), and one oxadicarbocyanine, 3,3'-diethyloxadicarbocyanine (DiOC2(4)), inhibit bovine heart mitochondrial NADH oxidase activity and one of them, DiOC6(3), inhibits Paracoccus denitrificans NADH oxidase activity. The mitochondrial I 50 values were 9 μM (DiOC2(3)), ∼1 μM (DiOC5(3)) and DiOC6(3)), and ∼3 μM (DiOC2(4)), whereas the I 50, value for P. denitrificans was ∼2 μM (DiOC6(3)). Neither succinate nor cytochrome oxidase (EC 1.9.3.1) activity was inhibited significantly by any of the compounds in either electron transport chain, localizing the inhibitory site of the oxacarbocyanine dyes to the respiratory chain segment between NADH and ubiquinone. With submitochondrial particles (SMP), NADH-dependent reduction of duroquinone and coenzyme Q 1 was inhibited markedly by all four compounds with DiOC6(3) being the most potent inhibitor, and the reduction of menadione was inhibited substantially by DiOC6(3). When purified complex I was used, NADH-dependent reduction of ferricyanide was inhibited by DiOC5(3) and coenzyme Q 1 reduction was inhibited by all oxacarbocyanines. With P. denitrificans membrane vesicles, DiOC6(3) substantially inhibited NADH-dependent reduction of coenzyme Q 1. All the oxacarbocyanines were more effective inhibitors with membrane preparations than with complex I, suggesting that membrane interactions play a role in inhibition. The mechanism of inhibition of the oxacarbocyanines appears to be similar to that of rotenone since (a) essentially only electron acceptors affected by rotenone were affected by the compounds, (b) inhibition of menadione reduction was diminished drastically with rotenone-saturated SMP, and (c) inhibition of coenzyme Q 1 was largely eliminated with rotenone-insensitive complex I, and P. denitrificans membrane vesicles.