INHIBITION OF MITOCHONDRIAL AND PARACOCCUS-DENITRIFICANS NADH UBIQUINONE REDUCTASE BY OXACARBOCYANINE DYES - A STRUCTURE-ACTIVITY STUDY

In this study, we determined that three structurally related oxacarboc yanine dyes, 3,3'-diethyloxacarbocyanine (DiOC2(3)), 3,3'-dipentyloxac arbocyanine (DiOC5(3)), and 3,3'-dihexyl-oxacarbocyanine (DiOC6(3)), a nd one oxadicarbocyanine, 3,3'-diethyloxadicarbocyanine (DiOC2(4)), in hibit bovine heart mitochondrial NADH oxidase activity and one of them , DiOC6(3), inhibits Paracoccus denitrificans NADH oxidase activity. T he mitochondrial I50 values were 9 muM (DiOC2(3)), approximately 1 muM (DiOC5(3)) and DiOC6(3)), and approximately 3 muM (DiOC2(4)), whereas the I50 value for P. denitrificans was approximately 2 muM (DiOC6(3)) . Neither succinate nor cytochrome oxidase (EC 1.9.3. 1) activity was inhibited significantly by any of the compounds in either electron tra nsport chain, localizing the inhibitory site of the oxacarbocyanine dy es to the respiratory chain segment between NADH and ubiquinone. With submitochondrial particles (SMP), NADH-dependent reduction of duroquin one and coenzyme Q1 was inhibited markedly by all four compounds with DiOC6(3) being the most potent inhibitor, and the reduction of menadio ne was inhibited substantially by DiOC6(3). When purified complex I wa s used, NADH-dependent reduction of ferricyanide was inhibited by DiOC 5(3) and coenzyme Q1 reduction was inhibited by all oxacarbocyanines. With P. denitrificans membrane vesicles, DiOC6(3) substantially inhibi ted NADH-dependent reduction of coenzyme Q1. All the oxacarbocyanines were more effective inhibitors with membrane preparations than with co mplex I, suggesting that membrane interactions play a role in inhibiti on. The mechanism of inhibition of the oxacarbocyanines appears to be similar to that of rotenone since (a) essentially only electron accept ors affected by rotenone were affected by the compounds, (b) inhibitio n of menadione reduction was diminished drastically with rotenone-satu rated SMP, and (c) inhibition of coenzyme Q1 was largely eliminated wi th rotenone-insensitive complex I, and P. denitrificans membrane vesic les.