ИСТИНА

The substrate (NAD+/NADH) binding site of the respiratory complex I is specifically blocked by NADH-OH, a derivative of NADH [1,2]. This tightly binding inhibitor prevents the proton motive force (pmf)-generating NADH:quinone reductase and/or reverse pmf-utilizing quinol:NAD+ (or :ferricyanide (Ferri)) reductase activities as well as non coupled FMN-mediated NADH:Ferri or :hexaammineruthenium III (HAR) reductase activities [3,4]. A simple procedure for measuring ATP-dependent reverse electron transfer (RET) from ubiquinol to HAR as catalyzed by coupled bovine heart submitochondrial particles or coupled Paracoccus denitrificans plasma membrane vesicles is introduced. ATP induces HAR-mediated oxygen consumption by the respiratory chain-inhibited particles supplemented with succinate. In contrast to pmf-dependent RET with NAD+ or Ferri, the reaction with HAR is insensitive to NADH-OH. The results suggest that a site (or mechanism) of HAR reduction in RET catalyzed by complex I is different from that for NAD+ or Ferri. Two possible explanations are discussed: (i) electron connection exists between reduced FMN and HAR different from that for NAD+ and Ferri; (ii) during RET HAR accepts electrons from iron-sulfur N-2 cluster –quinone junction site.