FMN site‐independent energy‐linked reverse electron transfer in mitochondrial respiratory complex I

A simple assay procedure for measuring ATP‐dependent reverse electron transfer from ubiquinol to hexaammineruthenium (III) (HAR) catalyzed by mitochondrial respiratory complex I is introduced. The specific activity of the enzyme in this reaction and its sensitivity to the standard inhibitors and uncoupling are the same as with other well‐known electron acceptors, NAD+ and ferricyanide. In contrast to the reactions with these acceptors, the energy‐dependent HAR reduction is not inhibited by NADH‐OH, the specific inhibitor of NADH‐binding site. These results suggest that a catalytically competent electron connection exists between HAR and a redox component of complex I that is different from flavin mononucleotide bound at the substrate‐binding site.