Decylubiquinol impedes mitochondrial respiratory chain complex I activity

We have studied the interaction of decylubiquinone, an effective substrate for respiratory chain complexes III and II, with complex I in mouse and human tissues. We found that its reduced form, decylubiquinol, severely impedes complex I activity, while the oxidized form, decylubiquinone acts as a potent acceptor for complex I electrons. This observation has obvious incidence on the assay conditions for complex I. In keeping with that, we found that the inhibition by the reduced form can be avoided by maintaining decylubiquinone under an oxidized form. Under these experimental conditions, a high complex I activity could be measured allowing to detect partial complex I deficiency. Use of these conditions is however restricted to tissues/cells with limited contaminating NADH dehydrogenase activities that are prone to react with redox active compounds.