Abstract
The homogenous distribution of vitamin E in lipid membranes is a prerequisite for its universal function as lipophilic antioxidant. Its antioxidant activity leads to the irreversible formation of α-tocopheryl quinone (TQ) in those membranes. Very little is known about the interference of TQ with redox-cycling enzymes normally interacting with ubiquinone (UQ), which exerts important bioenergetic functions in the mitochondrial respiratory chain. One of the most complex redox reactions of the respiratory chain is the interaction of reduced UQ (UQH 2) with the cytochrome bc 1 complex (ubiquinol:cytochrome c reductase, EC 1.10.2.2). The aim of this study was to elucidate the influence of TQ on the electron transfer from UQH 2 to cytochrome c via the isolated mitochondrial cytochrome bc 1 complex. Although TQ is present in substoichiometric amounts with respect to UQ in mitochondria and in our experiments with isolated bc 1 complex, we observed a decrease of the total electron transfer rate via the bc 1 complex with increasing amounts of TQ. Both reduced TQ (TQH 2) and UQH 2 are able to reduce b-cytochromes in the bc 1 complex, however, they act in a completely different way. While reduction of b-cytochromes by UQH 2 can occur both via the Q o and the Q i pocket of the cytochrome bc 1 complex, TQH 2 can preferably reduce b-cytochromes via the Q i pocket. These differences are also reflected by the extremely low turnover numbers of the bc 1 activity for TQ/TQH 2 compared to UQ/UQH 2 suggesting that TQ/TQH 2 acts as a weak competitive inhibitor for binding sites of UQ/UQH 2. In contrast, the oxidation properties of TQ and UQ are similar. Furthermore, oxidized TQ was observed to decrease the O 2 − release rate of UQH 2-consuming cytochrome bc 1 complex. These findings suggest that the irreversible oxidation of vitamin E to TQ in mitochondrial membranes causes a downregulation of respiratory activities as well as a lower O 2 − formation rate by the cytochrome bc 1 complex.
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