Fluidizing effect of endogenous ubiquinone in bovine heart mitochondrial membranes

Extraction of endogenous ubiquinone from lyophilized beef heart mitochondria results in increases of both the order parameter of the spin label 5-NS and of the rotational correlation time of 16-NS; reconstitution with the pentane extract results in restoration of the original spectral parameters. On the other hand, addition of purified ubiquinone homologs restores the original spectra only in the case of 16-NS, whereas the order parameter of 5-NS is restored by addition of mixed phospholipids. The same amounts of ubiquinone homologs incorporated in mixed phospholipid vesicles induce much lower effects. It is suggested that ubiquinone in mitochondria is intercalated with the lipid chains of the membrane in such a way to perturb the fluidity of the hydrophobic core.