The transverse organisation of ubiquinones in mitochondrial membranes as determined by fluorescence quenching. Evidence for a two-site model.

The transverse organisation of ubiquinone in mitochondrial membranes was investigated by quenching a set of fluorescent fatty acids. We show that the fluorescent moiety of the probes is located at a graded series of depths in the mitochondrial membrane. The probes sense the characteristics of the lipid phase and do not significantly perturb mitochondrial function as measured by the respiratory control ratio and the ADP/O ratio. The anthroyloxy fatty acids are readily quenched by ubiquinone-10. A recently developed method in the analysis of quenching data was used to obtain the subvolume of the membrane within which the quenching interactions are confined. The results indicate that ubiquinone-10 is restricted to two sites in the transverse plane of the membrane: one near the surface and the other close to the bilayer centre. The implications of these findings for the "two-pool" model of ubiquinone organisation are discussed.