Ubiquinone (coenzyme Q10, Figure 14) is so named because it is ubiquitous in virtually all living cells, excluding some bacteria and fungi, although the level is quite variable. Since most human tissues synthesize ubiquinone, it is not considered to be a vitamin.
Ubiquinone is primarily located in the inner mitochondrial membrane where it is essential for the production of the ATP required for all vital cellular functions (164). Until recently, ubiquinone was thought to function only in energy transduction; however, with the discovery that ubiquinone is also an antioxidant within subcellular membranes, new roles are now being recognized. Ubiquinone can regenerate reduced tocopherol, as depicted in Figure 3. In fact, within membranes the amount of ubiquinone is from three to thirty times that of tocopherol (165). Without ubiquinone, the regeneration of tocopherol would be very slow (166,167).