Oxidative stress is thought to play a significant role in biological pathways accelerating the skin aging process (premature skin aging). Our studies of fibroblasts from skin biopsies of young and old donors reveal a number of significant differences in their oxidative status. First, old donors show significantly higher levels of endogenous oxidative stress compared to young donors, as indicated by intracellular peroxides. Furthermore, the old donor group exhibits a higher UVA-modulated stress response in terms of increased protein tyrosine phosphorylation. We also demonstrate that UVA-induced oxidative stress significantly reduces the mitochondrial membrane potential (MMP) of old donors, leading to an enhancement of tyrosine phosphorylation. Our findings provide strong evidence that these effects are caused by oxidative inhibition of protein tyrosine phosphatase activity.We can substantially inhibit the UVA-induced depletion of MMP both in vitro and in vivo by incubation/topical application with the endogenous antioxidant and energizer coenzyme Q10 (CoQ10). In addition, we observe that CoQ10 bolsters DNA and hyaluronate synthesis in vitro by stabilizing mitochondrial protein complexes. Our in vivo investigations of the antioxidant status of elderly versus young skin using the ultra weak photon emission technique (UPE) shows a significantly diminished antioxidant state in elderly skin, which is improved by topical application of CoQ10. In placebo controlled clinical studies on elderly volunteers, we observe a significant reduction of facial wrinkles and fine lines on CoQ10 treated skin sites, thought to be a result of enhanced synthesis of hyaluronate. Both CoQ10 and its cellular reduced form ubiquinol are involved in the energy supplying system of the respiratory chain. Their dual function may serve to explain the substantial efficacy of topically applied CoQ10 in elderly skin. Oxidative stress is thought to play a significant role in biological pathways accelerating the skin aging process (premature skin aging). Our studies of fibroblasts from skin biopsies of young and old donors reveal a number of significant differences in their oxidative status. First, old donors show significantly higher levels of endogenous oxidative stress compared to young donors, as indicated by intracellular peroxides. Furthermore, the old donor group exhibits a higher UVA-modulated stress response in terms of increased protein tyrosine phosphorylation. We also demonstrate that UVA-induced oxidative stress significantly reduces the mitochondrial membrane potential (MMP) of old donors, leading to an enhancement of tyrosine phosphorylation. Our findings provide strong evidence that these effects are caused by oxidative inhibition of protein tyrosine phosphatase activity. We can substantially inhibit the UVA-induced depletion of MMP both in vitro and in vivo by incubation/topical application with the endogenous antioxidant and energizer coenzyme Q10 (CoQ10). In addition, we observe that CoQ10 bolsters DNA and hyaluronate synthesis in vitro by stabilizing mitochondrial protein complexes. Our in vivo investigations of the antioxidant status of elderly versus young skin using the ultra weak photon emission technique (UPE) shows a significantly diminished antioxidant state in elderly skin, which is improved by topical application of CoQ10. In placebo controlled clinical studies on elderly volunteers, we observe a significant reduction of facial wrinkles and fine lines on CoQ10 treated skin sites, thought to be a result of enhanced synthesis of hyaluronate. Both CoQ10 and its cellular reduced form ubiquinol are involved in the energy supplying system of the respiratory chain. Their dual function may serve to explain the substantial efficacy of topically applied CoQ10 in elderly skin.