Damage to skin tissue caused by ionizing radiation (IR) is a major problem that can lead to severe injury months to years after exposure, and poses serious complications for the long-term health of radiotherapy patients. The molecular and biochemical bases of early and late IR-induced skin injury are not well understood, but our preliminary studies have shown that mitochondria-generated reactive oxygen species (ROS) contribute to IR-induced skin pathologies. Our preliminary results also suggest a role for tetrahydrobiopterin (BH4), a major cofactor in numerous enzymes and mitochondrial function, as well as in the effects on skin observed following exposure to IR. Therefore, the overall goal of the proposed project is to begin to unravel the interactive mechanisms of IR-induced changes in mitochondrial function and BH4 bioavailability. In pursuit of this goal, we will test the central hypothesis that IR induces mitochondrial ROS generation, thus impairing BH4 metabolism and increasing mitochondrial dysfunction leading to skin tissue injury through the following specific aims. In Aim 1, we will determine the role of mitochondrial ROS and its relation to BH4 metabolism during IR-induced skin toxicity. Aim 2 will elucidate the role of BH4 availability through altered expression of GTP cyclohydrolase 1 feedback regulatory protein (GFRP) in skin and mitochondrial toxicity following IR exposure. In Aim 3, we will determine whether increasing BH4 bioavailability and/or mitochondrial function via pharmacologic interventions mitigates adverse skin pathologies following IR exposure. Successful completion of these specific aims will contribute to the mechanistic understanding of the role of mitochondriagenerated ROS and BH4 metabolism during IR-induced skin injury. The knowledge to be gained holds the potential for identifying novel targets for pharmacologic manipulation for the health problems arising from IR exposures.