Sepsis-induced acute kidney injury remains a major clinical problem with no effective therapy established to date. Although potential therapeutic targets have been identified and tested in clinical trials, none of them has proven to be effective, underscoring the complexity of sepsis pathophysiology. Work from our laboratory identified disturbances in cell-cell communication as a major feature underlying the complex pathophysiology of sepsis-induced kidney damage. For example, pathologic signaling from S1 tubules to downstream S2/S3 tubules was a major pathway of renal injury in sepsis. Indeed, we identified the S1 proximal tubular segment as a major sensor and sink for filtered endotoxin. This S1 uptake of endotoxin resulted in severe oxidative stress and damage to downstream S2/S3 segments. Targeted S1 transcriptomics and tissue metabolomics revealed that purine metabolism is markedly altered in septic S1 proximal tubules. This shift is catalyzed by xanthine oxidoreductase, a key enzyme in the purine degradation pathway. This enzyme functions as a dehydrogenase (XDH) in its native form but is readily converted to an oxidase (XO) through sulfhydryl oxidation or proteolytic modification. Whereas catalysis by XDH does not generate free radicals, XO is a strong pro-oxidant that generates deleterious reactive species. Furthermore, the pro-oxidant XO is secreted into the urinary lumen and can therefore cause damage to downstream segments. In contrast, XO expression in macrophages may be beneficial because it stimulates the bactericidal potential of these cells through generation of reactive species. It is one goal of this proposal to establish the relative roles of XO versus XDH in S1 and macrophages in septic kidneys. Moreover, because XO/XDH is involved in the final degradation of purines, the changes in XO/XDH activity will not only affect the generation of reactive species but will also have an upstream impact on the size of purine pools. It is the second goal of this proposal to determine the overall flux of purine metabolites and their contributions to purine homeostasis in sepsis.