Significance of hepatic xanthine oxidase and uric acid in aged and dietaryrestricted rats

Xanthine oxidase (XOD), one of the major intracellular sources of superoxid e production, is well characterized as a causative factor in ischemia/reper fusion related damage. In the present study, we investigated age-effect on the status of XOD, an enzyme interconvertible with xanthine dehydrogenase ( XDH) under oxidative stress. We also examined the modulation of the enzyme using the antioxidative action of dietary restriction (DR). We obtained evi dence showing XOD activity to be significantly increased by DR, peaking at 24 months, although no progressive, age-related changes were noticed. On th e other hand, while XDH activity decreased in ad libitum fed rats with age, DR maintained higher activity levels at 18 and 24 months of age. During ag ing, the conversion of XDH to XOD was slightly increased, as indicated by t he XOD/XDH ratio. One novel finding of the present study is DR's ability to elevate the uric acid level, which likely augments the anti-oxidative defe nse system, thereby buffering against oxidatively stressed conditions durin g aging. Based on what is known about the antioxidative abilities of DR and uric acid, we propose that the high uric acid levels we observed in DR rat s may well serve as part of a defense strategy to protect redox balance.