Significance of hepatic xanthine oxidase and uric acid in aged and dietary restricted rats.

Xanthine oxidase (XOD), one of the major intracellular sources of superoxide production, is well characterized as a causative factor in ischemia/reperfusion 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 anti-oxidative action of dietary restriction (DR). We obtained evidence showing XOD activity to be significantly increased by DR, peaking at 24 months, although no progressive, age-related changes were noticed. On the 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 aging, the conversion of XDH to XOD was slightly increased, as indicated by the 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 defense system, thereby buffering against oxidatively stressed conditions during 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 rats may well serve as part of a defense strategy to protect redox balance.