This article shows that insulin protects neurons against oxidative stress by, in part, an effect of preserving the intracellular uric acid level in neurons:
http://www.ncbi.nlm.nih.gov/pubmed/16140208 (Duarte et al., 2005)
They express the uric acid concentration in terms of percent of controls, and they only hint at the fact that the "standard" for intracellular urate is 476 uM. That's consistent with the concentration in the liver (100-300 uM, from references cited in this article):
http://ajpendo.physiology.org/cgi/content/full/280/2/E248 (pubmed: http://www.ncbi.nlm.nih.gov/pubmed/11158927?dopt=Abstract) (Nacide Ercan-Fang et al., 2001)
That article shows that urate can have a caffeine-like inhibitory effect on glycogen phosphorylase activity in the liver. It's difficult to find data on intracellular urate levels. It's surprising that there are so many articles on uric acid and so few that discuss intracellular concentrations and actual mechanisms by which its effects would occur. There's peroxynitrite scavenging, but I think that type of purine-analogue effect on glycogen phosphorylase is likely to be important. I mean, uric acid probably has many effects like that, effects that are non-antioxidant effects (which is not to say that peroxynitrite scavenging is not an important effect). But excessive accumulation of urate intracellularly, in response to high intracellular purine levels, could be both protective and harmful in some ways (depending on the conditions).
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