Serum urate as an antioxidant for ascorbic acid.

Urate serves as a potent antioxidant by means of radical scavenging and reducing activities. This antioxidant action is partly manifested by interaction with another powerful antioxidant, ascorbic acid, and is particularly evident in species that lack the ability to synthesize ascorbic acid. Urate not only behaves as a radical scavenger but also stabilizes ascorbate in biological fluids. This stabilizing effect appears to be due to an inhibition of iron-catalyzed oxidation of ascorbate. Ascorbate stabilization is particularly evident in human serum and is largely due to iron chelation by urate. Unlike radical-scavenging reactions, this protective effect of urate is not associated with its depletion because a stable, noncatalytic urate-iron complex is formed. Depletion of serum urate results in rapid subsequent oxidation of ascorbate, which is largely iron dependent. Sequential losses of urate and ascorbate significantly reduce the antioxidant capacity of serum.