Figure 4

Enzymatic activity of uricase (uric acid oxidase). Uricase oxidizes uric acid, which is sparingly soluble, to the highly soluble end product allantoin, which is readily excreted in the urine. In doing so, uricase generates not only intermediate forms of uric acid that are subject to further metabolism (including 5-hydroxyisourate), but also the oxidant hydrogen peroxide as a byproduct of the enzymatic reaction. During evolution, humans and higher primates lost expression of not only uricase, but also enzymes that rapidly degrade intermediate forms of uric acid generated by uric acid oxidation.