Spontaneous degradation of 5-HIU and OHCU. Uricase was pre-incubated with uric acid during 60 seconds to produce 5-HIU (data not shown) and degradation was followed at 312 nm. The degradation of both 5-HIU and OHCU was followed at 257 nm. Since spontaneous degradation of 5-HIU is complete in 2640 seconds (from 60 to 2700 seconds), as verified at 312 nm, after this point (marked with dash line), absorbance curve at 257 nm is exclusively due to the spontaneous degradation of OHCU. (X axis starts at 60 s).