The 13C-NMR spectra are characteristic and provide a convenient method to chart the pathway of uricase-catalysed oxidation of [5-13C]urate (1) to [4-13C]allantoin (5); 5-hydroxy[5-13C]isouric acid (2), its ring-opened derivative 3, and [4-13C]allantoin-5-carboxylate (4) were identified as consecutive intermediates in D2O/phosphate (pD 7.2).
