Quantitative separation of uric acid and allantoin from rat liver tissue

A simple procedure is described for the assay of liver uric acid and allantoin and their specific radioactivity after administration of a radioactive precursor. Uric acid was quantified by the uricase reaction in liver trichloroacetic acid (TCA) extracts. The 'true' allantoin content of the liver could be estimated only after precipitation with Hg-acetate, a step by which the standard allantoin was also quantitatively recovered. Crude extracts lead to the evaluation of 'apparent' allantoin. For the determination of specific radioactivity, the Hg-acetate precipitate was further purified by ion-exchange chromatography. The purity of the two metabolites was confirmed by ultraviolet absorbance spectra, HPLC, constancy of specific radioactivity and the absence of amino acids. The incorporation of [14C]formate into uric acid and allantoin in the liver was studied by this procedure. The radioactivity in allantoin was several-fold higher than that in uric acid up to 60 min after administration of the precursor. This quite unexpected result is not easily explained on the basis of current knowledge.