Studies on uric acid and related compounds. I. Quantitative determination of uric acid in biological fluids.

The available methods of determination of uric acid can be divided into three main groups: (a) the direct determination of the reducing power of uric acid; (b) the coprecipitation of the uric acid or its adsorption on a precipitating material, dissolving the acid, and determination of its reducing power; and (c) quantitative decomposition of uric acid by uricase. A survey of these methods has been given recently (1). The direct determination of the reducing power of uric acid is not specific at all, and the reaction is not stoichiometric. Therefore, slight variations in the preparation of the oxidizing reagent influence the results considerably. In addition, the non-linear relationship between concentration and optical density makes accurate measurement difficult. The method of precipitation is more specific, but again suffers from lack of a stoichiometric reaction in the final determination. The enzymatic determination is sufficiently specific and accurate (a), but requires exactly controlled conditions and new standards whenever the enzyme preparation is changed. Therefore the uricase method is not easy to handle and is unsuitable for routine work. None of the methods mentioned permits the quantitative analysis of substituted uric acids such as appear in serum and urine after ingestion of xanthines. The only method applicable so far to this problem involves complicated counter-current extractions (3). The lack of a quick and reliable method for uric acid determination, especially in plasma analysis, led Peters and Van Slyke to make the statement (4) that “values given for uric acid of whole blood, or even plasma, must be accepted with reservations.” Likewise uric acid clearance has not yet been satisfactorily measured owing to the lack of “analytical methods of undoubted specificity and accuracy” (4). It is the purpose of this series of investigations to study quantitatively the clearance of uric acid, the metabolism of xanthines, and the influence of these substances or their metabolites on uric acid excretion. Undoubtedly the most accurate analytical method for uric acid consists of the determination of the optical density at 290 rnp (5) ; however, numerous other metabolites absorb in this region, and, in urine, urochrome as well