A Method for the Determination of Allantoin in Rabbit Urine

During the past 10 years, the methods for the determination of uric acid have been gradually improved, so that short, accurate methods are now available for the estimation of this important catabolite. On the other hand, during the same period, there have been no marked improvements in the methods for allantoin, which are long, tedious, and probably not accurate, especially for small amounts of allantoin. As a consequence, most of the recent studies on purine metabolism have been made on human subjects, who excrete uric acid as the principal end-product of purine metabolism, rather than on those mammals in which allantoin is assumed to have a metabolic significance similar to that of uric acid. One group of methods for the estimation of allantoin depends upon the precipitation of allantoin from solution as a metallic salt. The methods of this type, which were published prior to that of Wiechowski (I), in 1909, may be dismissed without further consideration, since they have been found to be unreliable. The method of Wiechowski, which is the standard method at the present time, depends on the fact that if urine is diluted so that the urea concentration is 1 per cent or less, and is freed from chlorides and substances precipitated by phosphotungstic acid and basic lead acetate, the allantoin may be completely precipitated by mercuric acetate in the presence of a large amount of sodium acetate. The allantoin may be isolated and determined gravimetrically, or the nitrogen content of the mercury allantoin compound may be estimated. Handovsky (2), in 1914, modified the latter part of the procedure recommended by Wiechowski by the precipitation of the allantoin with a measured amount of a standard mercuric acetate-sodium acetate solution. After removal of the mercury allantoin salt by filtration, the excess of mercuric acetate in the filtrate is determined by titration with standard ammonium thiocyanate. The second group of methods is based on the fact that, under definite conditions, the allantoin nitrogen as well as the urea nitrogen of a urine may be hydrolyzed to ammonia. A separate determination of the pre-