618 G. SCHMIDT 



These authors proposed instead for the intermediary a hypothetical struc- 

 ture which represents a dehydration product of Schuler and Reindel's car- 

 boxylic acid, and which contains a tautomeric double bond in one of the 

 cyclic ureide moieties (II). According to these authors, uric acid is dehy- 

 drogenated to a carbonium compound (III) which isomerizes to an unstable 

 cyclic quaternary ammonium compound (IV). This is spontaneously 

 hydrolyzed to II. 



Evidence in favor of the nonenzymic nature of the decarboxylation reaction 

 ofuricolysis. The recent hypotheses regarding the nature of the intermediary 

 substance or substances^^^ of enzymic uricolysis depend largely on the as- 

 sumption of a nonenzymic terminal step of decarboxylation. Three reasons 

 are usually given in favor of the nonenzymic nature of this step and of the 

 enzymic homogeneity of uricase: (1) the preservation of the complete 

 enzyme system through numerous fractionation steps in all purification 

 procedures; (2) the chemical lability of the intermediary products; (3) the 

 observations initiated by Klempererj^** who found that, in addition to 

 allantoin, other end products are obtained during the enzymic uricolysis 

 in borate buffer. In such digests, urea, allantoin, alloxanic acid, and oxal- 

 uric acid were recently identified by chromatographic procedures.-" 



It is clear that none of these three reasons excludes the possibility of the 

 presence of a specific decarboxylase in the uricase preparations, although the 

 heterogeneity of the end products obtained in borate buffers renders their 

 direct enzymic formation unlikely. The spontaneous degradation of the 

 labile intermediary in the presence of borate could be explained by the pres- 

 ence of a borate-sensitive decarboxylase in the uricase system. 



Specificity. Uricase is a highly specific enzyme, uric acid being the only 

 purine derivative which is oxidized under its influence. Some uric acid de- 

 rivatives, however, are competitive inhibitors of uricase, such as 7-methyl- 

 uric acid and 1,3,7-trimethyluric acid.-*^ Owing to this high specificity, 

 uricase is used as a specific reagent for the determination of uric acid. Many 

 of the recent procedures for the purification^^^'^^^ of uricase were devised 

 for this purpose. 



pH Optimum. Keilin and Hartree^^^ found the pH optimum for the action 



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