Assays of Oxypurines

The oxypurines can be determined by their ultraviolet absorption, changes in absorption following reaction with enzymes, colorimetric reactions, or combinations of these methods. The assay of large quantities of reasonably pure oxypurines is quite simple practical problems, however, arise in the determination of small quantities in the presence of unknown impurities. No completely satisfactory methods for the determination of these compounds have been developed. One of the difiBculties in interpreting some of the results in the literature is to decide whether the cerebrations of the investigators exceed the limitations of their methods. 

A colorimetric method for determination of uric acid was introduced by Folin and Denis (F5) this initial method, which used phospho-tungstate, was modified by a series of investigators (B16, B32, F4). Brown 

The recent introduction of automatic analyzers with their standard and reproducible conditions has made the colorimetric method consistent and accurate, although the problem of the nonspecific blank still remains. This method is undoubtedly the quickest, easiest, and most reliable for routine analyses of serum uric acid (B40, 03). 

Methods have been devised for the determination of these compounds in the presence of uric acid through the use of uricase. The uricase is destroyed and the xanthine and hypoxanthine determined enzymatically. In the absence of uric acid (e.g., patients with xanthinuria), addition of xanthine oxidase converts the hypoxanthine and xanthine to uric acid and any method for uric acid determination could then be employed. The determination of hypoxanthine and xanthine by any method is complicated if the patient has received allopurinol, since allopurinol does not separate well from xanthine on ion-exchange resins and both allopurinol and alloxanthine are inhibitors of xanthine oxidase. Carefully 

Abderholden, E., and Schittenhehn, A., Der Ab und Aufbau Nucleinsauren in Tierischen Organismua. Z. Physiol. Chem. 47, 452-457 (1906). 

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