Studies on Uric Acid and Related Compounds Iii. Observations on the Specificity of Mammalian Xanthine Oxidases by Felix Bergmann

Xanthine oxidase (X0) has been thoroughly studied by many investigators and the range of possible substrates of this enzyme is well known (1, 2). In the early experiments, enzymatic activity was measured by decolorization of a suitable dyestuff such as methylene blue or by oxygen consumption. Therefore, the exact pathway of oxidation remained unknown for many substrates. The spectrophotometric method of Kalckar (3, 4), which is now being widely applied to hypoxanthine and xanthine, is much more specific. This method has been extended in the present and the following papers to identify the oxidation products of various purine derivatives unequivocally. In this way, the substrate specificity and other characteristic properties of xanthine oxidase have been determined. Our results enable us to draw certain conclusions about the mode of attachment of the substrate to the active center of the enzyme and about the mechanism of the dehydrogenation catalyzed by it. We have also found that among all methylated uric acids, which are formed after administration of methylated xanthines to animals, only the l-methyl derivative can be produced by direct action of X0. Thus, all other substituted uric acids must result from a different biochemical pathway. This problem will be dealt with in Paper IV.