Pteridine Metabolism in the Skin of the Tadpole, Rana Catesbeiana.

The apparent structural similarity between purines and pteridines has been the subject of a number of studies attempting to demonstrate a relationship between these compounds in living systems. That a relationship does, in fact, exist has been shown, not only by purine incorporation into a pteridine (l-3), but also by the ability of at least one organism (Alcaligines fuecalis) to degrade a pteridine into a purine (4, 5). The above investigations were confined, however, to microorganisms. In higher organisms (6), after injection of guanine-2-14C into the larvae of the amphibian, Xenopus, a labeled compound was found which upon oxidation was converted to 2-amino-4hydroxypteridine-6-carboxylic acid. 9 similar attempt by these same investigators to show purine incorporation into pteridines in the frog was unsuccessful (6). Because of the presence of large numbers of pteridines in frog skin (7), this animal seemed to offer a rather unique system for study not only of the biogenesis of the pteridine ring, but also of the interrelationship between individual pteridines. Since some success had been achieved with the young of Xenopus, the immature forms of the frog, i.e. tadpoles, were used. In the present work, it has been found that when sections of skin taken from the bullfrog tadpole are immersed in solutions of guanine-2-14C or guanine-2,4J4C, incorporation of the purine into at least three highly fluorescent compounds does indeed occur. To establish beyond doubt that these compounds were pteridines, they were isolated in crystalline form and characterized as 2-amino-4-hydroxypteridine-6-carboxylic acid (I), isoxanthopterin (11), and biopterin (111) (Formula 1). The characterization of these compounds confirmed the chromatographic work of Hama and Obika (7). The studies wit,h doubly labeled guanine have indicated quite clearly that the entire pyrimidine moiety of the purine was incorporated as a unit into the corresponding part of the pteridine in much the same way as it is incorporated into the pteridine 6methyl-%( I-n-ribityl)-2,4,7-triosohexahydropteridine isolated from the yeast, Eremothecium ashbyii (1, 8). The efficiency of incorporation of purines into pteridines, although high in the yeast, varies in tadpole skin from about 4 to 67, of the specific activity of the purine isotope. Such a seemingly low rate of incorporation may be due to the relatively large amount of pteridine present in the tissue init.ially, which effectively dilutes any new pteridine formed during the incubation period. Others (6), however, have speculated that the low rate of purine incorporation into a pteridine, or the absence of any incorporation, could be the result of an alternate mechanism of pteridine biogenesis, which resembles purine biosynthesis up to the point of ring closure but differs thereafter. It would be of interest to determine whether one pathway predominates in the immature form of the frog, and another as the animal matures. This might explain the inability to demonstrate purine incorporation into a pteridine in the adult animal (6).