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The catabolism of purine nucleotides proceeds by hydrolysis to the nucleoside and subsequently to the free base, which is further degraded. Deamination of guanine produces xanthine, and deamination of adenine produces hypoxanthine, the base corresponding to the nucleoside inosine, which is shown in Figure 23.23a. Hypoxanthine can be oxidized to xanthine, so this base is a common degradation product of both adenine and guanine. Xanthine is oxidized in turn to uric acid. In birds, some reptiles, insects, Dalmatian dogs, and primates (including humans), uric acid is the end product of purine metabolism and is excreted. In all other terrestrial animals, including all other mammals, allantoin is the product excreted, whereas allantoate is the product in fish. Allantoate is further degraded to glyoxylate and urea by microorganisms and some amphibians, as shown in Figure 23.23b. Gout is a disease in humans that is caused by the overproduction of uric acid. Deposits of uric acid (which is barely soluble in water) accumulate in the joints of the hands and feet. Allopurinol is a compound used to treat gout; it inhibits the degradation of hypoxanthine to xanthine and of xanthine to uric acid, preventing the buildup of uric acid deposits.
Salvage reactions are important in the metabolism of purine nucleotidesbecause of the amount of energy required for the synthesis of the purine bases. A free purine base that has been cleaved from a nucleotide can produce the corresponding nucleotide by reacting with the compound phosphoribosylpyro-phosphate (PRPP), formed by a transfer of a pyrophosphate group from ATP to ribose-5-phosphate (Figure 23.24).
Two different enzymes with different specificities with respect to the purine base catalyze salvage reactions. The reaction
Adenine + PRPP - > AMP + PPi
is catalyzed by adenine phosphoribosyltransferase. The corresponding reactions of guanine and hypoxanthine
Hypoxanthine + PRPP - > IMP + PPi
Guanine + PRPP - > GMP + PPi
are catalyzed by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Figure 23.25). A deficiency in HGPRT can result in a serious disorder, Lesch–Nyhan syndrome.
Purines are degraded to uric acid in primates (including humans) and further degraded in other organisms. Overproduction of uric acid causes gout in humans.
Salvage reactions exist for reuse of some of purines.
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