Purine free bases, catabolism

Figure 25-18 Pathways of catabolism of purine nucleotides, nucleosides, and free bases. Spiders excrete xanthine while mammals and birds excrete uric acid. Spiders and birds convert all of their excess nitrogen via the de novo pathway of Fig. 25-15 into purines. Many animals excrete allantoin, urea, or NH4+. Some legumes utilize the pathway marked by green arrows in their nitrogen transport via ureides.

FIGURE 23.23 The reactions of purine catabolism, (a) Purine nucleotides are converted to the free base and then to xanthine, (b) Catabolic reactions of xanthine. 

The ribonucleoside derivative of AIR, 5-aminounidazole ribonucleoside, has been isolated from culture filtrates of a purine-requiring mutant of E. coli (124). It was probable that the ribonucleotide was formed first within the bacterial cell, dephosphorylated, and released into the medium. The free base, 5-aminoimidazole, was a product of purine catabolism by microorganisms (114). 

Since there has been no evidence presented to support the hypothesis that free adenine can be formed de novo in biological systems from small molecule precursors, and furthermore, since purines have never been reported to have been essential dietary additions, the formation of nucleotides from free purines may be looked upon as a minor biosynthetic pathway. Undoubtedly, there is some utilization of free purines which are derived from the intestinal tract as well as from catabolic events within the cell. The term salvage pathway has been aptly applied to the reactions utilizing free bases for nucleic acid synthesis (206). 

Enzymic hydrolysis of nucleic acids produced mononucleotides, which, in turn, were successively degraded to nucleosides and free purine and P3rrimidine bases. The metabolism of the bases, cytosine, uracil, and thymine (Fig. 18), will be discussed in this section. Since there is little known of the intermediates involved in the catabolism of pyrimidine mononucleotides to free bases, this area will not be discussed. 

Excess purine nucleotides or those released from DNA and RNA by nucleases are catabolized first to nucleosides (loss of P.) and then to free purine bases (release of ribose or deoxyribose). Excess nucleoside monophosphates may accumulate when ... 

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