Purine bases, degradation

Cleavage at A or G If the DNA is first treated with acid, dimethyl sulfate methylates adenine at the 3-position as well as guanine at the 7-position (not shown). Subsequent reaction with OH and piperidine triggers degradation and displacement of the methylated A or G purine base and strand scission, essentially as indicated here for reaction of dimethyl sulfate with guanine. 

Human tissues can synthesize purines and pyrimidines from amphibolic intermediates. Ingested nucleic acids and nucleotides, which therefore are dietarily nonessential, are degraded in the intestinal tract to mononucleotides, which may be absorbed or converted to purine and pyrimidine bases. The purine bases are then oxidized to uric acid, which may be absorbed and excreted in the urine. While little or no dietary purine or pyrimidine is incorporated into tissue nucleic acids, injected compounds are incorporated. The incorporation of injected [ H] thymidine into newly synthesized DNA thus is used to measure the rate of DNA synthesis. 

Figure 34-8. Formation of uric acid from purine nucleosides byway of the purine bases hypoxanthine, xanthine, and guanine. Purine deoxyribonucleosides are degraded by the same catabolic pathwayand enzymes,all of which existin the mucosa of the mammalian gastrointestinal tract.

Figure 10.8 A summary of the reactions involved in the degradation of nucleic acid, nucleotides, nucleosides and purine and pyn midine bases. Nucleic add is hydrolysed by nucleases to form nucleotides, which are hydrolysed to nucleosides. The latter are split into ribose 1-phosphate and a base. The purine bases are converted to uric acid and ammonia. Uric acid is excreted. The pyrimidine bases are converted to 3-carbon intermediates (malo-nate semialdehyde and methylmalonate semialdehyde). The nitrogen is released as ammonia or used to convert oxoglutarate to glutamate.

Nucleic acid degradation in humans and many other animals leads to production of uric acid, which is then excreted. The process initially involves purine nucleotides, adenosine and guanosine, which are combinations of adenine or guanine with ribose (see Section 14.1). The purine bases are subsequently modified as shown. 

The nucleotides are among the most complex metabolites. Nucleotide biosynthesis is elaborate and requires a high energy input (see p. 188). Understandably, therefore, bases and nucleotides are not completely degraded, but instead mostly recycled. This is particularly true of the purine bases adenine and guanine. In the animal organism, some 90% of these bases are converted back into nucleoside monophosphates by linkage with phosphori-bosyl diphosphate (PRPP) (enzymes [1] and [2]). The proportion of pyrimidine bases that are recycled is much smaller. 

The principles underlying the degradation of purines (1) and pyrimidines (2) differ. In the human organism, purines are degraded into uric acid and excreted in this form. The purine ring remains intact in this process. In contrast, the ring of the pyrimidine bases (uracil, thymine, and cytosine) is broken down into small fragments, which can be returned to the metabolism or excreted (for further details, see p. 419). 

Purine (left). The purine nucleotide guano-sine monophosphate (CMP, 1) is degraded in two steps—first to the guanosine and then to guanine (Gua). Guanine is converted by deamination into another purine base, xanthine. 

In the most important degradative pathway for adenosine monophosphate (AMP), it is the nucleotide that deaminated, and inosine monophosphate (IMP) arises. In the same way as in GMP, the purine base hypoxanthine is released from IMP. A single enzyme, xanthine oxidase [3], then both converts hypoxanthine into xanthine and xanthine into uric acid. An 0X0 group is introduced into the substrate in each of these reaction steps. The oxo group is derived from molecular oxygen another reaction product is hydrogen peroxide (H2O2), which is toxic and has to be removed by peroxidases. 

Purine nucleotides are degraded by a pathway in which they lose their phosphate through the action of 5 -nucleotidase (Fig. 22-45). Adenylate yields adenosine, which is deaminated to inosine by adenosine deaminase, and inosine is hydrolyzed to hypoxanthine (its purine base) and D-ribose. Hypoxanthine is oxidized successively to xanthine and then uric acid by xanthine oxidase, a flavoenzyme with an atom of molybdenum and four iron-sulfur centers in its prosthetic group. Molecular oxygen is the electron acceptor in this complex reaction. 

Just as orotic acid is converted to a ribonucleotide in step e of Fig. 25-14, other free pyrimidine and purine bases can react with PRPP to give monoribonucleotides plus PP . The reversible reactions, which are catalyzed by phosphoribosyltransferases (ribonucleotide pyrophosphorylases), are important components of the salvage pathways by which purine and pyrimidine bases freed by the degradation of nucleic acids are recycled.273 However, thymine is usually not reused. Thymine will react with deoxribose 1-P to form thymidine plus inorganic phosphate (thymidine phosphorylase), and thymidine is rapidly... 

Figure 10.2 Tautomeric forms of uric acid. Although uric acid does not occur in nucleic acids (it is a degradation product of adenine and guanine), the tautomeric structures observed here are typical of all purine bases of this type. At pH 7, the keto forms predominate.

Figure 10.14 Degradation of purines to uric add and other excretion products. In human beings, uric add is the end product of purine base catabolism.

A different problem occurs with 7-methylguanosine. The purine base of this nucleoside is hydrolyzed at basic pH, and very significant degradation occurs in an hour at pH 9.5 at room temperature. The mixture of oxidized nucleoside and protein, therefore, is kept at pH 9.1 and at 0-4°, at which only very limited hydrolysis of the base occurs in 1 hr. A different reducing agent, tert-butylamine borane (Aldrich Chemical Co.), is used the reduction is done for only 30-60 min at 4°, and the product is separated from free nucleoside on a Sephadex G-25 column at 4°. The nucleotide of the 7-methylguanosine is more stable than the nucleoside, and it also has been used to induce antibody to the 7-methylguanine structure. ... 

In this case, the reducing agent was cyanoborohydride. To verify that the conjugate contains intact purine base, a difference spectrum of the hapten-protein conjugate minus protein should be obtained it should be very close to the spectrum of the intact nucleoside or nucleotide alone. Further, the antibodies should be specific for the intact base in comparison with the degradation product. ... 

Hydrolysis of DNA yields free purine bases and, under mild conditions, apurinic acid which can be further degraded by more severe conditions to pyrimidine sequences pPyp...Pyp (Michelson 1963, p. 368). Kirk (1963, 1967) used mild hydrolysis to liberate the purines which were then estimated after separation by column chromatography ... 

Reutilization of purine bases after conversion to their respective nucleotides constitutes salvage pathways. These pathways are particularly important in extrahepatic tissues. Purines arise from several sources intermediary metabolism of nucleotides, degradation of polynucleotides, and dietary intake. Quantitatively, the first two sources are the more important. Salvage occurs mainly by the phosphoribosyltransferase reaction ... 

B. Uric acid has a pKa of 5.4 and is ionized in the body to form urate. Urate is not very soluble in an aqueous enviromnent, and the quantity of urate in human blood is very close to the solubility range. Therefore, situations that lead to excessive degradation of purine bases can increase the urate concentration past the solubihty point and lead to the formation of urate crystals. The decreased body temperature found in the joints contributes to the formation of urate ays-tals under these conditions. 

Because urate is not very soluble in an aqueous environment and the concentration of urate in human blood is very close to saturation, conditions that lead to excessive degradation of purine bases can lead to the formation of urate crystals. 

The salvage pathways utilized by these pathogens are similar to those used in mammalian cells (Fig. 6.1). In many cases the pathways differ in certain aspects, perhaps in response to the purine composition of the parasite s host environment. These routes permit cells to utilize exogenous purines derived from the degradation of nucleic acids or nucleotides. In mammalian cells and many parasitic organisms, purine bases... 

The base hypoxanthine is not found in DNA but is produoed during degradation of the purine bases. It is found in oertain tRNA moleoules. Its nucleoside, inosine, is produced during synthesis of the purine nucleotides (see Chapter 41). 

V. DEGRADATION OF PURINE AND PYRIMIDINE BASES A. Purine Bases... 

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