Nucleic acid purine incorporation

Adenosine was shown to be incorporated into nucleic acid purines, but to only about half the extent of adenine itself (L2). The incorporation of guanosine was also studied and here again the guanine derivative was a poorer precursor than the adenine derivative, but the utilization of guanosine was considerably greater than that of the aglycone (L22). The administration of the adenine nucleotides revealed that these compounds, whether as the 3 isomer or the 5 isomer, were poorer precursors than the free base adenine (Wl). With all nucleotides studied, the incorporation of the 5 isomer was less extensive than of the 3 isomer (R9). Roll et al. have shown that guanine nucleotides are considerably... 

One of the first studies that indicated the possible interference of ametbopterin and aminopterin with purine synthesis was the demonstration by Skipper et al. (S21) that administration of these compounds to mice inhibited the incorporation of labeled formate into nucleic acid purines. Ametbopterin (4-amino-A -methyl-pteroylglutamic acid) has produced some inhibition of uric acid synthesis during the use of this compound for treatment of leukemia (K15, E24). Another inhibitor of... 

The origins of the ring nitrogens of uric acid were first studied in 1943 by Barnes and Schoenheimer. Little N from urea was incorporated into visceral nucleic acid purines of the rat, and urea could be eliminated as a potential precursor, thereby disproving Weiner s hjrpothesis. N-Ammon-ium citrate was a good precursor of purines, however, and because the specific activity of purines was considerably greater than that of histidine... 

Abrams and co-workers first showed that the precursors of nucleic acid purines were the same as those of uric acid by demonstrating the incorporation of N -glycine into adenine and guanine of the nucleic acids of growing yeast. Heinrich and Wilson, using the rat, also explored the precursors of purines in nucleic acids, and their results confirmed the earlier observations of Buchanan and collaborators for uric acid. It has now been well substantiated that formate, glycine, and CO2 aU con-... 

Adenine is inert in the nucleoside phosphorylase systems of both mammalian tissues and microorganisms, but isotopically labeled adenine is effectively incorporated into nucleic acid purines, both in rats " and in yeast.This poses a question as to the possible role of nucleoside phosphorylase in polynucleotide synthesis. It has been suggested that hypoxanthine or guanine nucleosides (or nucleotides) are synthesized first. Then an exchange reaction with free adenine (or a derivative) might occur, For example, adenine might react with inosine to form adenosine and hypoxanthine. Some known exchange reactions are discussed below. 

The occurrence of methylated derivatives of purines in nucleic acids raises the question of their biological significance. It is possible that methylated purines were incorporated accidentally into nucleic acid and merely replaced natural bases because they were present in the cell for other purposes. The methylated purines may represent constituents of unknown coenzymes or may be vital components of some nucleic acid of specialized function. Also, the possibility should not be overlooked that the methylated purines were not useful to the organism, but represented minor by-products of the usual nucleic acid purines. However, an intriguing possibility has been offered that a methylated base might participate in the template coding of an uncommon amino acid such as cysteine or tryptophan or even act to produce a correct termination of the polypeptide chain 226). 

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. 

Very recently the trans- [Pt(NH3)2Cl2] isomer has been shown to promote the formation of some highly unusual multiple strand nucleic acid structures (28). A parallel-stranded DNA duplex, psPtDNA, has been prepared which incorporates a frarcs-Pt(NH3)2+ crosslink. The pla-tinated strands were synthesized through the series of reactions illustrated in Fig. 3. The initial mono-functional G N7 adduct is formed at pH 3.5 by addition of rans-[Pt(NH3)2(H20)Cl]+ to the pyrimidine-rich sequence 5 -d(T4CT4G). After the pH is increased, normal anti-parallel duplex formation occurs upon addition of the complementary purine-rich strand, 5 -d(A4GA4G). This intermediate anti-parallel stranded... 

Seasonal variations in the metabolic fate of adenine nucleotides prelabelled with [8—1-4C] adenine were examined in leaf disks prepared at 1-month intervals, over the course of 1 year, from the shoots of tea plants (Camellia sinensis L. cv. Yabukita) which were growing under natural field conditions by Fujimori et al.33 Incorporation of radioactivity into nucleic acids and catabolites of purine nucleotides was found throughout the experimental period, but incorporation into theobromine and caffeine was found only in the young leaves harvested from April to June. Methy-lation of xanthosine, 7-methylxanthine, and theobromine was catalyzed by gel-filtered leaf extracts from young shoots (April to June), but the reactions could not be detected in extracts from leaves in which no synthesis of caffeine was observed in vivo. By contrast, the activity of 5-phosphoribosyl-1-pyrophosphate synthetase was still found in leaves harvested in July and August. 

Not all analogues become active against cancer cells through incorporation into nucleic acid. Some analogues block the synthesis of normal purine and pyrimidine nucleotides for example, 8-azaguanine blocks guanosine monophosphate (GMP) synthesis and 6-mercaptopurine inhibits adenosine monophosphate (AMP) syn-thesis. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

The subject of the incorporation of anticancer agents into macromolecules [13] and other compounds [336] has been reviewed. A number of purine analogues are incorporated into nucleic acid, but the incorporation of these compounds requires that they be anabolized to nucleoside mono-, di-, and triphosphates, and it is difficult to separate the metabolic effects of the nucleoside phosphates from the metabolic effects of the fraudulent polynucleotides. 

Thioguanine is a purine analogue structurally related to 6-mercaptopurine and azathioprine. Thioguanine interferes with several enzymes required for de novo purine synthesis, and its metabolites are incorporated into DNA and RNA, further impeding nucleic acid synthesis. The mechanism of action of thioguanine in psoriasis is not clearly understood it has been hypothesized to affect the proliferation and trafficking of lymphocytes as well as the proliferation of keratinocytes. 

Dietary purines are not an important source of uric acid. Quantitatively important amounts of purine are formed from amino acids, formate, and carbon dioxide in the body. Those purine ribonucleotides not incorporated into nucleic acids and derived from nucleic acid degradation are converted to xanthine or hypoxanthine and oxidized to uric acid (Figure 36-7). Allopurinol inhibits this last step, resulting in a fall in the plasma urate level and a decrease in the size of the urate pool. The more soluble xanthine and hypoxanthine are increased. 

A search for antimetabolites, i.e. analogues of essential metabolites that might displace the latter in vital processes, was proposed as a rational approach to the discovery of antibacterial agents, but it has had little success other than the achievements in the folic acid field (Section 1.06.6). Substances that resemble the components of nucleic acids have, however, had considerable success in the chemotherapy of cancer and of some virus diseases and in the suppression of the immune response. They may act by becoming incorporated in false nucleic acids or by blocking the synthesis of nucleic acids, nucleotides, nucleosides or of the pyrimidine and purine bases cytosine (88), thymine (89 R = Me), adenine (90) and guanine (91 X = CH). The simplest antimetabolites are analogues of these bases. 

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