Turnover of Nucleic Acids

Adenosine deaminase (ADA) is a ubiquitous enzyme that is essential for the breakdown of the purine base adenosine, from both food intake and the turnover of nucleic acids. ADA hydrolyzes adenosine and deoxyadenosine into inosine and deoxyinosine, respectively, via the removal of an amino group. Deficiency of the ADA enzyme results in the build-up of deoxyadenosine and deoxyATP (adenosine triphosphate), both of which inhibit the normal maturation and survival of lymphocytes. Most importantly, these metabolites affect the ability of T-cells to differentiate into mature T-cells [656430], [666686]. ADA deficiency results in a form of severe combined immunodeficiency (SCID), known as ADA-SCID [467343]. 

Secondary gout develops as a complication of hyperuricemia caused by another disorder (e.g., leukemia, chronic nephritis, polycythemia). This type of hyperuricemia usually is associated with abnormally rapid turnover of nucleic acids. The rare cases of gout in adolescents and children are usually of this type. 

Nyhan later described another patient who was a hyperexcretor and displayed the associated clinical symptoms. This child synthesized uric acid from labeled glycine 200 times as rapidly as did normal controls (N5). Another aspect of Nyhan s studies was the rapid decrease of labeled uric acid from patients given glycine- C. The fact that, a week after administration of the isotope, the radioactivity of uric acid in the control and the patients under study approached approximately the same value would seem to indicate that the initial large amount of labeled uric acid was formed by the synthesis of purine nucleotides, which were rapidly broken down and did not involve an increased turnover of nucleic acid per se. It reflects a labile nucleotide pool, not a highly labeled one. 

Uric acid that is produced in man is essentially the product of the action of the enzyme xanthine oxidase on xanthine and hypoxanthine. A tiny amount of uric acid may be ingested as part of the diet, but the great bulk is the result of the action of this enzyme on these two purines. These purines are themselves produced either as a result of the breakdown of cellular material in toto, the turnover of nucleic acids in the cells, or as a result of the intermediary metabolism of various purine nucleotide derivatives. These latter compounds are active in the flow of energy, in methyl group transfer reactions, and as part of the functional molecule of many vitamins. There is direct and indirect evidence that some of the uric acid derives from all these sources. Essentially this evidence consists of the demonstration that other parts of the nucleie acids are found in the urine, such as pyrimidine breakdown products (P9) and methylated purines, which are found only in nucleic acids. There is also isotopic evidence that some labeled purines appear in the urine too quickly after administration of radioactive precursors... 

Increased turnover of nucleic acids, e.g. rapidly growing malignancies or tissue damage. 

Since synthesis of pyrimidine and purine nucleotides de novo (anew) is energetically demanding, it may occur using heterocyclic bases from dietary sources or from those released by the turnover of nucleic acids. Such reactions, called salvage pathways (Figure 16.7) since they enable the reutilization of existing bases, facilitate considerable savings in ATP. 

The turnover of nucleic acids is a continuous process in which synthesis and degradation are carefully regulated. Degradation of RNA and DNA are effected by a variety of ribonucleases and deoxyribonucleases respectively. The nucleotides are converted to nucleosides by the action of... 

Desoxyribonucleic acid is wholly or mainly confined to cell nuclei. Since the desoxyribonucleic acid content of cell nuclei increases and decreases in the various stages of mitosis, we can expect ah appreciable turnover of nucleic acid to take place in growing tissue, and in such organs as well which secrete products containing desoxyribonucleic acid. To the latter belong, among others, thymus, spleen, and bone marrow. 

In earlier investigations of turnover of nucleic acid, acid-soluble and phosphatide components of tissue were extracted with trichloroacetic acid and with ethei -alcohol, and the activity of the residual part was determined. Such residues contain, beside thymonucleic apid, ribonucleic acid and possibly also phosphoproteins. Since the rate of renewal of ribonucleic acid is much larger than that of thymonucleic acid, no conclusion about the value for thymonucleic acid can be drawn from these experiments. 

Fohc acid is a precursor of several important enzyme cofactors required for the synthesis of nucleic acids (qv) and the metaboHsm of certain amino acids. Fohc acid deficiency results in an inabiUty to produce deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and certain proteins (qv). Megaloblastic anemia is a common symptom of folate deficiency owing to rapid red blood cell turnover and the high metaboHc requirement of hematopoietic tissue. One of the clinical signs of acute folate deficiency includes a red and painhil tongue. Vitamin B 2 folate share a common metaboHc pathway, the methionine synthase reaction. Therefore a differential diagnosis is required to measure foHc acid deficiency because both foHc acid and vitamin B 2 deficiency cause... 

Uric acid is the excreted end product of purine catabolism in primates, birds, and some other animals. A healthy adult human excretes uric acid at a rate of about 0.6 g/24 h the excreted product arises in part from ingested purines and in part from turnover of the purine nucleotides of nucleic acids. In most mammals and many other vertebrates, uric acid is further degraded to al-lantoin by the action of urate oxidase. In other organisms the pathway is further extended, as shown in Figure 22-45. 

Liver microsomes of vitamin E-deficient rabbit Increased TBA reactants Increased TBA, absorption Os Severe destruction of cytochrome b. Degeneration of microsomes increased turnover rate of nucleic acids effects on protein synthesis... 

An increased production of uric acid can result from clinical conditions in which there is a rapid increase in the rate of degradation of purine nucleotides. This degradation occurs as a result of the turnover or breakdown of nucleic acids and soluble nucleotides in the cell often associated with breakdown of the cell itself. Examples of this would include the acute leukemias and hemolytic anemias (2). In addition, the degradation of purine nucleotides can occur as a result of alterations in the energy of the cell which enhance the breakdown of ATP. Examples of this might include starvation, muscular exertion, and hypoxia. In some of these latter conditions related to the catabolism of purine nucleoside triphosphates, there may also be compensatory increase in the rate or purine biosynthesis de novo related to the release of feedback inhibition at the level of PRPP synthetase and/or PRPP amidotransferase. 

Xanthine oxidase does not act on biopterin (Forrest et al., 1956). The pattern of urinary excretion of biopterin might be expected, therefore, to differ not only from that of xanthopterin, but also from uric acid. The excretion of uric acid is elevated in leukemia, perhaps reflecting (Krakoff, 1957) an acceleration of nucleic acid ssmthesis. If this acceleration entailed parallel increases in the folic acid and other cofactors for making nucleic acid, then it would account for the increased excretion of xanthopterin in leukemia noted earlier. The turnover of biopterin in leukemia and in normal subjects is, one must emphasize, entirely unknown. 

Within the cell, the stabilizing effect of polyamines on nucleic add structures plays a rdle in the control of nucleic acid turnover, and DNA transcxiptioiL However. polyamines are also fixed on other cell structures bearing addic groups, mainly on the cell-membrane, and many of the polyamines effects can be attributed to a stabilization of the membrane, or a modification of its permeal ty. 

Roentgen rays were found to block formation of nucleic acid in organs of rapidly growing rats also. The percentage inhibition of the desoxyribonucleic acid turnover was found to be similar to that in normal organs of adult animals and in Jensen sarcoma. 

Gonsky R et al. Identification of rapid turnover transcripts overexpressed in thyroid tumors and thyroid cancer cell lines use of a targeted differential RNA display method to select for mRNA subsets. Nucleic Acids Res 1997 25 3823-3831. 

Enzymes useful for detection purposes in ELISA techniques (Chapter 26) also can be employed in the creation of highly sensitive DNA probes for hybridization assays. The attached enzyme molecule provides detectability for the oligonucleotide through turnover of substrates that can produce chromogenic or fluorescent products. Enzyme-based hybridization assays are perhaps the most common method of nonradioactive detection used in nucleic acid chemistry today. The sensitivity of enzyme-labeled probes can approach or equal that of radiolabeled nucleic acids, thus eliminating the need for radioactivity in most assay systems. 

Abstract This chapter updates but mostly supplements the author s Ange-wandte Review,111 setting in context recent advances based on protein and nucleic acid engineering. Systems qualify as a true enzyme mimics if there is experimental evidence for both the initial binding interaction and catalysis with turnover, generally in the shape of saturation kinetics. They are discussed under five broad headings mimics based on natural enzymes, on other proteins, on other biopolymers, on synthetic macromolecules and on small-molecule host-guest interactions. 

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