Deoxyribose 1-phosphate: radioenzymatic and spectrophotometric assays

Methods for the determination of intracellular levels of ribose phosphates

Ribose phosphates are either synthesized through the oxidative branch of the pentose phosphate pathway or stem from the phosphorolytic cleavage of the N-glycosidic bond of ribonucleosides. The two major pentose phosphates, ribose-5-phosphate and ribose-1-phosphate, can be readily interconverted by phosphopentomutase. Ribose-5-phosphate is also the direct precursor of 5-phosphoribosyl-1-pyrophosphate, which is used for both de novo and salvage synthesis of nucleotides. On the other hand, the phosphorolysis of deoxyribonucleosides is the major source of deoxyribose phosphates. While the destiny of the nucleobase stemming from nucleoside phosphorolysis has been extensively investigated, the fate of the sugar moiety has been somehow neglected. However, extensive advances have been made in elucidating the pathways by which the pentose phosphates, arising from nucleoside phosphorolysis, are either recycled, without opening of their furanosidic ring, or catabolized as a carbon and energy source. Nevertheless, many aspects of pentose phosphate metabolism, and the possible involvement of these compounds in a number of cellular processes still remain obscure. The comprehension of the role played by pentose phosphates may be greatly facilitated by the knowledge of their steady-state intracellular levels and of their changes in response to variations of intra- and extracellular signals.

Rapid disappearance of deoxyribose-1-phosphate in platelet derived endothelial cell growth factor/thymidine phosphorylase overexpressing cells

Platelet derived endothelial cell growth factor/thymidine phosphorylase (PD-ECGF/TP) catalyzes the phosphorolysis of thymidine (TdR) to thymine and deoxyribose-1-phosphate (dR-1-P) and has a pro-angiogenic effect for which dR-1-P may be responsible. Using a purine nucleoside phosphorylase based assay it was found that TdR incubation did not increase dR-1-P accumulation in colon cancer cell line Colo320 and its PD-ECGF/TP transfected variant Colo320TP1. The assay was linear up to 25,000pmol dR-1-P with complete recovery of dR-1-P from cellular extracts. There was a huge discrepancy between thymine production and the measured dR-1-P level, 0.05% of the expected value for dR-1-P was found, indicating that there was a rapid disappearance of dR-1-P. However, in cellular extracts, TdR incubation increased dR-1-P, measurable by trapping, which was inhibited by a thymidine phosphorylase inhibitor. dR-1-P directly added to cellular extracts disappeared within 5-10min. In conclusion, large amounts of dR-1-P are produced by Colo320TP1 cells, which rapidly disappear thus not resulting in a net accumulation of dR-1-P in these cells.

Catabolism of exogenous deoxyinosine in cultured epithelial amniotic cells

Uptake and catabolism of purine nucleosides have been commonly considered as means to salvage the purine ring for nucleic acid synthesis, usually neglecting the destiny of the pentose moiety. With the aim to ascertain if deoxyribose derived from exogenous DNA can be utilised as a carbon and energy source, we studied the catabolism of exogenous deoxyinosine in a cell line derived from human amnion epithelium (WISH). Intact WISH cells catabolise deoxyinosine by conversion into hypoxanthine. The nucleoside enters the cell through a nitrobenzylthioinosine-insensitive equilibrative transport. Deoxyinosine undergoes a phosphorolytic cleavage inside the cell. The purine base diffuses back to the external medium, while the phosphorylated pentose moiety can be further catabolised to glycolysis and citric acid cycle intermediates. Our results indicate that the catabolism of the deoxynucleoside can be considered mainly as a means to meet the carbon and energy requirements of growing cells.

Activation pathways of 5-fluorouracil in rat organs and in PC12 cells

Activation of the pyrimidine analogue 5-fluorouracil (5-FU) to the ribonucleotide level may occur through one of the following three pathways: 1) the 5-phosphoribosyl 1-pyrophosphate (PRPP)-mediated direct transfer of ribose 5-phosphate to 5-FU as catalysed by orotate phosphoribosyltransferase; 2) the ribose 1-phosphate (Rib1-P)-mediated addition of ribose by uridine phosphorylase, followed by the action of uridine kinase; and 3) the 2'-deoxyribose 1-phosphate (deoxyRib1-P)-mediated addition of deoxyribose, thought to be catalysed by thymidine phosphorylase, followed by the action of thymidine kinase. Many of the conclusions as to the precise pathways by which normal tissues and different cell lines activate uracil are indirectly derived from drug interactions affecting the availability of the substrates of the three pathways, or from measurement of activities of the enzymes metabolising 5-FU in normal tissues and tumours. In previous papers (Cappiello et al. Biochim Biophys Acta 1998;1425:273--81; Mascia et al. Biochim Biophys Acta 1999;1472:93--8), we assessed the molecular mechanisms by which the natural base uracil is salvaged in vitro to uracil ribonucleotides and deoxyribonucleotides in rat liver and brain. In this paper, we investigated the pathways of 5-FU activation to cytotoxic ribonucleotide and deoxyribonucleotide levels in normal rat tissues and PC12 cell extracts. The results clearly showed that normal rat tissues activated 5-FU mainly via the Rib1-P pathway, and to a lesser extent via the PRPP pathway. The deoxyRib1-P pathway was absent. PC12 cells activated 5-FU mainly via the PRPP pathway and to a lesser extent by the other two pathways.

In vitro recycling of alpha-D-ribose 1-phosphate for the salvage of purine bases

In this paper, we extend our previous observation on the mobilization of the ribose moiety from a purine nucleoside to a pyrimidine base, with subsequent pyrimidine nucleotides formation (Cappiello et al., Biochim. Biophys. Acta 1425 (1998) 273-281). The data show that, at least in vitro, also the reverse process is possible. In rat brain extracts, the activated ribose, stemming from uridine as ribose 1-phosphate, can be used to salvage adenine and hypoxanthine to their respective nucleotides. Since the salvage of purine bases is a 5-phosphoribosyl 1-pyrophosphate-dependent process, catalyzed by adenine phosphoribosyltransferase and hypoxanthine guanine phosphoribosyltransferase, our results imply that Rib-1P must be transformed into 5-phosphoribosyl 1-pyrophosphate, via the successive action of phosphopentomutase and 5-phosphoribosyl 1-pyrophosphate synthetase; and,in fact, no adenosine could be found as an intermediate when rat brain extracts were incubated with adenine, Rib-1P and ATP, showing that adenine salvage does not imply adenine ribosylation, followed by adenosine phosphorylation. Taken together with our previous results on the Rib-1P-dependent salvage of pyrimidine nucleotides, our results give a clear picture of the in vitro Rib-1P recycling, for both purine and pyrimidine salvage.

In vitro assessment of salvage pathways for pyrimidine bases in rat liver and brain

In this paper we extend our previous observation on the mobilization of the ribose moiety from guanosine to xanthine catalyzed by rat liver extracts (Giorgelli et al., Biochim. Biophys. Acta 1335 (1997) 16-22). The data show that in rat liver and brain extracts the activated ribose, stemming from inosine and guanosine phosphorolysis as ribose 1-phosphate, can be used to salvage uracil to uracil nucleotides. Uridine is an intermediate. The salvage process occurs even in the presence of excess inorganic phosphate suggesting that uridine phosphorylase may function in vivo as an anabolic enzyme. Ribose 5-phosphate cannot substitute for inosine, guanosine or ribose 1-phosphate as ribose donor. When inorganic phosphate was substituted with arsenate, hindering the formation of ribose 1-phosphate, no ribose transfer could be observed. A similar pathway occurs at the deoxy level. The deoxyribose moiety of deoxyinosine can be used to salvage thymine to thymine nucleotides, again in the presence of excess inorganic phosphate. Our results introduce a novel aspect of the salvage pathway, in which ribose 1-phosphate seems to play a pivotal role.

Channelling of deoxyribose moiety of exogenous DNA into carbohydrate metabolism: role of deoxyriboaldolase

In bacteria, the addition of (deoxy)nucleosides or (deoxy)ribose to the growth medium causes induction of enzymes involved in their catabolism, leading to the utilisation of the pentose moiety as carbon and energy source. In this respect, deoxyriboaldolase appears the key enzyme, allowing the utilisation of deoxyribose 5-P through glycolysis. We observed that not only deoxynucleosides, but also DNA added to the growth medium of Bacillus cereus induced deoxyriboaldolase; furthermore, the switch of the culture from aerobic to anaerobic conditions caused a further increase in enzyme activity, leading to a more efficient channelling of deoxyribose 5-P into glycolysis, probably as a response to the low energy yield of the sugar fermentation. In eukaryotes, the catabolism of (deoxy)nucleosides is well known. However, the research in this field has been mainly devoted to the salvage of the bases formed by the action of nucleoside phosphorylases, whereas the metabolic fate of the sugar moiety has been largely neglected. Our results indicate that the deoxyriboaldolase activity is present in the liver of several vertebrates and in a number of cell lines. We discuss our observations looking at the nucleic acids not only as informational molecules, but also as a not negligible source of readily usable phosphorylated sugar.

8-azaguanosine-5'-monophosphate synthesis via nucleoside kinase in cultured Chinese hamster lung fibroblasts

Cultured chinese hamster lung fibroblasts, and a variant clone selected for resistance to 8-azaguanine, that lacks hypoxanthine-guanine phosphoribosyl transferase (EC 2.4.2.8), have been tested for the ability to convert 8-azaguanine into 8-azaguanosine-5'-monophosphate via purine nucleoside phosphorylase and nucleoside kinase. Purine nucleoside phosphorylase of both cell types is able to synthesize 8-azaguanosine from 8-azaguanine with the same efficiency. Wild type cells possess a nucleoside kinase activity acting on 8-azaguanosine, but this activity is considerably lower in the cells displaying resistance to the base analog. Our lines of evidence demonstrate that purine nucleoside phosphorylase and nucleoside kinase constitute a possible way of synthesis of the cytotoxic mononucleotide of 8-azaguanine, and, in fact, cells selected for resistance to the base analog show an impairement in the nucleoside kinase activity.