An improved enzymatic radioassay for ribose 1-phosphate

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.

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.

Purine nucleosides as cell-specific modulators of 5-fluorouracil metabolism and cytotoxicity

Purine nucleosides and ribose-5-phosphate (Rib-5-P) were used to modulate the metabolism and cytotoxicity of 5-fluorouracil (5-FU) in order to get a better understanding of the mechanism of action of 5-FU. In extracts from five different cell lines both Rib-5-P and inosine were relatively good precursors for Rib-1-P, but deoxyinosine was a moderate to poor precursor for deoxyRib-1-P. In the human colon carcinoma WiDr and the human epithelial intestinal Intestine 407 inosine enhanced Rib-1-P concentrations 3-6-fold. Incubation with deoxyinosine resulted in the appearance of deoxyRib-1-P in both cell lines in levels comparable to those of Rib-1-P. dIMP had the same effect as deoxyinosine in Intestine 407 cells, but not in WiDr cells. Both inosine and deoxyinosine caused a depletion of phosphoribosyl-pyrophosphate. In WiDr cells deoxyinosine (0.1-1.0 mM) clearly potentiated the growth inhibition by 0.1-0.5 microM 5-FU after 24 h of culture, but growth between 24 and 48 h was normal. In Intestine 407 cells the potentiation of 5-FU cytotoxicity by deoxyinosine was even more pronounced at 48 h than at 24 h. In WiDr cells dIMP did not potentiate 5-FU cytotoxicity, but in Intestine 407 cells the effect was comparable to that of deoxyinosine. The lack of potentiation in WiDr was accompanied by a low metabolism of dIMP. Growth inhibition by 5-FU and deoxyinosine could be reversed by thymidine in Intestine 407 cells but not completely in WiDr cells. Since the predominant target of the deoxyinosine-5-FU combination was thymidylate synthase, we analyzed the inhibition of this enzyme by FdUMP and the retention of the inhibition in cell culture. In both cell lines FdUMP was a potent competitive inhibitor of thymidylate synthase with a Ki of between 0.5 and 2 nM. Culture of cells in the presence of 5-FU and deoxyinosine resulted in an almost complete inhibition of thymidylate synthase activity after 24 h but after 48 h the activity was partly recovered. In Intestine 407 cells replenishment of the culture medium at 24 h even enhanced the recovery. Analysis of 5-FU anabolism into nucleic acids demonstrated that deoxyinosine inhibited the incorporation of 5-FU into RNA. It is concluded that in Intestine 407 cells addition of deoxyinosine enhanced the effects of 5-FU on growth inhibition due to increased formation of FdUMP leading to enhanced inhibition of thymidylate synthase.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of uridine phosphorylase in the anabolism of 5-fluorouracil

The activities of enzymes responsible for activating 5-fluorouracil (FUra) to 5-fluorouridine-5'-monophosphate (FUMP) were compared in normal and tumor tissues of rodents to assess the potential capacity of uridine phosphorylase to anabolize FUra to the nucleoside in the presence of ribose-1-phosphate (R-1-P). The activity of the alternative pathway to FUMP with a pyrimidine phosphoribosyltransferase [FUra + 1-pyrophosphoribosyl-5-phosphate (PPRP)] was approximately 15 to 17 nmoles/mg protein/hr in bone marrow from mice and rats and ranged from 28 to 47 nmoles/mg protein/hr in tumor tissues. Uridine phosphorylase [measured as the formation of 5-fluorouridine (FUrd) from FUra and R-1-P] was 35-230 nmoles/mg/hr in bone marrow and in two FUra-sensitive solid tumors, colon tumor No. 38 in mice and RPMI colon tumor in rats; the activity of uridine phosphorylase from L5178Y ascites leukemic cells was notably lower, 8 nmoles/mg/hr. Levels of uridine kinase ranged from 55 to 187 nmoles/mg protein/hr. Thus, the activities of the enzymes of the two-step FUra activating pathway were high compared to the PPRP-dependent activity in all tissues except L5178Y; also, the FUra-sensitive tumors yielded extracts with 1.5 to 6.5 times greater enzyme activity than the corresponding activity in bone marrow. Uridine phosphorylase was partially purified from rat liver, RPMI rat tumor and colon tumor No. 38; the apparent Km of FUra averaged 50 microM, almost 9-fold lower than that of uracil, and the apparent Km of R-1-P for condensation with FUra was 33 microM. The tissue concentration of R-1-P was greater than 70 microM in kidney and liver of rodents and somewhat less in spleen. Colon tumor No. 38 and RPMI colon tumor had 12 and 20 microM R-1-P, respectively, but these low values may reflect low tumor viability. The high levels of uridine phosphorylase and uridine kinase activities in normal tissues and even higher levels in tissues from FUra-sensitive tumors, as well as the sufficient concentration of R-1-P relative to its kinetic constant, suggest that FUra metabolism by the two-step pathway to FUMP may be a significant factor in the activity and selectivity of FUra.

Uridine phosphorylase from Novikoff rat hepatoma cells: purification, kinetic properties, and its role in uracil anabolism

Uridine phosphorylase activity was detected in sonic extracts of six different mammalian cell lines and, in conjunction with uridine kinase, provides a route for the conversion of uracil to UMP via uridine. Uracil phosphoribosyl transferase activity was not detected in any of eight different mammalian cell lines. Uridine phosphorylase was purified 5,330-fold from Novikoff rat hepatoma cells by ammonium sulfate precipitation, DEAE-Sephadex chromatography, hydroxyapatite chromatography, and Sephadex G-200 fractionation. The molecular weight of the enzyme by gel filtration was approximately 45,000. The kinetics of the purified enzyme were analyzed with respect to all four substrates at saturating cosubstrate concentration, yielding the parameters KmUra = 360 microM, KmRib-1-P = 88 microM, KmUrd = 16 micron, and KmPi = 130 microM. However, in intact cells the phosphorolysis of uridine proceeded with an apparent Km of 231 microM. Novikoff cells treated with 0.5 mM inosine exhibited an increase in uracil uptake rate which was proportional to an observed increase in intracellular ribose-1-phosphate. Nevertheless, in cells whose de novo synthesis of pyrimidines was blocked by pyrazofurin or N-(phosphonacetyl)-L-aspartate ("PALA"), the uptake of uracil was insufficient to support proliferation, even when enhanced by inosine. These observations are consistent with the kinetic characteristics of the enzyme and provide evidence that the intracellular level of ribose-1-phosphate plays a rate-limiting role in the uptake of uracil mediated by uridine phosphorylase.

Uridine phosphorylase from Acholeplasma laidlawii: purification and kinetic properties

Uridine phosphorylase was purified 1,370-fold from sonicated extracts of Acholeplasma laidlawii by ammonium sulfate precipitation, DEAE-Sephadex column chromatography, hydroxylapatite chromatography, and Sephadex G-200 fractionation. The molecular weight of the enzyme as determined by gel filtration was approximately 65,000. [U-14C]ribose-1-phosphate (Rib-1-P), prepared enzymatically from [U-14C]inosine, was utilized in initial velocity studies of uridine synthesis, which indicated a sequential reaction with a KmUra of 110 microM and a KmRib-1-P of 17 microM. The kinetics of uridine cleavage were assessed at a saturating cosubstrate concentration, resulting in a KmUrd of 170 microM and a KmPi of 120 microM. These results indicate that an intracellular flux from uracil to uridine is kinetically feasible. However, such flux would be metabolically unproductive, since the low affinity of uridine kinase (KmUrd = 3.2 mM) precludes the operation of uridine phosphorylase and uridine kinase in tandem to convert uracil to UMP. We conclude that uridine phosphorylase performs only a catabolic function in A. laidlawii.