Evidence of a new metabolic pathway of 5-fluorouracil in Escherichia coli from in vivo 19F-NMR spectroscopy

Two distinct metabolic pathways of 5-fluorouracil are proposed in Escherichia coli. The first metabolic pathway is a reductive degradation with the formation of dihydrofluorouracil as the first metabolite. The second metabolic pathway is shown to be a hydroxylating degradation, possibly with the formation of 5-hydro-6-hydroxy-5-fluorouracil as the first metabolite. The metabolites of both pathways undergo subsequent hydrolytic degradation with fluoride ion as the common final product. The chemical structures of these metabolites were partially identified by 19F-NMR. The results show a close resemblance between these two metabolic pathways with in vivo pyrimidine biodegradation. The reductive degradation has been proposed by several laboratories, whereas the hydroxy degradation has not been reported before. Both the reductive and hydroxy pathways are demonstrated in this report, to be independent reactions.     

Specific incorporation of 5-fluorocytidine into Escherichia coli RNA

RNAs isolated from Escherichia coli B grown in the presence of 5-fluorouracil have high levels of the analog replacing uridine and uridine-derived modified nucleosides. Cytidine has also been shown to be replaced in these RNAs by 5-fluorocytidine, a metabolic product of 5-fluorouracil, but to a considerably lesser extent. When 5-fluorocytidine is added to cultured of E. coli B little 5-fluorocytidine (0.20 mol%) is incorporated into cellular RNAs because of the active cytosine/cytidine deaminase activities. Addition of the cytidine deaminase inhibitor tetrahydrouridine (70 micrograms/ml) increases 5-fluorocytidine incorporation to about 3 mol% in tRNAs, but does not eliminate 5-fluorouridine incorporation. E. coli mutants lacking cytosine/cytidine deaminase activities are able to more than double the extent of 5-fluorocytidine incorporation into their transfer and ribosomal RNAs, replacing cytidine with no detectable 5-fluorouridine incorporation. Levels of 5-methyluridine, pseudouridine and dihydrouridine in tRNAs are not affected. These fluorocytidine-containing tRNAs show amino acid-accepting activities similar to control tRNAs. Fluorocytidine was found to be quite susceptible to deamination under alkaline conditions. Its conversion to primarily 5-fluorouridine follows pseudo-first-order reaction kinetics with a half-life of 10 h in 0.3 M KOH at 37 degrees C. This instability in alkali probably explains why 5-fluorocytidine was not found earlier in RNAs isolated from cells treated with 5-fluorouridine, since most early RNA hydrolyses were carried out in alkali. It may also explain the mild mutagenic properties observed in some systems following 5-fluorouridine treatment. Initial 19F-NMR measurements in fluorocytidine-containing tRNAs indicate that this modified tRNA may be useful in future structural studies of tRNAs and in probing tRNA-protein complexes.     

The fluorinated anesthetic halothane as a potential NMR biologic probe

Fluorinated anesthetics such as halothane preferentially partition into hydrophobic environments such as cell membranes. The 19F-NMR spectrum of halothane in a rat adenocarcinoma (with known altered lipid metabolism and membrane composition) shows an altered chemical shift pattern compared to the anesthetic in normal tissue. In eight tumor samples examined, the 19F-NMR spectra exhibit two distinct resonances, compared to a single resonance observed in normal tissues. This is explained by an enhanced or altered hydrophobic component in the tumor tissue giving rise to two discrete halothane environments. Another fluorinated anesthetic, isoflurane, shows similar behavior in distinguishing normal from diseased tissue. Given the large chemical shift range of fluorine and the inherent sensitivity of this nucleus, 19F-NMR spectra of fluorinated anesthetics can also be used to follow anesthetic degradation by the liver. The ability of fluorinated anesthetics to discriminate tissues and to monitor metabolic processes is potentially useful for in vivo 19F-NMR surface coil and imaging studies.     

Does 29-mer RNA hairpin of the HIV-1 TAR RNA sequence bind magnesium? 19F-NMR and modelling studies.

5-Fluorouridine residues have been introduced into functionally important bulge and loop regions of 29-mer HIV-1 TAR RNA hairpins I and II to study Mg2+ and Ca2+ binding using 19F-NMR spectroscopy. There was no substantial binding detected up to 20-molar excess in case of both cations, whereas association of argininamide, used as a reference ligand, could be detected at less than 1-molar excess. The deltadelta 19F value of 1.93 ppm observed for (F)U23 upon argininamide binding is in agreement with former NMR studies of TAR RNA/argininamide complex. However, obtained results do not confirm U38 x A27 x U23 base-triple formation. The unmodified HIV-1 TAR RNA hairpin resulted from 600 ps in aqua molecular dynamics simulation was subjected to a molecular mechanics modelling of Mg+ binding.     

The fluorinated anesthetic halothane as a potential NMR biologic probe

Fluorinated anesthetics such as halothane preferentially partition into hydrophobic environments such as cell membranes. The ¹⁹F-NMR spectrum of halothane in a rat adenocarcinoma (with known altered lipid metabolism and membrane composition) shows an altered chemical shift pattern compared to the anesthetic in normal tissue. In eight tumor samples examined, the ¹⁹F-NMR spectra exhibit two distinct resonances, compared to a single resonance observed in normal tissues. This is explained by an enhanced or altered hydrophobic component in the tumor tissue giving rise to two discrete halothane environments. Another fluorinated anesthetic, isoflurane, shows similar behavior in distinguishing normal from diseased tissue. Given the large chemical shift range of fluorine and the inherent sensitivity of this nucleus, ¹⁹F-NMR spectra of fluorinated anesthetics can also be used to follow anesthetic degradation by the liver. The ability of fluorinated anesthetics to discriminate tissues and to monitor metabolic processes is potentially useful for in vivo ¹⁹F-NMR surface coil and imaging studies.     

Incorporation of 19F-substituted aromatic amino acids into membrane proteins from chromatophores of Rhodospirillum rubrum G9+

Chromatophore membranes were isolated from cells of the carotenoidless mutant Rhodospirillum rubrum G9⁺ grown in the presence of several fluorinated aromatic amino acids, solubilized using SDS and the extent of incorporation analyzed using high-resolution ¹⁹F-NMR spectroscopy. 3- and 4-¹⁹F-phenylalanine, 6-¹⁹F-tryptophan and 3-¹⁹F-tyrosine were biosynthetically incorporated into membrane proteins whereas 5-¹⁹F-tryptophan and 2-¹⁹F-phenylalanine were inhibitors of cell growth. The polypeptide chains of the major chromatophore membrane protein the light-harvesting complex, were isolated and shown by high-resolution ¹⁹F-NMR to contain 3-¹⁹F-phenylalanine, which is known to be situated principally within the membrane hydrocarbon layer. Broad-band ¹⁹F-NMR spectra of 3-¹⁹F-phenylalanine-labelled chromatophores showed the phenyl ring to be immobilized within the membrane.     

5-fluorouracil modulation of dihydrofolate reductase RNA levels in methotrexate-resistant KB cells

Cytotoxicity and growth inhibition by 5-fluorouracil in methotrexate-resistant dihydrofolate reductase gene-amplified KB cells in the presence of 30 microM thymidine correlates with incorporation of this fluorinated pyrimidine into RNA. Growth of these cells over several generations in the presence of inhibitory concentrations of 5-fluorouracil does not depress the steady state levels of either 18 or 28 S RNA but actually causes an increase. Similarly the rates of RNA and protein synthesis in 5-fluorouracil-treated cells are not decreased. The level of dihydrofolate reductase RNA from 5-fluorouracil-treated cells increases in a dose-dependent manner correlated with 5-fluorouracil incorporation into RNA. The qualitative size distribution of the dihydrofolate reductase RNA species is unaffected when examined by the Northern blotting technique indicating an RNA processing lesion is not induced by 5-fluorouracil incorporation into RNA. As the dose of dihydrofolate reductase RNA increases, there is no change in the level of dihydrofolate reductase specific activity, but the level of enzyme activity per cell increases. The relevance of these phenomena to the mechanism of 5-fluorouracil effect on RNA and relevance to combination chemotherapy with methotrexate are discussed.     

Visualization of different types of hapten-cell membrane interaction by fluorine high-field nuclear magnetic resonance

Using a high field spectrometer (5.9 Tesla) 19F-NMR spectrum of soluble material from 4 trifluoromethyl 2,6 dinitrobenzene sulphonate (CF3-DNBS) treated murine lymphocytes was recorded. CF3-DNBS is a fluorinated analog of 2,4,6 trinitrobenzene sulphonate (TNBS), and these compounds have been found to create cross-reacting antigenic modifications of cell surface. At least 4 distinguishable signals have been detected, and we think that 19F-NMR could be used to study, at a molecular level, some immunochemical problems concerning modification with TNBS.     

Raman and 19F(1H) nuclear Overhauser evidence for a rigid solution conformation of Escherichia coli 5-fluorouracil 5S ribonucleic acid

Escherichia coli cells grown on a medium containing 5-fluorouracil (FU) produce 5S RNA whose uracil residues are approximately 80% replaced by FU. The Raman spectra of native and FU-5S RNA are very similar, confirming similar solution conformations for the two species and a highly base-stacked structure in solution. The 254-MHz 19F NMR spectrum of FU-5S RNA shows that the 20-odd FU residues reside in at least ten distinct chemical environments, suggesting a highly ordered structure. Comparison of theoretical and experimental 19F(1H) nuclear Overhauser enhancements demonstrates definitively that virtually all the labeled uracils are bound to a rigid macromolecular frame, with a rotational correlation time of about 19 ns or longer. Since these uracils are widely distributed throughout the nucleotide primary sequence, it may be concluded that the entire FU-5S RNA solution structure is relatively rigid, in agreement with the most recently proposed "cloverleaf" secondary structural model for native prokaryotic 5S RNA.     

Evidence that intracellular synthesis of 5-fluorouridine-5'-phosphate from 5-fluorouracil and 5-fluorouridine is compartmentalized

L1210 cells were exposed to equitoxic concentrations of [14C]5-fluorouracil and [3H]5-fluorouridine for 4 hours. The RNA from these cells was separated into cytosolic and nuclear fractions, and then further fractionated by chromatography on poly-U Sepharose, Sephadex G-200 and DEAE-cellulose. The ratio of tritium to carbon-14 incorporated into various species of RNA differed by as much as 6-fold, indicating that the respective 5-fluorouridine-5'-monophosphates synthesized from the two precursors are localized in separate pools that do not mix rapidly.     

Effects of 5-fluorouracil on dihydrofolate reductase and dihydrofolate reductase mRNA from methotrexate-resistant KB cells

Growth of methotrexate-resistant dihydrofolate reductase gene-amplified KB cells in the presence of 5-fluorouracil results in an increase in dihydrofolate reductase mRNA. This increase can be solely attributed to a species of RNA of approximately 3.5 kilobase pairs in size. Although dihydrofolate reductase enzyme activity increases per cell with increasing 5-fluorouracil, there is a decrease of enzyme activity per mg of protein (Dolnick, B. J., and Pink, J. J. (1983) J. Biol. Chem. 258, 13299-13306). The rate of in vivo enzyme synthesis, as assayed by immunoprecipitation and supported by gel electrophoresis, does not decrease and may in fact increase with increasing 5-fluorouracil. Translation of purified dihydrofolate reductase mRNA in vitro shows that the rate of translation is unaffected by 5-fluorouracil incorporation into mRNA. The inhibition of dihydrofolate reductase by a monospecific polyclonal antiserum is reduced with extracts from 5-fluorouracil-treated cells. Inhibition of dihydrofolate reductase by methotrexate is significantly reduced in extracts from 5-fluorouracil-treated cells compared to control extracts. Tight binding of [3H]methotrexate is also different in extracts from 5-fluorouracil-treated cells. This data supports the hypothesis of translational miscoding during protein synthesis as a major mechanism of 5-fluorouracil-mediated cytotoxicity and suggests a new mechanism of 5-fluorouracil-methotrexate antagonism.     

色氨酸类似物标记法研究DsbA蛋白的结构环境

用生物标记的方法将色氨酸类似物标记在DsbA蛋白中的色氨酸位置,分析标记蛋白质的谱学性质、色氨酸结构环境和潜在应用前景.5-OH-Trp标记的DsbA蛋白具有315 nm激发的荧光发射光谱;¹⁹F-NMR 能分辨5-F-Trp标记的DsbA蛋白的两个F-Trp残基(Trp76和Trp126),Trp76化学位移变化反映二硫键交换引起的结构转化.进一步将利用标记蛋白的独特荧光和¹⁹F-NMR性质,研究DsbA蛋白的氧化还原及与底物蛋白的结合作用.     

Fluorine-19 nuclear magnetic resonance as a probe of the solution structure of mutants of 5-fluorouracil-substituted Escherichia coli valine tRNA.

In order to utilize 19F nuclear magnetic resonance (NMR) to probe the solution structure of Escherichia coli tRNAVal labeled by incorporation of 5-fluorouracil, we have assigned its 19F spectrum. We describe here assignments made by examining the spectra of a series of tRNAVal mutants with nucleotide substitutions for individual 5-fluorouracil residues. The result of base replacements on the structure and function of the tRNA are also characterized. Mutants were prepared by oligonucleotide-directed mutagenesis of a cloned tRNAVal gene, and the tRNAs transcribed in vitro by bacteriophage T7 RNA polymerase. By identifying the missing peak in the 19F NMR spectrum of each tRNA variant we were able to assign resonances from fluorouracil residues in loop and stem regions of the tRNA. As a result of the assignment of FU33, FU34 and FU29, temperature-dependent spectral shifts could be attributed to changes in anticodon loop and stem conformation. Observation of a magnesium ion-dependent splitting of the resonance assigned to FU64 suggested that the T-arm of tRNAVal can exist in two conformations in slow exchange on the NMR time scale. Replacement of most 5-fluorouracil residues in loops and stems had little effect on the structure of tRNAVal; few shifts in the 19F NMR spectrum of the mutant tRNAs were noted. However, replacing the FU29.A41 base-pair in the anticodon stem with C29.G41 induced conformational changes in the anticodon loop as well as in the P-10 loop. Effects of nucleotide substitution on aminoacylation were determined by comparing the Vmax and Km values of tRNAVal mutants with those of the wild-type tRNA. Nucleotide substitution at the 3' end of the anticodon (position 36) reduced the aminoacylation efficiency (Vmax/Km) of tRNAVal by three orders of magnitude. Base replacement at the 5' end of the anticodon (position 34) had only a small negative effect on the aminoacylation efficiency. Substitution of the FU29.A41 base-pair increased the Km value 20-fold, while Vmax remained almost unchanged. The FU4.A69 base-pair in the acceptor stem, could readily be replaced with little effect on the aminoacylation efficiency of E. coli tRNAVal, indicating that this base-pair is not an identity element of the tRNA, as suggested by others.     

Solid-state 19F-NMR analysis of 19F-labeled tryptophan in gramicidin A in oriented membranes

The response of membrane-associated peptides toward the lipid environment or other binding partners can be monitored by solid-state NMR of suitably labeled side chains. Tryptophan is a prominent amino acid in transmembrane helices, and its (19)F-labeled analogues are generally biocompatible and cause little structural perturbation. Hence, we use 5F-Trp as a highly sensitive NMR probe to monitor the conformation and dynamics of the indole ring. To establish this (19)F-NMR strategy, gramicidin A was labeled with 5F-Trp in position 13 or 15, whose chi(1)/chi(2) torsion angles are known from previous (2)H-NMR studies. First, the alignment of the (19)F chemical shift anisotropy tensor within the membrane was deduced by lineshape analysis of oriented samples. Next, the three principal axes of the (19)F chemical shift anisotropy tensor were assigned within the molecular frame of the indole ring. Finally, determination of chi(1)/chi(2) for 5F-Trp in the lipid gel phase showed that the side chain alignment differs by up to 20 degrees from its known conformation in the liquid crystalline state. The sensitivity gain of (19)F-NMR and the reduction in the amount of material was at least 10-fold compared with previous (2)H-NMR studies on the same system and 100-fold compared with (15)N-NMR.     

Doxorubicin and 5-fluorouracil resistant hepatic cancer cells demonstrate stem-like properties

The efficacy of hepatocellular carcinoma (HCC) treatment is very low because of the high percentage of recurrence and resistance to anticancer agents. Hepatic cancer stem cells (HCSCs) are considered the origin of such recurrence and resistance. Our aim was to evaluate the stemness of doxorubicin and 5-fluorouracil resistant hepatic cancer cells and establish the new method to isolate the HCSCs from primary cultured HCC tumors. HCC biopsies were used to establish primary cultures. Then, primary cells were selected for HCSCs by culture in medium supplemented with doxorubicin (0, 0.1, 0.25, 0.5 or 1 μg/mL), 5-fluorouracil (0, 0.1, 0.25, 0.5 or 1 μg/mL) or their combination. Selection was confirmed by detection of HCSC markers such as CD133, CD13, CD90, and the side population was identified by rhodamine 123 efflux. The cell population with the strongest expression of these markers was used to evaluate the cell cycle, gene expression profile, tumor sphere formation, marker protein expression, and in vivo tumorigenesis. Selective culture of primary cells in medium supplemented with 0.5 μg/mL doxorubicin and 1 μg/mL 5-fluorouracil selected cancer cells with the highest stemness properties. Selected cells strongly expressed CD13, CD133, CD90, and CD326, efflux rhodamine 123 and formed tumor spheres in suspension. Moreover, selected cells were induced to differentiate into cells with high expression of CD19 and AFP (alpha-fetoprotein), and importantly, could form tumors in NOD/SCID mice upon injection of 1 × 10⁵ cells/mouse. Selective culture with doxorubicin and 5-fluorouracil will enrich HCSCs, is an easy method to obtain HCSCs that can be used to develop better therapeutic strategies for patients with HCC, and particularly HCSC-targeting therapy.     

Nucleotide sequence of nuclear 5.7S RNA of mouse cells

Viable tumor cells were examined by ¹⁹F-NMR spectroscopy after treatment with 4-trifluoromethyl 2,6-dinitrophenyl sulphonate (CF₃-DNBS), which is an analog of 2,4,6-trinitrophenyl sulphonate (TNBS). The presence of a strong ¹⁹F-NMR signal from treated cells suggested the binding of the “probe”. Treatment of labelled cells with proteolytic enzymes significantly decreased the signals, suggesting that the label was essentially bound to the cell surface macromolecules. A proportion of the material bound to the cell was removable by dialysis of cell extracts against a structural analog, suggesting that some CF₃-DNBS-cell membrane bonds were not covalent. The existence of such non-covalent bonds has also been confirmed with soluble proteins. By the same approach, it was also found that ¹⁴C-TNBS formed covalent and non-covalent bonds with tumor cells.     

19F-nuclear magnetic resonance: measurements of [O2] and pH in biological systems.

19F-nuclear magnetic resonance (NMR) has been used to determine both intracellular pH and oxygen concentrations in cell suspensions. Oxygen concentrations in Paracoccus denitrificans and insulinoma cells, RINm5F, in the NMR probe can be monitored directly by 1/T1 measurements of perfluorotripropylamine (FTPA)/lecithin emulsion added to the suspensions. With FTPA oxygen monitoring, we investigated the relative aeration capabilities of two types of NMR chambers. Both normal and transformed eucaryotic cells can be maintained in either chamber for at least 1-2 h at cytocrits of up to 20-25%, with 30% oxygen saturation and cell viabilities of 90-95%. Similar concentrations of procaryotes were maintained aerobic with high FTPA concentrations in the more efficient of the two NMR chambers. A new precursor molecule for the 19F-NMR pH indicator difluoromethylalanine, the para-chlorophenyl ester, has been tested and used in RINm5F cells and P. denitrificans, neither of which hydrolyzes methyl esters.     

The effect of 5-fluorouracil on the mitotic cycle and DNA synthesis in the epithelium of mouse small intestine.

The examinations were performed on 42 mice of the Porton strain. The experimental animals were injected intraperitoneally with the dose of 75 mg of 5-fluorouracil per kg body weight. The first experimental group received injections of [3H]thymidine within 48 hours and the second group within 96 hours of the injection of 5-fluorouracil. Two mice from each group were killed at within 1, 2, 4, 8, 12, 24, and 48 hours of the [3H]thymidine injection. Calculations of the mitotic index and time of duration of individual phases of the mitotic cycle in epithelial cells of the small intestine were based on application of the autoradiographic method. These studies lead to the conclusion that 5-fluorouracil disturbs the course of metabolic processes in the cell, which are also related with the distribution of the genetic material. Histological examinations show that 5-fluorouracil produces profound morphological changes in the intestine, which affect both the intestinal epithelium and the connective tissue stroma. The autoradiographic tests revealed a considerable suppression of the mitotic activity of the epithelium of intestinal crypts. Moreover, it was shown that 5-fluorouracil inhibits the mitotic activity of the intestinal epithelium by diminishing the number of cells capable of entering into mitosis. Nevertheless, by 96 hours following introduction of a single dose of 5-fluorouracil normal morphological structure and mitotic activity of the intestinal wall cells are restored.