A micro-extraction technique using a new digitally controlled syringe combined with UHPLC for assessment of urinary biomarkers of oxidatively damaged DNA.
(17/29)
Purification and characterization of 5-hydroxymethyluracil-DNA glycosylase from calf thymus. Its possible role in the maintenance of methylated cytosine residues.
(18/29)
5-Hydroxymethyluracil (HmUra) residues formed by the oxidation of thymine are removed from DNA through the action of a DNA glycosylase activity. This activity was purified over 1870-fold from calf thymus and found to be distinct from uracil (Ura)-DNA glycosylase. The HmUra-DNA glycosylase has a molecular weight of 38,000, a pH optimum of 6.7-6.8 and an apparent Km of 0.73 +/- 0.04 microM. These values are similar to those reported for other mammalian DNA glycosylases. The enzyme removed HmUra residues from single- and double-stranded DNA with almost equal efficiency. HmUra-DNA glycosylase activity was not product inhibited by free HmUra. The DNA glycosylase activity was inhibited by Mg2+, but the purest enzyme fractions contained a Mg2+-dependent apurinic/apyrimidinic endonuclease activity. HmUra-DNA glycosylase and the recently described 5-hydroxymethylcytosine (HmCyt)-DNA glycosylase (Cannon, S. V., Cummings, A. C., and Teebor, G. W. (1988) Biochem. Biophys. Res. Commun. 151, 1173-1179) are unique among known DNA glycosylases in being present in mammalian cells and absent from bacteria. These DNA glycosylase activities were shown here to reside on different proteins. We suggest that the major function of HmUra-DNA glycosylase, together with HmCyt-DNA glycosylase, is the maintenance of methylated cytosine residues in the DNA of higher organisms. (+info)
Effects of 5-hydroxymethyluracil and 3-aminobenzamide on the repair and toxicity of 5-hydroxymethyl-2'-deoxyuridine in mammalian cells.
(19/29)
5-Hydroxymethyl-2'-deoxyuridine (HmdUrd), a cytotoxic analogue of thymidine, has been proposed for use as an anticancer agent. HmdUrd is incorporated into DNA and then removed at a rate of 30-40% per day. The removal of HmdUrd from DNA has been attributed to the action of 5-hydroxymethyluracil-DNA glycosylase (HmUra-DNA glycosylase). We demonstrated the release of [3H]HmUra into the growth medium of V79 Chinese hamster cells that had incorporated [3H]HmdUrd into their DNA. The amount of [3H]HmUra recovered from the growth medium was equal to the amount of [3H]HmdUrd lost from DNA. These experiments confirmed that the initial step of repair of HmUra in DNA was mediated by DNA glycosylase activity. A combination of HmUra and HmdUrd resulted in increased uptake of HmdUrd by cells and increased cytotoxicity. The increased incorporation of HmdUrd into DNA was not due to inhibition of repair. 3-Aminobenzamide, an inhibitor of poly(ADP-ribose) synthesis, was cytotoxic to cells which incorporated and repaired HmdUrd. The extent of toxicity was directly related to the number of HmUra residues in DNA. HeLa cells, known to be resistant to the toxic effects of HmdUrd, do not incorporate HmdUrd into their DNA. HeLa cells were resistant to the toxic effects of 3-aminobenzamide, confirming that the absence of HmdUrd in their DNA was not due to an accelerated rate of repair. These experiments indicate that the potential therapeutic antineoplastic properties of HmdUrd may be enhanced by using HmUra to increase the incorporation of HmdUrd into DNA and 3-aminobenzamide to interfere with repair of HmUra in DNA. (+info)
Mutagenicity of 5-hydroxymethyl-2'-deoxyuridine to Chinese hamster cells.
(20/29)
5-Hydroxymethyluracil (HmUra) is formed from thymine in DNA through the action of ionizing radiation or reactive oxygen species generated by activated leukocytes. HmUra is removed from DNA by a specific DNA glycosylase, suggesting that it is also formed from endogenously generated reactive oxygen species and that its formation in DNA is potentially deleterious. To determine whether HmUra residues in DNA are mutagenic, hamster V79 cells were grown in the presence of 5-hydroxymethyl-2'-deoxyuridine (HmdUrd) which is incorporated into DNA, and mutagenicity at the ouabain- and thioguanine-resistant loci was determined. Levels of substitution ranged from 1/500 to 1/5,000 HmUra residues/thymine residues. There was slight mutagenicity at the thioguanine-resistant locus but none at the ouabain-resistant locus. The mutagenicity of HmdUrd, expressed as a function of HmUra substitution in DNA, was 1/30,000 in the hypoxanthine-guanine-phosphoribosyltransferase target gene. This low frequency indicates that the oxidation of thymine to HmUra in a preexisting AT base pair does not contribute significantly to the mutagenicity of ionizing radiation, because the yield of HmUra formed in DNA at mutagenic doses of radiation is too low. To determine whether repair of HmUra might be inhibited by ionizing radiation, cells were grown in medium containing HmdUrd and exposed to as much as 5 Gy of gamma-irradiation, and the removal of HmUra from DNA was measured. No inhibition of repair was noted. Preirradiation of cells neither accelerated the rate of repair nor raised the level of HmUra-DNA glycosylase activity, indicating that repair of HmUra was not induced by this type of oxidative stress. Although the mutagenicity of HmUra residues in DNA is low, even a rare mutation might be sufficiently deleterious to higher organisms to promote the development of HmUra-DNA glycosylase activity. (+info)
Toxicity of 3-aminobenzamide to Chinese hamster cells containing 5-hydroxymethyluracil in their DNA.
(21/29)
V79 cells incorporated 5-hydroxymethyl-2'-deoxyuridine (HmdUrd) into their DNA linearly over a wide range of concentrations and time. Cells grew normally when 0.03% of thymidine residues were replaced with HmdUrd. At this level of substitution, 5-hydroxymethyluracil (HmUra) was removed from DNA at a rate of 30-40%/24 h. Concentrations of HmdUrd in the growth medium which produced higher levels of substitution reduced survival and caused cells to delay their transit through S phase. However, the treatment of HmdUrd-containing cells with 3-aminobenzamide caused extensive cell death. At levels of HmdUrd substitution compatible with near 90% survival, the addition of 3-aminobenzamide, an inhibitor of poly (adenosine diphosphoribose) synthesis, killed over 90% of the cells. This toxicity was not due to inhibition of the removal of HmUra from DNA. Cells killed by this combination of agents arrested in the G2 phase of the cell cycle. We conclude that the toxicity of HmdUrd resulted primarily from the repair of the HmUra residue in DNA and not from any intrinsic toxicity of the HmUra residue itself. We also conclude that the cytotoxicity of 3-aminobenzamide resulted from interference with the completion of DNA repair following base (HmUra) excision. Since HmUra is also formed in DNA through the action of ionizing radiation, it may be among the components of radiation-induced DNA damage which sensitizes cells to 3-aminobenzamide. (+info)
DNA binding by the bacteriophage SPO1-encoded type II DNA-binding protein, transcription factor 1. Site-specific binding requires 5-hydroxymethyluracil-containing DNA.
(22/29)
The bacteriophage SPO1-encoded Type II DNA-binding protein, transcription factor 1 (TF1), forms complexes with specific sites in SPO1 DNA. We have investigated the binding of TF1 to one of its preferred sites in which the normal 5-hydroxymethyluracil (hmUra) of SPO1 DNA has been replaced by thymine and have also investigated the binding of a bacterial Type II DNA-binding protein (from Bacillus stearothermophilus) to the hmUra- and thymine-containing forms of the same DNA segment. Our results show that TF1 binds selectively to this high affinity binding site only in hmUra-containing DNA and that the bacterial Type II DNA-binding protein interacts nonspecifically with both forms of DNA. (+info)
Resistance of bacteriophage H1 to restriction and modification by Bacillus subtilis R.
(23/29)
H1, a 5-hydroxymethyluracil (HMU)-containing Bacillus subtilis bacteriophage, was neither restricted nor modified upon infection of B. subtilis R cells. In vitro, H1 DNA was not restricted by BsuR under standard conditions (200 mM salt), although the expected frequency of -GGCC- cleavage sites was approximately 250. However, four specific sites were cleaved under nonstandard conditions (low salt or high pH) or in the presence of organic solvents, like dimethyl sulfoxide and glycerol. After the substitution of thymine for HMU by DNA cloning in B. subtilis, a BsuR cleavage site was restricted and modified under standard conditions. No additional sites were detected after shotgun-cloning of about 11% of the chromosome. The nucleotide sequence of a cleavage site was found to be 5'. .C-A-Hmu-A-A-C-Hmu-Hmu-Hmu-G-G-C-C-Hmu-A-G-. . .3', which shows the presence of a bona fide BsuR (GGCC) recognition sequence, flanked by (Hmu-A)-rich sequences. The results suggested that the resistance of H1 to restriction and modification by B. subtilis R was due to (i) a strong bias against the GGCC-recognition sequence and (ii) protection of the four remaining GGCC sites as a consequence of HMU-A base pairs flanking the sites. (+info)
Markedly different ascorbate dependencies of the sequential alpha-ketoglutarate dioxygenase reactions catalyzed by an essentially homogeneous thymine 7-hydroxylase from Rhodotorula glutinis.
(24/29)
The alpha-ketoglutarate dioxygenase, thymine 7-hydroxylase (EC 1.14.11.6), has been purified from cultures of Rhodotorula glutinis grown with thymine as a nitrogen source. The purification scheme developed yielded essentially homogeneous preparations of the 7-hydroxylase and also purified another alpha-ketoglutarate dioxygenase, pyrimidine deoxyribonucleoside 2'-hydroxylase (EC 1.14.11.3). The purity of the 7-hydroxylase was determined with analytical disc gel electrophoresis in which runs were varied with respect to pH, extent of cross-linking, and the presence of sodium dodecyl sulfate-mercaptoethanol. The 7-hydroxylase apparently exists as a monomer since its molecular weight was 42,700 when determined by molecular gel filtration chromatography and was 40,300 when determined by analytical disc gel electrophoresis under denaturing conditions. Gel filtration chromatography under nondenaturing conditions was used to show that the 2'-hydroxylase has a molecular weight of 64,600. The essentially homogeneous preparations of the 7-hydroxylase were shown to catalyze the thymine-, 5-hydroxymethyluracil-, and 5-formyluracil-dependent oxygenations that are coupled to the decarboxylation of alpha-ketoglutarate, as well as a putative uncoupled decarboxylation which is dependent on uracil. Furthermore, these enzyme preparations were used to show that ATP stimulated the 7-hydroxylase reaction in the absence of ascorbate. Even though it is attractive to consider the four pyrimidine-dependent reactions as being catalyzed by the same active site, they were shown to differ markedly in their dependencies on ascorbate or ATP. The effects of ascorbate and ATP on these reactions, and on the 2'-hydroxylase reaction, are discussed in terms of the possible roles of ascorbate and ATP. (+info)