Purification and characterization of an endonuclease from Micrococcus luteus that acts on depurinated and carcinogen-modified DNA

Detection of base damage in DNA in human blood exposed to ionizing radiation at biologically relevant doses

The alkaline elution technique for the detection of DNA damage has been adapted to allow application on unlabelled blood cells. Both the induction and subsequent repair have been studied of two classes of DNA damage, viz, single-strand breaks and base damage recognized by the gamma-endonuclease activity in a cell-free extract of Micrococcus luteus bacteria. The high sensitivity of the assay permitted the measurement of induction and repair of base damage after in vitro exposure of full blood under aerobic conditions to biologically relevant doses of gamma-rays (1.5-4.5 Gy). After a radiation dose of 3 Gy about 50% of the base damage was removed within 1.5 h of repair. Base damage could still be detected at 24 h after exposure to 15 Gy.

The DNA cleavage induced by a chromium(V) complex and by chromate and glutathione is mediated by activated oxygen species

The number of strand breaks induced by the combination of chromate and glutathione (GSH) in PM2 DNA was effectively reduced upon addition of the hydroxyl radical scavengers dimethyl sulphoxide (DMSO), formate and benzoate. Administration of catalase also led to a depression of DNA degradation whereas superoxide dismutase (SOD) had very little influence. Essentially the same results were obtained in experiments employing a chromium(V) complex Na4(GSH)4Cr.8H20, which is an intermediate chromium species isolated from the reduction of chromate by glutathione. DNA cleavage was dependent on the presence of iron (FeCl3). When compared with the number of breaks produced by FeCl3 and GSH alone, chromate stimulated the generation of single-strand breaks. These findings suggest that hydroxyl radicals are one ultimate DNA cleaving agent in both reactions. A reaction scheme for the production of hydroxyl radicals is proposed.

Comparison of the cytotoxicity and DNA-damaging properties of 2,4-D and U 46 D fluid (dimethylammonium salt of 2,4-D)

U 46 D Fluid (the dimethylammonium salt of 2,4-dichlorophenoxyacetic acid in a commercial formulation) was more toxic to human fibroblasts than 2,4-dichlorophenoxyacetic acid (2,4-D). Moreover, U 46 D Fluid induced single-strand breaks at apurinic/apyrimidinic (AP) sites of heat-acid treated PM2 DNA while 2,4-D did not. The 1H-NMR spectrum of the dimethylammonium (DMA) salt of 2,4-D indicated the formation of a complex via a six-membered aggregate of the DMA ion and the acid group of 2,4-D. The role of complex formation of the DMA salt of 2,4-D as a cause of single-strand breaking activity and increased cytotoxicity is discussed.

The role of hydroxyl radicals in tetrachlorohydroquinone induced DNA strand break formation in PM2 DNA and human fibroblasts

Tetrachlorohydroquinone (TCHQ), which has previously been identified as a metabolite of pentachlorophenol, induces DNA strand breaks in isolated DNA and in human fibroblasts. Strand break formation in PM2 DNA is prevented by the addition of catalase and the hydroxyl radical scavengers DMSO, ethanol and mannitol, whereas addition of SOD reduced SSB only slightly. Oxygen radicals are formed by the autoxidation of TCHQ to the tetrachlorosemiquinone radical. Desferrioxamine (0.2 mM) completely abolished strand break formation, whereas the metal chelator DETAPAC (1 mM) reduced SSB by only 8.5%. The formation of the semiquinone radical at physiological conditions is shown by ESR spectroscopy. Exposure of human fibroblasts to TCHQ also leads to DNA single strand breaks measured by the alkaline elution assay. These were reduced by addition of 5% DMSO. This indicates that at least part of the strand break formation in human cells is also due to the action of hydroxyl radicals.

Generation of PM2 DNA breaks in the course of reduction of chromium(VI) by glutathione

The carcinogen chromate is efficiently taken up and reduced to chromium(III) compounds by various biological systems. To test the possible DNA damage induced in the course of chromium(VI) reduction, we used a combination of chromate with the reductant glutathione (GSH) as well as a green complex of chromium(V), which is formed in the reaction of chromate with GSH. The combination of chromate and glutathione was found to cause single-strand breaks in supercoiled circular DNA of the bacteriophage PM2. The green chromium(V) complex Na4(GSH)4Cr(V).8H2O, prepared from chromate and glutathione, also cleaved supercoiled PM2 DNA. No DNA-degrading effects were observed with either chromate or the final product of the reaction with GSH, a purple anionic chromium(III) GSH complex. The nature of the buffering agents revealed a strong influence on the extent of DNA strand breaks produced by chromate and GSH. A variation of the GSH concentration in the reaction with chromate and PM2 DNA, performed in sodium phosphate-buffered solutions showed an initial increase in the number of strand breaks at GSH concentrations up to 1 mM followed by a decline at higher GSH concentrations. Since neither chromate, when administered individually, nor the final product of chromium(VI) reduction, the purple chromium(III) GSH complex, produced any detectable DNA cleavage, the critical steps leading to DNA strand breaks occur in the course of the conversion of chromium(VI) to chromium(III) by GSH, the most abundant intracellular low molecular thiol. Moreover, the demonstration that DNA cleavage is induced in the presence of the chromium(V) complex identifies chromium(V) as the oxidation state of the metal, which is involved in the steps leading to DNA-damaging effects of chromate.

Photoalkylated DNA and ultraviolet-irradiated DNA are incised at cytosines by endonuclease III

Photoalkylation, the ultraviolet irradiation of DNA with isopropanol and di-tert-butylperoxide, causes a variety of base alterations. These include 8-(2-hydroxy-2-propyl)guanines, 8-(2-hydroxy-2-propyl)adenines and thymine dimers. An E. coli endonuclease against photoalkylated DNA was assayed by conversion of superhelical PM2 phage DNA to the nicked form. Enzyme activities were compared between extracts of strain BW9109 (xth-), lacking exonuclease III activity, and strain BW434 (xth-,nth-), deficient in both exonuclease III and endonuclease III. The endonuclease level in the double mutant against substrate photoalkylated DNA was under 20% of the activity in the mutant lacking only exonuclease III. Irradiation of the DNA substrate in the absence of isopropanol did not affect the activity in either strain. Analysis by polyacrylamide gel electrophoresis identified the sites of DNA cleavage by purified E. coli endonuclease III as cytosines, both in DNA irradiated at biologically significant wavelengths and in photoalkylated DNA. Neither 8-(2-hydroxy-2-propyl)purines, pyrimidine dimers, uracils nor 6-4'-(pyrimidin-2'-one)pyrimidines were substrates for the enzyme.

Detection of DNA damage in human cells and tissue using sequencing techniques

Methods have been developed that permit both identification and location of sites of alterations is defined DNA sequences. These methods can be extended to human tissues using the alphoid segment, which comprises 1% of the human genome. This segment can be isolated in ample quantities from human cells and tissues. Once purified and end labeled, this defined segment can be used to detect sites of altered DNA moieties by combining Maxam-Gilbert sequencing protocols with appropriate enzymatic probes and chemical techniques. These studies can be performed in cultured cells or in tissues obtained by surgical excision or autopsy.

DNA-damaging properties and cytotoxicity in human fibroblasts of tetrachlorohydroquinone, a pentachlorophenol metabolite

The DNA-damaging potential of pentachlorophenol (PCP) and its metabolite tetrachlorohydroquinone (TCH) was investigated. TCH was found to bind covalently to calf-thymus DNA and to cause single-strand breaks in PM2 DNA. No DNA-damaging effects were observed for PCP. Exposure of human fibroblasts to PCP and TCH showed that TCH is more toxic, when colony-forming ability after exposure to the agent is used as a measure of toxicity. In the evaluation of the mutagenic and carcinogenic potential of PCP the metabolite TCH should be taken into consideration.

Apurinic endonuclease from Saccharomyces cerevisiae

An endonuclease cleaving depurinated and alkylated double-stranded DNA has been purified 500-fold from Saccharomyces cerevisiae, strain MB 1052. The enzyme has an Mr of 31 000 +/- 2000, a sedimentation value of 3.2S and a diffusion coefficient of 9.5 X 10-7 cm2/s. The enzyme was active only at apurinic/apyridiminic sites, regardless of whether they were produced by heating the DNA at acidic pH or by alkylation with the ultimate carcinogen methyl methanesulphonate. Native DNA was not acted upon. U.v.-irradiated DNA and DNA treated with the ultimate carcinogen N-acetoxy-2-acetylaminofluorene were cleaved to an extent related to the extent of apurinic/apyridiminic sites. Enzymic activity was not dependent upon Mg2+, but was stimulated approx. 3-fold by 4mM-Mg2+. The enzyme did not bind to DEAE-cellulose or CM-cellulose at KCl concentrations greater than 160 mM. The endonuclease was obtained free of exonuclease and 3-methyladenine-DNA glycosylase activity in five chromatographic steps.

Purification and properties of a mouse-cell DNA-repair endonuclease, which recognizes lesions in DNA induced by ultraviolet light, depurination, gamma-rays, and OsO4 treatment

A DNA-repair endonuclease has been purified 117-fold from mouse plasmacytoma cells (line MPC-11) by gel filtration, followed by ion-exchange and affinity chromatography. Its molecular weight was determined by gel filtration to be 28,000 +/- 2000. The enzyme recognizes apurinic and apyrimidinic sites induced by acid and gamma-rays in DNA, as well as another type of lesion(s) which is introduced into DNA by both ultraviolet irradiation and OsO4. Quantitative measurements of the number of nicks the purified DNA-repair endonuclease makes in DNA treated with various amounts of OsO4 and ultraviolet light suggests that the endonuclease may act on 5,6-dihydroxydihydrothymine lesions. The endonuclease activity was sensitive to the ionic strength and was most active in the presence of 100 mM KCl, whereas the presence of divalent cations did not stimulate the activity.

Micrococcus luteus endonucleases for apurinic/apyrimidinic sites in deoxyribonucleic acid. 1. Purification and general properties

Two chromatographically distinct endonucleases from Micrococcus luteus, specific for apurinic and apyrimidinic sites (AP-endonucleases A and B), have been extensively purified and characterized. Both are free from DNA glycosylase, unspecific endonuclease, and phosphatase activities. The two enzymes behave as monomeric proteins of approximately 35000 daltons. In addition to their different chromatographic properties on CM-cellulose, P-cellulose, hydroxylapatite, and DNA--Sepharose, both AP-endonucleases can be distinguished as follows: AP-endonuclease A has an isoelectric point of 4.8, shows a half-life of 4 min at 45 degrees C, reacts optimally at pH 7.5 and has a KM value of 2.3 X 10(-6) M. AP-endonuclease B has a pI of 8.8, is more stable at 45 degrees C (half-life of 10 min), and reacts optimally between pH 6.5 and pH 8.5; its KM value is 3.7 X 10(-6) M.

Purification and properties of the major apurinic/apyrimidinic endodeoxyribonuclease of rat-liver chromatin

Two nucleases active on alkylated-depurinated DNA have been extracted from rat liver chromatin with 1 M KCl. The major enzyme was purified to near homogeneity; it has a molecular weight of 12 500 (although some dimerization might occur), needs Mg2+ or Mn2+ for activity. The endonuclease activity is specific for apurinic/apyrimidinic sites in DNA; the enzyme has no associated exonuclease activity.

Defective excision repair in a mutant of Micrococcus radiodurans hypermutable by some monofunctional alkylating agents

The lethal and mutagenic effects of methyl methanesulphonate (MMS), ethyl methanesulphonate (EMS), and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) can be dissociated in a mitomycin C (MTC)-sensitive mutant, strain 302, of Micrococcus radiodurans. As regards lethality 302 is extremely sensitive, compared with the wild type, to MTC and decarbamoyl MTC (DCMTC), slightly sensitive to EMS, MNNG, nitrous acid, 7-bromomethylbenz[alpha]anthracene (BrMBA), and N-acetoxy-N-2-acetylaminofluorene (AAAF), and resistant to MMS, hydroxylamine, and ICR 191G. As regards mutability it is, compared to the wild type, very sensitive to MMS, EMS, and MNNG, and slightly sensitive to hydroxylamine and nitrous acid but not to any other agent examined. Alkaline sucrose gradient studies indicate the 302 does not incise DNA containing BrMBA adducts, although it does incise DNA damaged by AAAF but probably not to the same extent as wild type. We put forward the hypothesis that the hypermutability of 302 is due to the non-removal of bases or nucleotides, modified in exocyclic positions, which have altered base-pairing capabilities, while lethality results from the non-removal of bases or nucleotides, also modified in exocyclic positions, which no longer form hydrogen-bonded base pairs.

Correlation of the colony-forming abilities of xeroderma pigmentosum fibroblasts with repair-specific DNA incision reactions catalyzed by cell-free extracts

Several normal and XP group A fiblast cell lines were exposed to the weakly carcinogenic and toxic agent methyl methanesulfonate, and the differences in their abilities to form colonies were determined. The XP group A cell lines investigated exhibited higher sensitivity towards methyl methanesulfonate than normal cell lines. Correspondingly, cell-free extracts of the same XP cell lines differed from normal ones in cleaving methyl methane-sulfonate-treated double-stranded DNA less rapidly. Since depurinated DNA was cleaved by XP and normal cell lines at equal rates, it was concluded that the differences observed with methylated DNA were due to a reaction preceding cleavage at apurinic sites. In control experiments using extracts from Chinese hamster ovary cells liberation of m3Ade was observed indicating the presence of 3-methyl-adenine DNA glycosylase activity. Furthermore, extracts from a normal fibroblast line liberated small amounts of m3Ade, whereas the one of a XP group A cell line was found to be less effective. The possible role of 3-methyl-adenine DNA glycosylase activity as a rate-limiting factor in the incision step has been discussed.

Modification of PM2 DNA with N-acetoxy-2-acetylaminofluorene: changes in buoyant density, electrophoretic mobility and electron microscopic structure

PM2 DNA, forms I and II, were chemically modified with the ultimate carcinogen N-acetoxy-2-acetylaminofluorene. The products were subjected to 3 analytical procedures: (i) buoyant density centrifugation in the presence of the intercalating dyes ethidium bromide or propidium diiodide; (ii) agarose gel electrophoresis: and (iii) electron microscopy. It was found that carcinogen-modified DNAs banded at higher densities in the presence of either dye. This effect was far more pronounced with form I DNA than with form II. Form I DNA showed a concentration-dependent decrease of migration velocity in agarose gel electrophoresis after modification; form II DNA exhibited a similar but smaller effect. Electron microscopy of form I DNA did not reveal a consistent correlation between the extent of chemical modification and the loss of superhelical turns. Form II DNA exhibited characteristic alterations such as aggregation of several PM2 molecules into network-like structures, kinky configuration, and probably single-stranded bubbles extending over 600-1500 bases.

The excision of N-methyl-N-nitrosourea-induced lesions from the DNA of Chinese hamster cells as measured by the loss of sites sensitive to an enzyme extract that excises 3-methylpurines but not O6-methylguanine

An enzyme extract from Micrococcus luteus excises 3-methyladenine and 3-methylguanine but not O6-methylguanine, 7-methylguanine, 1-methyladenine or 7-methyladenine from DNA reacted with N-methyl-N-nitrosourea. The extract was used to detect lesions in the DNA of Chinese hamster cells treated in culture with N-methyl-N-nitrosourea. It was concluded that 3-methyladenine is excised from these cells with a half-life of about 2.3 h.