Repair of cyclic nucleic acid adducts and adverse effects of apurinic sites.
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Alkylating Agents↗
Biomedical subjects
J Laval
Publications and source records attributed to J Laval.
The Escherichia coli O6-methylguanine-DNA methyltransferase does not repair promutagenic O6-methylguanine residues when present in Z-DNA.
The repair of O6-methylguanine present in N-methylnitrosourea (MNU)-treated alternating polynucleotides MNU-poly(dG-dC) X poly(dG-dC) and MNU-poly(dG-me5dC) X poly(dG-me5dC] was investigated using O6-methylguanine-DNA methyltransferase purified from Escherichia coli. Both modified polynucleotides are equally good substrates for the DNA methyltransferase when they are in the B-form. The substrate properties of the MNU-treated polynucleotides do not differ from those of MNU-treated DNA. One of these modified polynucleotides, MNU-poly(dG-me5dC) X (dG-me5dC), can adopt the Z-conformation under physiological conditions. The conformational transition of the poly(dG-me5dC) X poly(dG-me5dC) from the B-form to the Z-form was monitored by the modification of its spectroscopic properties and by the specific binding of antibodies raised against Z-DNA. The O6-methylguanine residues are repaired in MNU-poly(dG-me5dC) X poly(dG-me5dC) in B-form. At variance, the conversion of this template to the Z-form completely inhibits the repair of the O6-methylguanine residues. The cooperative transition from the Z- to the B-form of MNU-poly(dG-me5dC) X poly(dG-me5dC), mediated by intercalating drugs such as ethidium bromide, restores the ability of MNU-poly(dG-me5dC) X poly(dG-me5dC) to be substrate for the transferase. These results imply that the promutagenic DNA lesion O6-methylguanine persists in Z-DNA fragments and suggest that DNA conformation modulates the extent of DNA repair and, as a result, plays an important role in determining the mutagenic potency of chemical carcinogens.
DNA Repair↗
Excision of uracil residues in DNA: mechanism of action of Escherichia coli and Micrococcus luteus uracil-DNA glycosylases.
Various octadeoxynucleotides containing uracil at different positions were synthesized and submitted to the action of Escherichia coli and Micrococcus luteus uracil-DNA glycosylases. A uracil residue situated at the 5'-end was excised by the M.luteus enzyme but not by the E.coli one. Uracil residues located at the ultimate and penultimate positions at the 3'-end were not cleaved by either enzymes. At the other central positions, uracil was eliminated with different initial velocities. Single stranded phi X 174 DNA fragments were used to study the influence of the sequence. Cytosine bases were deaminated to give uracil by bisulfite treatment. It was shown that the initial excision velocity of two vicinal uracil residues was decreased. The same observation was made for two uracils separated by one base. A hypothetical scheme is suggested to explain the mechanism of action of uracil-DNA glycosylases.
Base Sequence↗
Long-term effect of somatostatin 14 on mouse stomach, antrum, intestine and exocrine pancreas.
Mice were injected 3 times a day for 12 days with 8 micrograms/kg of somatostatin 14 which caused a hypoplasia of parietal and goblet cells, a hypotrophy and hypofunctionality of pancreatic acinar cells with a decrease in lipase and chymotrypsin activities, a decrease in the secretory fuction of the Brunner gland and in the number of dark granules of G cells. Neither villous and microvillous areas nor brush border hydrolase activities were affected. The number of peptic cells and Paneth cells increase as the level of pepsin and lysozyme. Mice were injected 4 times per hour with 2 micrograms/kg of somatostatin. 2 h after the first injection of somatostatin and 90 min after a single injection of tritiated thymidine, fundic, antral, jejunal and ileal labelling indexes strongly decrease (maximal effect in ileum). The inhibitory effect of somatostatin on the digestive epithelial cell proliferation compared to its long-term action only directed on specific cell types evokes probable compensatory mechanisms induced to maintain the equilibrium of the digestive epithelia.
Animals↗
Comparative study of histological and kinetic variations of the digestive mucosa and pancreatic parenchyma after hypophysectomy in the rat. Light and electron microscopic study.
Variations of the pancreatic parenchyma, the gastric mucosa and the intestinal mucosa were studied in adult male Wistar rats on day 8 and 15 after hypophysectomy. All results were compared with those obtained in pair-fed control rats. Hypophysectomy affected small intestine as well as gastric mucosa. Hypotrophy was observed on day 8 as most of the morphological parameters reached the maximal decrease. By contrast, hypoplasy occurred on day 15, when the labeling index (LI) decreased significantly. In the intestine, however, a decrease of the LI was observed only for the upper proliferative cells of the crypts. In the gastric mucosa, the LI was reduced only in the proliferative zone containing progenitor cells (isthmic region). Consequently, the cell differentiation is not similarly affected on all levels of the digestive tract.
Animals↗
Two rotameric forms of open ring 7-methylguanine are present in alkylated polynucleotides.
High performance liquid chromatography analysis of imidazole open ring 7-methylguanine, 2-6 diamino-4-hydroxy-5N-methyl-formamidopyrimidine (rom7G), showed two well-separated peaks (fI and fII) of the same magnitude. Rechromatography of each isolated component indicated that they are slowly interconverted to give a 1:1 mixture. NMR analysis demonstrated that the two species observed on reversed phase HPLC are rotational isomers. Thermodynamic measurements strongly suggested that the equilibrium can be assigned to rotation around the N-methyl formamido bond. The two species, fI and fII, separated by HPLC were identified as rotamers E and Z, respectively. The structures of fI and fII were also determined. A polynucleotide containing rom7G was obtained by alkaline treatment of poly (dGC) containing 7-methylguanine. In order to study its structure within the polynucleotide, rom7G was enzymatically excized by E.coli rom7G-DNA glycosylase. The analysis of the products released by the enzyme showed a 1:4 mixture of the two rotamers favoring the Z form (fII).
Alkylation↗
Enzymatic repair of O-alkylated thymidine residues in DNA: involvement of a O4-methylthymine-DNA methyltransferase and a O2-methylthymine DNA glycosylase.
Alkylation of poly(dT) by N-[methyl-3H] (N-nitrosomethylurea) and subsequent annealing with poly(dA) yield a substrate containing O2 and O4-methylthymidine, 3-methylthymidine and phosphotriesters. In an in vitro assay using this substrate, cell extracts from Escherichia coli catalyse i) the transfer of the O4-methyl present in O4 methylthymidine to a protein which becomes alkylated; ii) the release of O2-methylthymine by a glycosylase activity. The two DNA repair activities described above appear to be involved in the adaptive response.
Alkylation↗
3-Methyladenine residues in DNA induce the SOS function sfiA in Escherichia coli.
The induction by methylating agents of the SOS function sfiA was measured by means of a sfiA::lac operon fusion in Escherichia coli mutants defective in alkylation repair. The sfiA operon was turned on at a 10-fold lower concentration of methylmethane sulfonate or dimethyl sulfate in tagA strains, lacking specific 3-methyladenine-DNA glycosylase, than in wild-type strains. In contrast, the induction of sfiA by u.v. light was not affected by a tagA mutation. We confirm that tagA strains specifically accumulate 3-methyladenine in their DNA. We conclude that the persistence of 3-methyladenine in E. coli DNA most likely induces the SOS functions. Results on in vitro DNA synthesis further suggest that this induction is due to an unscheduled arrest of DNA synthesis at this lesion.
Adenine↗
Adaptive response in mammalian cells: crossreactivity of different pretreatments on cytotoxicity as contrasted to mutagenicity.
Pretreatment of H4 (rat hepatoma) cells for 48 hr with low nontoxic doses of alkylating agents [methyl methanesulfonate (MMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and N-methyl-N-nitrosourea] renders the cells more resistant to the toxic effect of these compounds. Crossreactivity for survival is also observed with the different alkylating agents tested. Pretreatment with MNNG enables the cells to be less mutated than control cultures during a subsequent challenge with high doses of this compound. However, pretreatment with MMS does not modify the mutation frequency of cells challenged with either MMS or MNNG. The adaptive response to mutagenesis is correlated with a faster and more efficient removal of O6-methylguanine in MNNG-pretreated cells as compared to control cultures, whereas the disappearance of this lesion is not modified in MMS-pretreated cells. As MMS produces less methylation at the O6 position of guanine and more methylation at the N7 position in comparison to MNNG, the results suggest that: (i) N7-methylguanine is not implicated in the adaptive response and (ii) adaptation to mutagenesis can be correlated with the amount of O6-methylguanine induced during the pretreatment. The effect of pretreatment on other O-alkylated derivatives is not known.
Acclimatization↗
Enzymatic methylation of chemically alkylated DNA and poly(dG-dC) X poly(dG-dC) in B and Z forms.
The enzymatic methylation of chemically alkylated DNA and of poly(dG-dC) X poly(dG-dC) by beef brain DNA(cytosine-5-)-methyltransferase have been tested. The alkylation by dimethylsulfate, which yields mostly 7 methylguanine (m7G) and 3 methyladenine (m3A) do not affect the enzymatic methylation. The dimethylsulfate alkylated poly(dG-dC) X poly(dG-dC) converted into the Z-form in the presence of MgCl2, is just as well methylated as the native or the alkylated polynucleotide in the B-form. The alkylation of DNA or of poly(dG-dC) X poly(dG-dC) by methylnitrosourea yields, in addition to the above base modifications described for dimethylsulfate, methylphosphotriesters and O6-methylguanine. The enzymatic methylation of these substrates modified by methylnitrosourea is decreased. This decrease is proportional to the extent of the chemical alkylation of the substrate.
Alkylation↗
Imidazole open ring 7-methylguanine: an inhibitor of DNA synthesis.
Guanine methylated at the N7 position (me7G) is susceptible to cleavage of the imidazole ring yielding: 2,6-diamino-4-hydroxy-5N-methyl-formamidopyrimidine (rom7G). DNA synthesis catalysed by E.coli DNA polymerase I, using as templates poly(dGC) containing either me7G or rom7G, show that rom7G blocks DNA chain elongation. It implies a potential killing effect. Furthermore rom7G does not induce mispairing with either dAMP or dTMP. me7G does not affect DNA synthesis. The results suggest that, beside AP-sites, rom7G is a potential killing lesion in cells treated by alkylating agents.
Catalysis↗
Control of cyclic AMP accumulation in dog acini: its relation to amylase release.
In dispersed acini from dog pancreas, exogenous derivatives of cAMP stimulated the amylase release; 8-Br-cAMP was a more potent stimulant than Bt2-cAMP, and induced a potentiation of the amylase release stimulated by cerulein. Both secretin and vasoactive intestinal peptide (VIP), increased cellular cAMP, with a greater potency for secretin, and supramaximal concentration of secretin plus VIP increased cellular cAMP to the same degree as that obtained with secretin alone. 125I-VIP-binding studies showed that in dog pancreatic acini there is a 95% decrease in the number of VIP-preferring receptors with minor changes in receptor affinity as compared to guinea-pig pancreas. The absence of effect between secretin or VIP and cerulein in stimulating amylase release could likely be explained by the low number in VIP-preferring receptors on dog pancreatic acini.
8-Bromo Cyclic Adenosine Monophosphate↗
Coding properties of poly(deoxycytidylic acid) templates containing uracil or apyrimidinic sites: in vitro modulation of mutagenesis by deoxyribonucleic acid repair enzymes.
Heat treatment of poly(deoxycytidylic acid)-[poly(dC)] induces the formation of dUMP residues, which code for dAMP when replicated by Escherichia coli DNA polymerases I and III. The specificity of dUMP coding properties is indicated by the quantitative relation between the dAMP incorporated and the frequency of dUMP residues in the heat-treated poly(dC). The dAMP incorporation is prevented by preincubation of uracil containing poly(dC) with uracil-DNA glycosylase. The excision of uracil by uracil-DNA glycosylase leads to the formation of apyrimidinic sites (AP sites), which are barely replicated in vitro under physiological conditions. However, the alteration of E. coli DNA polymerase I fidelity of replication by Mn2+ greatly stimulates the replication of AP sites. There is a preferential incorporation of dAMP, as compared to dTMP, opposite the AP sites. The dAMP incorporation is prevented by preincubation of poly(dC) containing AP sites with Micrococcus luteus AP endonuclease B. The results show a close association between DNA repair by base excision and the prevention of mutagenic processes in vitro. Furthermore, since the alteration of DNA polymerase fidelity allows some replication of the noncoding DNA lesion (AP site), this could imply a role in SOS-induced mutagenesis in vivo.
DNA Ligases↗
Dopamine-stimulated cyclic AMP and binding of [3H] dopamine in acini from dog pancreas.
Dispersed acini from dog pancreas were used to examine the ability of dopamine to increase cyclic AMP cellular content and the binding of [3H] dopamine. Cyclic AMP accumulation caused by dopamine was detected at 1.10(-8) M and was half-maximal at 7.9 +/- 3.4 10(-7) M. The increase at 1.10(-5) M, (7.5-fold) was equal to the half-maximal increase caused by secretin at 1.10(-9) M. Haloperidol, a dopaminergic receptor antagonist inhibited cyclic AMP accumulation caused by dopamine. The IC50 value for haloperidol, calculated from the inhibition of cyclic AMP increase caused by 1.10(-5) M dopamine was 2.3 +/- 0.9.10(-6) M. Haloperidol did not alter basal or secretin-stimulated cyclic AMP content. [3H] Dopamine binding was studied on the same batch of cells as cyclic AMP accumulation. At 37 degrees C, it was rapid, reversible, saturable and stereospecific. The Kd value for high affinity binding sites was 0.43 +/- 0.1.10(-7) M and 4.7 +/- 1.6.10(-7) M for low affinity binding sites. The concentration of drugs necessary to inhibit specific binding of dopamine by 50% was 1.2 +/- 0.4.10(-7) M epinine, 4.1 +/- 1.8.10(-6) M fluphenazine, 8.0 +/- 1.6.10(-6) M haloperidol, 4.2 +/- 1.2.10(-6) M cis-flupenthixol, 2.7 +/- 4.0.10(-5) M trans-flupenthixol, less than 1.10(-5) M apomorphine, sulpiride, naloxone and isoproterenol.
Amylases↗
Uracil-DNA glycosylase. Purification and properties of uracil-DNA glycosylase from Micrococcus luteus.
A uracil-DNA-glycosylase from Micrococcus luteus has been purified more than 3,000-fold. The enzyme preparation appears homogeneous, according to the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It is devoid of nonspecific endonucleases, specific endonucleases for apurinic and apyrimidinic sites, 3-methyladenine or 7-methylguanine-DNA-glycosylases. It behaves as a monomer protein of 19,400 daltons. It has an isoelectric point of 7.0 +/- 0.1. It has an optimal activity between pH 5.0 and 7.0. It has no cofactor requirement and is not inhibited by EDTA. Uracil-DNA-glycosylase is highly specific for DNA containing dUMP residues, releasing uracil as product of the reaction. It is 2-fold more active on single-stranded DNA than on double-stranded DNA. If it releases uracil dimers from ultraviolet-irradiated PBS1 DNA, it is at the threshold of the detection. The apparent Km is 7 X 10(-8) M, and uracil acts as a noncompetitive inhibitor with a Ki of 3.2 X 10(-4) M. Cis-syn cyclogbutadiuracil also is a potent inhibitor, while some analogs, produced by x-irradiation of uracil and thymine, are weak inhibitors. Spermine, between 10 and 400 microM, increases the enzymatic activity by 50% and is not inhibitory at other concentrations. Spermidine activates the enzyme at concentrations of 40 to 120 microM, but becomes inhibitory at 200 and 400 microM. A new finding is that drugs which intercalate in DNA, such as ethidium bromide and ellipticine, cause a 2- to 2.5-fold activation of this enzyme activity. The concentrations giving maximal activation depend on the drug. The enzyme does not behave as a processive enzyme during uracil excision.
DNA↗
In vitro enzymatic recognition of DNA modified by O,O'-diacetyl or O-acetyl derivatives of the carcinogen 4-hydroxyaminoquinoline-1-oxide.
Purified DNA was modified in vitro by 3H-labelled O-acetyl or O,O'-diacetyl-4-hydroxyaminoquinoline-1-oxide (Ac4HAQO or di Ac-4HAQO). It was then subjected to the action of the single-stranded DNA specific nuclease S1 and the digested fractions were analysed. For both types of modified DNA, the release of non-modified nucleotides was faster than the release of modified nucleotides. This result is at variance with that obtained with acetoxy-acetylaminofluorene-modified DNA: in the latter case, the modified nucleotides were preferentially released. The results suggest that the S1 endonuclease can recognize different conformational changes in DNA, which depend on the carcinogen used. The enzymatic activity (or activities) present in Micrococcus luteus cell extracts released ethanol-soluble products from Ac-4HAQO modified DNA.
4-Hydroxyaminoquinoline-1-oxide↗
Histological variations of jejunal and ileal mucosa on days 8 and 15 after hypophysectomy in rat: morphometrical analysis on light and electron microscopy.
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Animals↗
Specific nicking of DNA at apurinic sites by peptides containing aromatic residues.
Tripeptides containing aromatic residues between basic ones, such as Lys-Trp-Lys and Lys-Tyr-Lys can nick supercoiled or relaxed DNAs containing apurinic/apyrimidinic sites (AP-sites). The nicking, which is ionic strength-dependent, occurs at AP-sites, since native PM2 DNA is not hydrolyzed. The nicking activity of the tripeptides at AP-sites occurs in total darkness. An activation energy of 21 +/- 2 kcal . mol-1 has been calculated for the incision of PM2 DNA by Lys-Trp-Lys. Tripeptides without aromatic residues, such as Lys-Ala-Lys-O-Methyl and Lys-Lys-Lys, can nick apurinic DNA, although with a much lower efficiency. Relaxed depurinated PM2 DNA is a poor substrate for the tripeptide, indicating that single-stranded regions are better recognition sites. The nicking of the DNA backbone probably occurs by beta elimination, since reduced AP-sites do not act as substrate. The termini generated are 3'-hydroxyl and 5'-phosphoryl.
Apurinic Acid↗