Strand breakage

Perhaps the simplest coordination system known to produce DNA cleavage is the cuprous-1,10-phenanthroline system which was shown to cleave DNA in an oxygen dependent reaction [144]. The co-requirement for activity is a reducing agent, such as a thiol, and molecular oxygen. Subsequent work showed that this cleavage is remarkably specific for the 1,10-phenanthroline system, other Cu chelators such as neocupreine and 2,2 -bipyridine being ineffective [145—147]. The neocupreine (2,9-dimethyl-1,10-phenanthroline) complex is more stable in the Cu(I) state relative to Cu(II) [148], and actually inhibits the reaction. 

Mechanistic work showed that catalase and some hydroxyl radical 

This system has also been shown to be dependent on the secondary structure of DNA, the A, B, and Z forms reacting at different rates [150]. The likely explanation is that the faster reacting B DNA forms a more stable complex with the catalyst. This artificial DNase activity has also been compared with cleavage by micrococcal nuclease, and shown to recognize the same sites but not all those cleaved by DNase 1, again implying some local conformational preferences [151]. Chromatin structure has also been probed [152]. 

A further demonstration of metal catalyzed redox chemistry on DNA is that of the light induced cleavage of DNA by Co(III)—phenanthroline complexes [153]. The likelihood is that reduction of Co(III) to Co(II) initiates the oxygen activation leading to cleavage. 

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Depurination - N7 alkylation may cause cleavage of the imidazole ring and excision of the guanine residue, leading to DNA strand breakage. 

Chromosome function Quinolones Metronidazole (also ) Nitrofu rantoin Rifampicin (also ) 5-Fluorocytosine Inhibit DNA gyrase DNA strand breakage DNA strand breakage Inhibits RNA polymerase Inhibits DNA synthesis No action on mammalian equivalent Requires anaerobic conditions not present in mammalian cells No action on mammalian equivalent Converted to active form in fungi... 

It has been known for many years that neither Oa nor H2O2 causes any strand breakage in DNA, if the reaction mixture is carefully freed of transition metal ions (see e.g. Rowley and Halliwell, 1983). Our later work... 

Strand Breakage DNA Base Modification Deoxyribose Fragmentation... 

Topoisomerase An enzyme that uncoils DNA during replication by cutting the DNA. Topoisomerase inhibitors prevent DNA from sealing the cut, which causes DNA strand breakage. 

Bleomycin -antitumor antibiotic that causes DNA strand breakage -dose-related pneumonitis -mucocutaneous effects (stomatitis, mucositis) -acute pulmonary edema -fever in 50% -hyperpigmentation (can rarely be DLT)... 

Carboplatin -atypical alkylating agent leading to DNA strand breakage during replication -bone marrow suppression—particularly thrombocytopenia -nausea and vomiting -liver function test abnormalities -uncommon neurotoxicity, ototoxicity... 

Xanthine oxidase, a widely used source of superoxide, has been frequently applied for the study of the effects of superoxide on DNA oxidation. Rozenberg-Arska et al. [30] have shown that xanthine oxidase plus excess iron induced chromosomal and plasmid DNA injury, which was supposedly mediated by hydroxyl radicals. Ito et al. [31] compared the inactivation of Bacillus subtilis transforming DNA by potassium superoxide and the xanthine xanthine oxidase system. It was found that xanthine oxidase but not K02 was a source of free radical mediated DNA inactivation apparently due to the conversion of superoxide to hydroxyl radicals in the presence of iron ions. Deno and Fridovich [32] also supposed that the single strand scission formation after exposure of DNA plasmid to xanthine oxidase was mediated by hydroxyl radical formation. Oxygen radicals produced by xanthine oxidase induced DNA strand breakage in promotable and nonpromotable JB6 mouse epidermal cells [33]. 

The possible prooxidant effects of a major lipophilic antioxidant vitamin E (a-tocopherol) have already been discussed in Chapter 25. Yamashita et al. [82] showed that a-tocopherol induced extensive DNA damage including base modification and strand breakage in the... 

It is known that peroxynitrite is able to induce DNA strand breakage, which activates nuclear enzyme poly(ADP ribose) synthase (PARS). Szabo et al. [257] showed that the inhibition of PARS by oral treatment with lipophilic inhibitor 5-iodo-6-amino-l,2-benzopyr-one delayed the onset of arthritis in rats. It is possible that infrared pulse laser therapy can be useful for the treatment of RA patients [258]. 

Hassoun et al. (1993) examined the effects of various pesticides on lipid peroxidation and DNA single strand breakage in the hepatic cells of female Sprague-Dawley rats. Animals were dosed orally once with endrin at 4.5 mg/kg, lindane at 30 mg/kg, chlordane at 120 mg/kg, or DDT (dichlorodiphenyl trichloro-ethane) at 40 mg/kg, or vehicle only (com oil, control). At 6, 12, and 24 hours post-dosing, 4 animals from each group were sacrificed, their livers removed, and prepared for lipid peroxidation assay. Lipid peroxidation was measured calorimetrically by determining the amount of thiobarbituric acid reactive substances (TBARS) formed. Exposure to endrin resulted in a 14.5% increase in hepatic mitochondrial... 

This enables one to use aliphatic systems as precursors to the radicals X-Y whose solvolytic (= redox) behavior can then be studied. Equations 2a, c describe what may be called oxidative solvolysis . This reaction sequence, the first step of which is in many cases induced by the OH radical, is of great importance in radical (and radiation) chemistry. It extends from /8-elimination reactions of monomeric radicals [6, 7] to the mechanism of DNA strand breakage [8]. An example for Eq. 2 in which it is shown that the radical XY can be produced by either step a or b is given in section 3.3. 

Figure 21.11 Role of p53 as a tumour suppressor gene. p53 induces a cell with damaged DNA either to initiate apoptosis, or arrest the cell cycle, to give time for damaged DNA to be repaired. Damage can be, for example, a mutation, DNA strand breakage or chromosomal rearrangement.

Kluza J, Mazinghien R, Irwin H, Hartley JA, Bailly C. (2006) Relationships between DNA strand breakage and apoptotic progression upon treatment of HL-60 leukemia cells with tafluposide or etoposide. Anti-Cancer Drugs 17 155-164. 

The fourth endpoint is DNA repair. One can determine whether DNA has been damaged in a chemical sense by measuring, by any of several techniques, whether certain characteristic changes in DNA, mainly strand breakage or resynthesis, take place after treatment. 

These substances are analogues of thymine (azidothymidine, stavudine), adenine (didanosine), cytosine (lami-vudine, zaldtabine), and guanine (car-bovir, a metabolite of abacavir). They have in common an abnormal sugar moiety. Like the natural nucleosides, they undergo triphosphorylation, giving rise to nucleotides that both inhibit reverse transcriptase and cause strand breakage following incorporation into viral DNA. 

Cystostatic antibiotics insert themselves into the DNA double strand this may lead to strand breakage (e.g., with bleomycin). The anthracycline antibiotics daunorubkin and adriamycin (doxorubicin) may induce cardiomyopathy. Bleomycin can also cause pulmonary fibrosis. 

The epipodophyllotoxins, etopo-side and teniposide, interact with topo-isomerase 11, which functions to split, transpose, and reseal DNA strands (p. 274) these agents cause strand breakage by inhibiting resealing. 

The mammalian topoisomerase II enzyme catalyzes the double-strand breakage of DNA to allow the second strand passage and thereby control the topology... 

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