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Further research is needed to de term ine the mechan ism by wh ich mammalian repair is induced by these agents and to establish whether NER can be induced in response to other carcinogens.AP sites, whether chemically induced or generated by glycosylases, are repaired by BER. The most frequently observed mutation induced by AFB in several experimental systems is the G to T transversion.However, recent studies indicate that different mammalian cells show unique preferences for the base inserted opposite an abasic site, and thymidine is preferentially incorporated opposite AP sites in mouse broblast NIHT cells. Although most of the mutations induced by AFB are targeted to the site of the lesion, a signicant number occur at the base to the modied guanine residue, and some are even found several bases away. Therefore, mutational hotspots do not always correspond with hotspots for adduct formation. This is consistent with structural models demonstrating that the aatoxin moiety intercalates into DNA on the face of the guanine residue, resulting in helical distortion and disruption of base pairing to the modied guanine. Differences in DNA repair activity also correlate with tissue differences in susceptibility to AFB carcinogenicity in the mouse. Mouse lung has much lower DNA repair activity for AFB damage than does liver.In addition, treatment of mice with a tumourigenic dose of AFB leads to tissuespecic changes in repair activity that also correlate with susceptibility.Additional investigation of the mechanism by which AFB can modify repair activity is of signicant scientic interest, not only to improve our understanding of the mechanism of AFB carcinogenesis, but to further understand the mechanisms involved in response to other carcinogens.Recognition and cleavage of oxidatively damaged DNA, J.Res. Toxicol. Oxidative stress, an imbalance toward the prooxidant side of the prooxidantantioxidant homeostasis, occurs in several brain neurodegenerative disorders.Among these neurodegenerative brain disorders are those in which protein aggregation is observed, including AD, PD, and stroke.The cytotoxicity of A brils is also implicated as an oxidative mechanism.There is considerable evidence consistent with the importance of oxidative stress in the pathology of AD. Evidence supporting the notion of freeradical oxidative stress in the AD brain includes increased redoxactive metal ions in the AD brain, increased lipid peroxidation detected by decreased levels of polyunsaturated fatty acids and increased levels of the lipid peroxidation products, acrolein, TBARS, isoprostanes, and neuroprostanes, increased protein oxidation, increased oxidation of DNA and RNA, and decreased activity of oxidatively prone enzymes, such as glutamine synthetase. Oxidation of proteins normally is caused by free radicals, and this process, from a chemical thermodynamics standpoint, is an exothermic event.Oxidative reactions of peptides are mediated mainly by the hydroxyl radical sidechain oxidation. Backbone oxidation is initiated by carbon abstraction of hydrogen by the free radical, leading to the formation of a carboncentered radical.In the presence of oxygen, this radical is converted to a peroxyl radical.This can lead to the formation of an alkoxyl radical and subsequent hydroxylation of the peptide backbone.The oxidation of amino acid side chains greatly depends on their structure.An important oxidative process with profound functional and structural consequences involves the irreversible nitration of tyrosine residues by peroxynitrite.

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