Characterization of intracellular DNA strand breaks induced by neocarzinostatin in Escherichia coli cells

Neocarzinostatin-mediated DNA damage in a model AGT.ACT site: mechanistic studies of thiol-sensitive partitioning of C4' DNA damage products

Double-strand (DS) DNA damage caused by neocarzinostatin (NCS) has been studied in the trinucleotide AGT-ACT sequence in an AP-1 transcription factor binding site. There are strong similarities between bistranded lesions produced at AGT.ACT and AGC-GCT, including the fact that DS lesions outnumber SS lesions on the AGT and AGC strands, while SS exceed DS on the ACT and GCT strands. Structure-function studies revealed that a variety of different thiols produced bistranded lesions in this model by predominantly C4'-hydrogen atom abstraction (84-93%) at the T of AGT and C5'-hydrogen atom abstraction (87-91%) at the T of ACT. Single-strand (SS) lesions were found to represent a variable mixture of C4' and C5' chemistry. The C4'-hydroxylated abasic site occurred in both SS and DS lesions at both sites and accounted for most of the DS damage at AGT (60-83%); the remaining damage consisted of 3'-phosphoglycolate- and 3'-phosphate-ended fragments. The nature of the thiol was found to affect the partitioning of the breakdown products arising from C4' and, to a lesser extent, C5' hydrogen atom abstraction. Production of 3'-phosphoglycolate residues, restricted mainly to the T of AGT in bistranded lesions, correlated with the incidence of direct DS breaks in the AGT.ACT model and in plasmid DNA and appeared to be influenced by the reducing power of the thiol activator. Furthermore, hydrazine and sodium borohydride both inhibited the formation of glycolate, an effect that was exploited to determine the rate constant for 3'-phosphoglycolate formation: 0.06 min-1 at 0 degree C, pH 7.4. Under anaerobic conditions, the nitroaromatic radiation sensitizer misonidazole caused a large increase in glycolate production in both SS and DS lesions formed by NCS, which suggests that the formation of 3'-phosphoglycolate, like 3'-formylphosphate generated by C5' chemistry, involves an oxyradical intermediate. The pathways for DNA damage involving C4' and C5' hydrogen atom abstraction thus share many common features, several of which are consistent with a mechanism for the production of NCS-mediated bistranded lesions at AGT.ACT that involves a tetraoxide bridge joining the lesions on opposite strands of DNA.

Influence of thiol structure on neocarzinostatin activation and expression of DNA damage

Neocarzinostatin (NCS) is an enediyne antitumor antibiotic that cleaves DNA following a thiol-induced electronic rearrangement to a diradical form. Structure-function studies with 11 thiol-containing compounds were undertaken to clarify the role of the thiol in NCS-mediated DNA damage. The rates of activation of NCS in the presence of DNA with the various thiols approximated a Brønsted relation (beta = 0.43, r2 = 0.86), which suggests that the basicity/nucleophilicity of the thiol is important to NCS activation. However, an additional contribution to NCS activation may arise from the affinity of the thiol for DNA, since there is a correlation between the concentration of thiol producing maximal DNA damage, assessed by quantitating the topologic forms of plasmid pBR322 following treatment with NCS, and the apparent ability of the thiol to bind to DNA by hydrophobic or electrostatic interactions. The overall second-order rate constants for the activation of NCS were found to be inversely correlated with the thiol optima; a plot of the former versus the reciprocal of the optimal thiol concentration revealed a first-order rate constant of activation of 0.013 s-1 in the presence of DNA. This indicates that maximal DNA damage occurs when NCS is activated with a half-life of 52 s, a relatively slow rate of activation that suggests that NCS binds to DNA before undergoing activation by thiol. Finally, an analysis of strand breaks in pBR322 shows that thiols possessing a carboxylate moiety produce larger quantities of bistranded DNA lesions than their esterified or non-carboxylate-containing counterparts.

Molecular models of neocarzinostatin damage of DNA: analysis of sequence dependence in 5'GAGCG:5'CGCTC

Model building and molecular mechanics and dynamics calculations have been performed on a number of complexes of the post-activated form of the neocarzinostatin chromophore (NCS) with the B-DNA oligomer 5'GAGCG:5'CGCTC. Stable structures with the naphthoic acid moiety intercalated at all base pairs can be constructed. The observed bistranded lesions consisting of an abasic site at the Cyt residue in AGC and a direct break at the Thy residue on the complementary strand can be explained by assuming that NCS in the (R,R) form intercalates between the Ade2-Thy9/Gua3-Cyt8 base step with its 'diradical' core oriented towards the 3'-end of the (+) strand. Sites at C5', C4' and C1' in the minor groove are within a short enough distance from the two radical centers on NCS to permit hydrogen atom abstraction and the formation of the bistranded lesions. Strand cleavage at Thy9 may occur as a single lesion if NCS is intercalated into the Gua3-Cyt8/Cyt4-Gua7 base step with its active core towards the 3'-end of the (-) strand. The results are analyzed, and the utility and limitations of this type of model building are discussed.

Free radical mechanisms in neocarzinostatin-induced DNA damage

The molecular mechanisms by which the antitumor protein antibiotic, neocarzinostatin, interacts with DNA and causes DNA sugar damage is discussed. Physical binding of the nonprotein chromophore of neocarzinostatin to DNA, involving an intercalative process and dependent on the microheterogeneity of DNA structure, is followed by thiol activation of the drug to a probable radical species. The latter attacks the deoxyribose, especially at thymidylate residues, by abstracting a hydrogen atom from C-5' to generate a carbon-centered radical on the DNA. This nascent form of DNA damage either reacts with dioxygen to form a peroxyl radical derivative, which eventuates in a strand break with a nucleoside 5'-aldehyde at the 5'-end or reacts with the bound drug to form a novel drug-deoxyribose covalent adduct. Nitroaromatic radiation sensitizers can substitute for dioxygen, but the DNA damage products are different. Similarities between the various biological effects of neocarzinostatin and ionizing radiation are reviewed.