DNA strand scission in E. coli by electronically excited state molecules generated by enzymatic systems

Generation of electronically excited triplet species at the cellular level: a potential source of genotoxicity

Selected enzymatic systems can efficiently produce a product in the electronically excited triplet state. Earlier, only the formation of electronically excited singlet species was known. The formation of triplet species has been demonstrated with both normal substrates/metabolites and with xenobiotics, even at the cellular level. Triplet excited species have intrinsically much longer lifetimes than excited singlets, whereby they can be potentially important agents for normal and/or deleterious processes, including mutagenesis. Enzymically generated triplet species can damage DNA, even when protein coated, as in the case of the lambda-phage of Escherichia coli. Some evidence of damage by triplet species has also been reported for intact cells. Triplet excited species may produce their effects through type I and/or type II dark photosensitization, that is, the events may be started by H abstraction and/or singlet oxygen/superoxide ion production. The induction of lipid peroxidation, with concomitant clastogenic effects, appears to be of special importance.

Different lethal effects by enzyme-generated triplet indole-3-aldehyde in different Escherichia coli strains

Strains of Escherichia coli which lack 4-thiouridine (S4U) exhibit a higher survival rate than their wild-type parents which contain S4U after treatment with enzyme-generated triplet indole-3-aldehyde. In a similar manner to results obtained with monochromatic 334 nm UV light, the survival is related to single-strand breakage of DNA in E. coli containing the pBR 322 plasmid. The effects of the excited states generated by an enzymatic system suggest that S4U is an important chromophore in the lethal effects observed. The results also suggest that the energy transferred from triplet indole-3-aldehyde to S4U may also be passed from S4U of t-RNA to DNA, possibly through a singlet oxygen intermediate generated by excited S4U, resulting in a decrease in the survival rate of E. coli containing S4U. These results emphasize the importance of excited states in biological systems.

Bleomycin-metal interaction: ferrous iron-initiated chemiluminescence

Chemiluminescence often accompanies the spontaneous degradation of intermediates in an electronically excited state. The interaction of iron with bleomycin results in the activation of bleomycin to a reactive intermediate which can alter DNA or undergo self-inactivation. This report demonstrates that the interaction of ferrous iron with bleomycin results in chemiluminescence, that this response is iron-specific and that the presence of DNA prevents the generation of chemiluminescence. These observations suggest that the activated bleomycin intermediate may be in an electronically excited state.