Relationship between DNA acid-solubility and frequency of single-strand breaks near apurinic sites

Inactivation and repair of double-stranded DNA damaged by the aziridinyl nitroimidazole RSU 1069

Incubation of RSU 1069 in the presence of biologically active double-stranded phi X174 DNA resulted in, depending on pH, ionic strength and concentration of drug, inactivation of the DNA. A variety of lesions are induced including a high number of single-strand breaks and alkali-labile lesions, which are at most partly lethal. The main inactivating damage consists probably of base damage, induced by alkylation. A considerable part of the damage induced by RSU 1069 can be repaired by the various repair enzymes of the bacterial host of the phi X174 DNA. Finally the damage (pattern) depends considerably on the ionic composition of the reaction solution, which can be explained by an equilibrium model presented in this paper.

The ability of liver extracts from different-aged rats to repair ‘mis-instructive’ and ‘non-instructive’ lesions of DNA

The ability to repair 'mis-instructive', O6-methylguanine, and 'non-instructive', AP sites, DNA lesions in Fischer 344 rat livers at various ages was determined. Different behaviours were observed. While the AP-endodesoxyribonuclease enzymes displayed a high constant level throughout the animals' lifetime, the O6-methylguanine-DNA methyltransferase activity presented a stepwise modulation (DNA normalisation of results): the O6-MT activity significantly increased within the first month of animal life and enhanced again after 6 months reaching a maximum plateau in the 12-18-month-old animals. Thereafter a net significant decrease of O6-MT enzyme was detected in the 24-month-old group. While the repair of the widely formed AP sites appeared uniformly efficient like 'house keeping' functions, the removal of the rare precancerous O6-methylguanine is age-dependent indicating a decreased protection of the youngest and oldest animals against this 'mis-instructive' damage. However, any extrapolation of the age-associated cancer risk needs further assessment.

Interaction of RSU 1069 and 1137 with DNA in vitro. Biological implications and mechanistic aspects

We have examined the capacity of the nitroimidazole aziridine antitumour drug RSU 1069 to react with DNA in vitro in order to get a better understanding of its mechanism of action. Moreover, we have utilized biologically active phi X174 DNA to investigate the biological relevance of the chemical DNA modification induced by the drug. Incubation of RSU 1069 in the presence of single-stranded phi X174 DNA resulted in extensive inactivation of the DNA, which is dependent on the concentration of drug and temperature. Only about 2% of the inactivating damage can be attributed to strand breakage. The main damage most probably consists of base damage, of which a part is non-lethal and alkali-labile which in turn can be converted into lethal lesion and subsequently into a break applying a post-incubation alkali treatment. Furthermore, from the dependence of the inactivation and also the formation of breaks on pH and ionic strength, it is concluded that the reaction most probably takes place between a protonated RSU 1069 and a negative DNA coil and that the damage pattern reflects the difference in reactivity of RSU 1069 with the phosphate groups and the bases in DNA. Comparison between RSU 1069 and its ring-open hydrolysis product RSU 1137 revealed that (lethal) damage induced in the DNA must be ascribed to the alkylating properties of the aziridine moiety.

The use of thioglycollate to demonstrate DNA AP (apurinic/apyrimidinic-site) lyase activities. Biological consequences of thiol addition to the 5' product of a beta-elimination reaction at an AP site in DNA

Thioglycollate reacts with the 5' product of AP lyase activity on apurinic/apyrimidinic (AP) sites in DNA. The 3'-terminal thioglycollate-unsaturated sugar 5-phosphate adduct can be released by the use of Escherichia coli endonuclease IV or endonuclease VI, and identified by DEAE-Sephadex chromatography. In contrast, the mammalian AP endonuclease is unable to excise a 3'-terminal thiol-unsaturated sugar adduct; this lesion, which must sometimes occur in vivo, might be irreparable and have pathological consequences.

Characterization of damage in gamma-irradiated and OsO4-treated DNA using methoxyamine

Unlabelled and radiolabelled methoxyamine have been used to characterize DNA damage caused by gamma-rays or by the chemical reagent osmium tetroxide (OsO4). Both treatments introduce in DNA a number of methoxyamine-binding sites proportional to the dose. Whereas the number of these sites remains constant after the OsO4 treatment it increases during postirradiation incubation; the postirradiation appearance of methoxyamine-binding sites is enhanced by the presence of methoxyamine. OsO4 treatment and gamma-irradiation also induce the formation of alkali-labile sites in DNA. Whereas the number of these sites remains constant after OsO4 treatment, it increases during postirradiation incubation and an alkaline medium accelerates their formation. A fraction of the alkali-labile sites found in gamma-irradiated DNA is methoxyamine-labile; by contrast, the OsO4-treated DNA is stable in the presence of methoxyamine.

Formation, detection and repair of AP sites

The paper is an outline review of the main aspects concerning the formation and repair of AP (apurinic/apyrimidinic) sites in DNA as well as some of the chemical properties allowing their quantitative determination. A new method for the measurement of AP sites based on their reaction with [14C]methoxyamine is described. It has been applied to the measurement of AP sites produced in DNA either by physical (gamma-rays) or chemical (methyl methanesulphonate, osmium tetroxide) agents. The method has also been used to quantify the excision of abnormal bases from DNA under the action of specific DNA glycosylases and to prevent the chemical or enzymatic degradation of DNA containing AP sites. The paper contains data about the purification and characterization of uracil-DNA glycosylase and AP endodeoxyribonuclease from carrot cells, two enzymes involved in the first steps of base excision repair through AP site intermediates. The biological effects of unrepaired AP sites are also discussed.