Mutation of a unique aspartate residue abolishes the catalytic activity but not substrate binding of the mouse N-methylpurine-DNA glycosylase (MPG)

Repair kinetics of acrolein- and (E)-4-hydroxy-2-nonenal-derived DNA adducts in human colon cell extracts

ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) play a role in the pathogenesis of colon cancer. Upon oxidation, PUFAs generate α,β-unsaturated aldehydes or enals, such as acrolein (Acr) and (E)-4-hydroxy-2-nonenal (HNE), which can form cyclic adducts of deoxyguanosine (Acr-dG and HNE-dG, respectively) in DNA. Both Acr-dG and HNE-dG adducts have been detected in human and animal tissues and are potentially mutagenic and carcinogenic. In vivo levels of Acr-dG in DNA are at least two orders of magnitude higher than those of HNE-dG. In addition to the facile reaction with Acr, the higher levels of Acr-dG than HNE-dG in vivo may be due to a lower rate of repair. Previous studies have shown that HNE-dG adducts are repaired by the NER pathway (Choudhury et al. [42]). We hypothesize that Acr-dG adducts are repaired at a slower rate than HNE-dG and that HNE-dG in DNA may influence the repair of Acr-dG. In this study, using a DNA repair synthesis assay and a LC-MS/MS method, we showed that Acr-dG in a plasmid DNA is repaired by NER proteins, but it is repaired at a much slower rate than HNE-dG in human colon cell extracts, and the slow repair of Acr-dG is likely due to poor recognition/excision of the lesions in DNA. Furthermore, using a plasmid DNA containing both adducts we found the repair of Acr-dG is significantly inhibited by HNE-dG, however, the repair of HNE-dG is not much affected by Acr-dG. This study demonstrates that the NER repair efficiencies of the two major structurally-related in vivo cyclic DNA adducts from lipid oxidation vary greatly. More importantly, the repair of Acr-dG can be significantly retarded by the presence of HNE-dG in DNA. Therefore, this study provides a mechanistic explanation for the higher levels of Acr-dG than HNE-dG observed in tissue DNA.

Discrimination of lesion removal of N-methylpurine-DNA glycosylase revealed by a potent neutralizing monoclonal antibody

N-Methylpurine-DNA glycosylase (MPG), a ubiquitous DNA repair enzyme, initiates excision repair of several N-alkylpurine adducts, induced by alkylating chemotherapeutics, and deaminated and lipid peroxidation-induced purine adducts. We have generated monoclonal antibodies (moAbs) against human MPG. Twelve independent hybridoma clones were characterized, which, except 520-16A, are identical based on epitope exclusion assay. Four moAbs, including 520-2A, 520-3A, 520-16A, and 520-26A, have high affinity (K(D) approximately 0.3-1.6nM), and their subtypes were IgG(2a), IgG(1), IgG(2a), and IgG(2b), respectively. moAb 520-3A recognizes the sequence (52)AQAPCPRERCLGPP(66)T, an epitope exclusively present in the N-terminal extension of human MPG. We found that moAb 520-3A significantly inhibited MPG's enzymatic activity towards different substrates, such as hypoxanthine, 1,N(6)ethenoadenine and methylated bases, which represent different classes of DNA damage, however, with different efficiencies. Real-time binding experiments using surface plasmon resonance (SPR) spectroscopy showed that the pronounced inhibition of activity was not in the substrate-binding step. Single turnover kinetics (STO) revealed that the inhibition was at the catalytic step. Since we found that this antibody has an epitope in the N-terminal tail, the latter appears to have an important role in substrate discrimination, however, with a differential effect on different substrates.

Magnesium, Essential for Base Excision Repair Enzymes, Inhibits Substrate Binding of N-Methylpurine-DNA Glycosylase

N-Methylpurine-DNA glycosylase (MPG) initiates base excision repair in DNA by removing a wide variety of alkylated, deaminated, and lipid peroxidation-induced purine adducts. MPG activity and other DNA glycosylases do not have an absolute requirement for a cofactor. In contrast, all downstream activities of major base excision repair proteins, such as apurinic/apyrimidinic endonuclease, DNA polymerase beta, and ligases, require Mg(2+). Here we have demonstrated that Mg(2+) can be significantly inhibitory toward MPG activity depending on its concentration but independent of substrate type. The pre-steady-state kinetics suggests that Mg(2+) at high but physiologic concentrations decreases the amount of active enzyme concentrations. Steady-state inhibition kinetics showed that Mg(2+) affected K(m), but not V(max), and the inhibition could be reversed by EDTA but not by DNA. At low concentration, Mg(2+) stimulated the enzyme activity only with hypoxanthine but not ethenoadenine. Real-time binding experiments using surface plasmon resonance spectroscopy showed that the pronounced inhibition of activity was due to inhibition in substrate binding. Nonetheless, the glycosidic bond cleavage step was not affected. These results altogether suggest that Mg(2+) inhibits MPG activity by abrogating substrate binding. Because Mg(2+) is an absolute requirement for the downstream activities of the major base excision repair enzymes, it may act as a regulator for the base excision repair pathway for efficient and balanced repair of damaged bases, which are often less toxic and/or mutagenic than their subsequent repair product intermediates.

Repair kinetics of trans-4-hydroxynonenal-induced cyclic 1,N2-propanodeoxyguanine DNA adducts by human cell nuclear extracts

trans-4-Hydroxynonenal (HNE) is a major peroxidation product of omega-6 polyunsaturated fatty acids. The reaction of HNE with DNA produces four diastereomeric 1,N(2)-gamma-hydroxypropano adducts of deoxyguanosine (HNE-dG); background levels of these adducts have been detected in tissues of animals and humans. There is evidence to suggest that these adducts are mutagenic and involved in liver carcinogenesis in patients with Wilson's disease and in other human cancers. Here, we present biochemical evidence that in human cell nuclear extracts the HNE-dG adducts are repaired by the nucleotide excision repair (NER) pathway. To investigate the recognition and repair of HNE-dG adducts in human cell extracts, we prepared plasmid DNA substrates modified by HNE. [(32)P]-Postlabeling/HPLC determined that the HNE-dG adduct levels were approximately 1200/10(6) dG of plasmid DNA substrate. We used this substrate in an in vitro repair-synthesis assay to study the complete repair of HNE-induced DNA adducts in cell-free extracts. We observed that nuclear extracts from HeLa cells incorporated a significant amount of alpha[(32)P]dCTP in DNA that contained HNE-dG adducts by comparison with UV-irradiated DNA as the positive control. Such repair synthesis for UV damage or HNE-dG adducts did not occur in XPA cell nuclear extracts that lack the capacity for NER. However, XPA cells complemented with XPA protein restored repair synthesis for both of these adducts. To verify that HNE-dG adducts in DNA were indeed repaired, we measured HNE-dG adducts in the post-repaired DNA substrates by the [(32)P]-postlabeling/HPLC method, showing that 50-60% of HNE-dG adducts were removed from the HeLa cell nuclear extracts after 3 h at 30 degrees C. The repair kinetics indicated that the excision rate is faster than the rate of gap-filling/DNA synthesis. Furthermore, the HNE-dG adduct isomers 2 and 4 appeared to be repaired more efficiently at early time points than isomers 1 and 3.