Prokaryotic expression and characterization of human AP DNA endonuclease

The expression of major human apurinic/apyrimidinic DNA endonuclease (APEX) from its cDNA in E. coli (DH5 alpha) was attempted in order to obtain a biologically active recombinant APEX. E. coli cells were transformed by a prokaryotic translation vector (pGEX-4T-3) harboring APEX cDNA. GST-APEX fusion protein with a molecular weight of 6.3 KDa was induced by IPTG (1.0 mM) treatment. Western blot immunodetection identified the induced protein as the GST-APEX fusion protein. The survival rate of E. coli cells (DH5 alpha) transformed with pGEX-4T-3-APEX increased when the cells were treated with N-diethyl-N-nitrosamine (DENA) or 3'-methyl-4-monomethylaminoazobenzene (3'-MeMAB), indicating that APEX expression had a protective effect on the cytotoxicity of these carcinogens. The fusion protein extracted from E. coli cells and purified by GSH-agarose gel affinity chromatography exhibited APEX activity. Treatment of thrombin to the GST-APEX fusion protein and affinity purification followed by Sephacryl S-100 gel filtration resulted in APEX peptide with MW 36 KDa, which exhibited AP DNA repair activity (8,7000 EU/mg protein). N-ethylmaleimide (0.1 mM) or AMP (0.98 mM) inhibited APEX activity by 50% and kinetic analysis indicated that the recombinant APEX (rAPEX) had a Km value of 0.022 microM (AP sites for AP DNA) and the Ki value was 0.48 mM for AMP. These results indicated that E. coli cells expressing biologically active GST-APEX were resistant to the cell damage caused by chemical carcinogens and that rAPEX purified from E. coli cells transformed with APEX cDNA-inserted translation vector was similar to native APEX in some properties.

Adaptive Change of AP DNA Endonuclease Against Genotoxic Agents in Normal and Transformed Cells / 대한암학회지

PURPOSE: AP DNA endonuclease (APE), an enzyme responsible for the repair of damaged DNAs, is essential for the maintenance of genetic information of cells. Deficiency of APE in certain hereditary skin tumor and senescent cells has been implicated but the regulation of APE activity as well as the expression of APE gene in response to DNA damage has not been well documented. Genotoxic agents including ultimate carcinogens that can damage DNA were treated to cultured normal and transformed human cells and adaptive response of APE gene expression to these treatments was measured in order to evaluate the role of APE in chemical carcinogenesis. MATERIALS AND METHODS: Hydroxyl radical ('OH) generated from H2O2 (60 uM) through Fenton reaction, each 100 uM of N-nitrosomethylurea (NMU), 3-methyl-4-monomethyl- aminoazobenzene (3'-MeMAB) and N-acetoxy-2-acetaminofluorene (AAAF) were treated to umbilical cord blood cells (UCBC), HepG2 cells and HL-60 cells. APEX mRNA and APEX protein contents expressed in these cells exposed to each of these agents were measured by Northern blot hybridization and Western blot immunodetection analysis. The changes of APE activity in cells exposed to these genetoxic agents were measured. RESULTS: Treatment of H2O2 (60 uM) to UCBC, HepG2, and HL-60 cells increased APE activity significantly and pretreatment of a catalytic agent for OH, FeSO4 (60 pM) to the cells prior to H2O2 exposure did not further increase the APE activity in cells. Adaptive response to H2O2 in HL-60 cells increased in proportion to the concentration of H2O2 up to 60 pM. However, further increase in H2O2 concentration had no effect on the enzyme activity. Treatment of NMU (100 pM), 3-MeMAB (100 pM) and AAAF (100 pM) to these cells brought about a slight increase in the APE activity. APEX mRNA expression in UCBC and HepG2 cells exposed to H2O2, NMU, 3-MeMAB was markedly increased in APEX mRNA expression. APEX mRNA expression was also increased in HL-60 cells exposed to H2O2 (60 pM) and 3-MeMAB (100 uM) but NMU (100 pM) exposure to the cells resulted in a slight increase of it (Fig. 2). APEX protein expression was increased in all UCBC, HepG2 and HL-60 cells exposed to these genotoxic agents (Fig. 3). CONCLUSION: These results implicate that exposure of genotoxic agents to the cultured cells may cause DNA damage and lead to adaptive increase in APE activity as well as APE gene expression. It is probable that APE gene is transcriptionally regulated in response to the exposure of H2O2 or 3-MeMAB in cultured human cells as a consequence of activation of DNA repair system for the adaptation to the crisis.

Studies on rat liver nuclear DNA damaged by chemical carcinogen (3'-Me DAB) and AP DNA endonuclease. I. Purification and some properties of AP DNA endonucleases in rat liver chromatin

Three kinds of apurinic/apyrimidinic (AP) DNA endonuclease, APcI, APcII, APcIII, were purified from rat liver chromatin through 1M KCl extraction, DEAE-trisacryl ion exchange chromatography. Sephadex G-150 gel filtration and AP DNA cellulose affinity chromatography. Activities of the purified APcI, APcII and APcIII were 62.5, 83.3 and 52.0 EU/mg of protein, respectively. Molecular weights of APcI, APcII and APcIII, each consisting of a single polypeptide, were 30,000, 42,000 and 13,000, and isoelectric points of them were 7.2, 6.3 and 6.2, respectively. Three enzymes showed different substrate specificities; APcI acted only on AP DNA, and APcII acted on both AP DNA and UV DNA, while APcIII acted on 3'-methyl-4-monomethylaminoazobenzene (3'-Me MAB) DNA adduct as well as AP DNA and UV DNA. These results indicate that three kinds of AP DNA endonuclease present in rat liver chromatin have structural and functional diversities.

Studies on rat liver nuclear DNA damaged by chemical carcinogen (3'-Me DAB) and AP DNA endonuclease. II. Kinetic properties of AP DNA endonucleases in rat liver chromatin

An experiment was designed to investigate the reaction mechanism of AP (apurinic or apyrimidinic) DNA endonucleases (APcI, APcII, APcIII) purified from rat liver chromatin. Sulfhydryl compounds (2-mercaptoethanol, dithiothreitol) brought about optimal activities of AP DNA endonucleases and N-ethylmaleimide or HgCl2 inhibited the enzyme activities, indicating the presence of sulfhydryl group at or near the active sites of the enzymes. Mg2+ was essential and 4mM of Mg2+ was sufficient for the optimal activities of AP DNA endonucleases. Km values of APcI, APcII and APcIII for the substrate (E. coli chromosomal AP DNA) were 0.53, 0.27 and 0.36 microM AP sites, respectively. AMP was the most potent inhibitor among adenine nucleotides tested and the inhibition was uncompetitive with respective to the substrate. The Ki values of APcI, APcII and APcIII were 0.35, 0.54 and 0.41mM, respectively. The degree of nick translation of AP DNAs nicked by APcI, APcII and APcIII with Klenow fragment in the presence and absence of T4 polynucleotide kinase or alkaline phosphatase were the same, suggesting that all 3 AP DNA endonucleases excise the phosphodiester bond of AP DNA strand to release 3-hydroxyl nucleotides and 5-phosphomonoester nucleotides.