Purification and properties of a mouse-cell DNA-repair endonuclease, which recognizes lesions in DNA induced by ultraviolet light, depurination, gamma-rays, and OsO4 treatment

Incision activity of human apurinic endonuclease (Ape) at abasic site analogs in DNA

The major apurinic/apyrimidinic (AP) endonuclease of human cells, the Ape protein, incises DNA adjacent to abasic sites to initiate DNA repair and counteract the cytotoxic and mutagenic effects of AP sites. Here we address the determinants of Ape AP endonuclease activity using duplex DNA substrates that contain synthetic analogs of AP sites: tetrahydrofuranyl (F), propanediol (P), ethanediol (E), or 2-(aminobutyl)-1,3-propanediol (Q). The last of these, a branched abasic structure, was a poor substrate for which Ape had kcat > 1000-fold lower than for F. In contrast, the specificity constant (kcat/Km) for E or P of Ape purified from HeLa cells was only 5-8-fold lower than for F. Positioning a phosphorothioate ester immediately 5' to F inhibited Ape incision activity 20-fold (Rp isomer) or > 10,000-fold (Sp isomer). Although Ape did not have detectable endonuclease activity toward single-stranded substrates or unmodified double-stranded DNA, the enzyme displayed a low level of 3'-exonuclease activity for duplex DNA (< 0.03% of its AP endonuclease activity), which was influenced by the reaction conditions. The base positioned opposite F did not dramatically affect the cleavage efficiency of Ape, but an F:F arrangement was cleaved at approximately one-third of the efficiency of F:C. A 3'-mismatch diminished P and E cleavage only slightly and F not at all. A 5'-mismatch reduced the Ape cleavage rate 4-10-fold for F and approximately 100-fold for P and E. A series of substrates with F at different positions along the oligonucleotide showed that Ape requires > or = 4 base pairs 5' to the abasic site and > or = 3 base pairs on the 3'-side. The implications of these results for substrate recognition by Ape are discussed.

Purification to homogeneity and characterization of a redoxyendonuclease from calf thymus

A redoxyendonuclease from calf thymus was purified to apparent homogeneity. The redoxyendonuclease recognized and induced cleavage of DNA damaged by ultraviolet light. The enzyme preparation produced a single band of a relative molecular mass of approximately 34 kDa upon SDS/PAGE. The apurinic/apyrimidinic endonuclease and the DNA glycosylase activities remained associated in the apparently homogeneous preparation of the enzyme. The redoxyendonuclease activity displayed a broad pH optimum between pH 5.0-8.5 and exhibited no requirement for divalent cations. By application of FPLC columns Mono-S, Mono-Q and Mono-P, the isoelectric point (pI) of the enzyme was found to be approximately 8.0. Using the DNA sequencing procedure of Maxam and Gilbert [Maxam, A. M. & Gilbert, W. (1980) Methods Enzymol. 65, 499-560] the purified enzyme was found to incise ultraviolet-light-irradiated DNA at pyrimidine sites as observed previously with a more crude form of the enzyme. While the most frequently cleavaged sites for the crude preparation were at cytosine residues, the apparently homogeneous enzyme preparation frequently induced cleavage sites at both cytosine and guanine residues. Predominant incision induced by the apparently homogeneous preparation was observed at guanine residues when a particular DNA sequence was used as substrate. Furthermore, the 16 N-terminal amino acid residues of the purified enzyme were identified. The sequence did not show any significant similarity to other known proteins.

Cloning and expression of APE, the cDNA encoding the major human apurinic endonuclease: definition of a family of DNA repair enzymes

Abasic (AP) sites are common, potentially mutagenic DNA damages that are attacked by AP endonucleases. The biological roles of these enzymes in metazoans have not been tested. We have cloned the human cDNA (APE) that encodes the main nuclear AP endonuclease. The predicted Ape protein, which contains likely nuclear transport signals, is a member of a family of DNA repair enzymes that includes two bacterial AP endonucleases (ExoA protein of Streptococcus pneumoniae and exonuclease III of Escherichia coli) and Rrp1 protein of Drosophila melanogaster. Purified Ape protein lacks the 3'-exonuclease activity against undamaged DNA that is found in the bacterial and Drosophila enzymes, but the lack of obvious amino acid changes to account for this difference suggests that the various enzyme functions evolved by fine tuning a conserved active site. Expression of the active human enzyme in AP endonuclease-deficient E. coli conferred significant resistance to killing by the DNA-alkylating agent methyl methanesulfonate. The APE cDNA provides a molecular tool for analyzing the role of this central enzyme in maintaining genetic stability in humans.

Two distinct human DNA diesterases that hydrolyze 3'-blocking deoxyribose fragments from oxidized DNA

Mammalian cells were investigated for enzymes that help correct oxidative damages in DNA. We focused on 3'-repair diesterases, which process DNA ends at oxidative strand breaks by removing 3'-blocking fragments of deoxyribose that prevent DNA repair synthesis. Two enzymes were found in a variety of mouse, bovine and human tissues and cultured cells. The two activities were purified to differing degrees from HeLa cells. One enzyme had the properties of the known HeLa AP endonuclease (Mr approximately 38,000, with identical substrate specificity and reaction requirements, and cross-reactivity with anti-HeLa AP endonuclease antiserum) and is presumed identical to that protein. The second activity did not interact with anti-HeLa AP endonuclease antibodies and had relatively less AP endonuclease activity. This second enzyme may have been detected in other studies but never characterized. In addition to the 3'-repair diesterase and AP endonuclease, this partially purified preparation also harbored DNA 3'-phosphatase and 3'-deoxyribose diesterase activities. It is unknown whether all activities detected in the second preparation are due to a single protein, although activity against undamaged DNA was not detected. The in vivo roles of these two widely distributed 3'-repair diesterase/AP endonucleases have not been determined, but with the characterizations presented here such questions may now be focused.

An endonuclease activity in human polymorphonuclear neutrophils that removes 8-hydroxyguanine residues from DNA+

An endonuclease that specifically removes 8-hydroxyguanine (oh8Gua) from DNA has been isolated from Escherichia coli. As the amount of oh8Gua produced in DNA of X-ray-irradiated mice is known to decrease with time after irradiation, an attempt was made to find a similar activity in human polymorphonuclear neutrophils (PMNs) using a synthetic dsDNA containing oh8Gua as a substrate. The PMN enzyme was isolated free of other DNases, and found to cleave the substrate DNA simultaneously at 2 sites, the phosphodiester bonds 5' and 3' to oh8Gua, producing free hydroxyl and phosphate groups, respectively. The enzyme showed almost no activity on DNAs containing other kinds of modified base tested or mismatched DNA. Thus human cells also contain an endonuclease that specifically removes oh8Gua residues from DNA.

The enzymology of apurinic/apyrimidinic endonucleases

Studies on the enzymology of apurinic/apyrimidinic (AP) endonucleases from procaryotic and eucaryotic organisms are reviewed. Emphasis will be placed on the enzymes from Escherichia coli from which a considerable portion of our knowledge has been derived. Recent studies on similar enzymes from eucaryotes will be discussed as well. In addition, we will discuss the chemical and physical properties of AP sites and review studies on peptides and acridine derivatives which incise DNA at AP sites.

Mammalian mitochondrial endonuclease activities specific for ultraviolet-irradiated DNA

Mitochondrial forms of uracil DNA glycosylase and UV endonuclease have been purified and characterized from the mouse plasmacytoma cell line, MPC-11. As in other cell types, the mitochondrial uracil DNA glycosylase has properties very similar to those of the nuclear enzyme, although in this case the mitochondrial activity was also distinguishable by extreme sensitivity to dilution. Three mitochondrial UV endonuclease activities are also similar to nuclear enzymes; however, the relative amounts of these enzyme activities in the mitochondria is significantly different from that in the nucleus. In particular, mitochondria contain a much higher proportion of an activity analogous to UV endonuclease III. Nuclear UV endonuclease III activity is absent from XP group D fibroblasts and XP group D lymphoblasts have reduced, but detectable levels of the mitochondrial form of this enzyme. This residual activity differs in its properties from the normal mitochondrial form of UV endonuclease III, however. The presence of these enzyme activities which function in base excision repair suggests that such DNA repair occurs in mitochondria. Alternatively, these enzymes might act to mark damaged mitochondrial genomes for subsequent degradation.

Cellular and molecular biological aspects of human bronchogenic carcinogenesis

This is a time of rapid progress in the field of human bronchogenic carcinogenesis due to recent advances in cellular and molecular biology. Important developments over the last 10 years include establishment of methods for culturing NHBE cells under defined conditions, and molecular biological and biochemical epidemiological techniques for identifying genetic changes that are associated with malignant transformation of these cells. Most progress in defining genes associated with human carcinogenesis has been due to discoveries related to oncogenes and more recently, tumor suppressor genes. As was described in Section II.B.3.a, we now know that oncogene products serve as growth factors, growth factor receptors, and cytosolic and nuclear regulatory proteins. In addition, although the actions of putative tumor suppressor genes are less well understood, the first isolated tumor suppressor gene Rb, interacts with the products of DNA viruses which, in turn, are involved in regulation of transcription as was described in Section II.B.3.b. Thus, not surprisingly, both oncogenes and tumor suppressor genes code for classes of proteins that are known to play an important role in regulation of cell proliferation. Recently, a second gene that appears to possess tumor suppression activity (p53) has been identified on the short arm of chromosome 17 (17p). The initial data suggesting a possible tumor suppressor gene on chromosome 17p came from cytogenetic and RFLP studies associating loss of heterozygosity in the chromosome 17p13 region with tumor cells and tissues. Since the p53 gene is located in this region it was evaluated and found to be frequently or always altered in several types of tumor cells. Recently, it was determined that introduction of the wild-type p53 gene into NIH3T3 cells will inhibit subsequent malignant transformation. Thus, the preponderance of evidence now supports the hypothesis that while mutated p53 acts as an oncogene, the wild-type p53 gene codes for a tumor suppressor function. The role of balance between oncogenes and tumor suppressor genes in control of proliferation is presently an active area of investigation. As discussed, introduction of a chromosome containing a tumor suppressor gene will suppress tumorigenicity of a malignant cell line, even though that cell line possesses an active c-Ha-ras oncogene. Whether or not the level of expression of an activated oncogene is related to tumorigenicity is presently being investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

Action of a mammalian AP-endonuclease on DNAs of defined sequences

An apurinic/apyrimidinic (AP) specific endonuclease from mouse plasmacytoma cells (line MPC-11), was observed to cleave apurinic sites in oligonucleotides 9, 11, 12, 39 and 40 nucleotides in length. However, the enzyme failed to cleave AP-sites in two oligonucleotides 7 nucleotides in length. The maximum rates of digestion observed on these short single-stranded DNA (ssDNA) fragments were approximately 1/30 of the rates observed on double-stranded DNA (dsDNA). In studies using the Maxam-Gilbert DNA sequencing analysis, apurinic sites in purine-rich regions were preferentially cleaved in dsDNA but not in ssDNA, indicating that the enzyme has a sequence preference on dsDNA. These results suggest that some sites on DNA might be more efficiently repaired than others.

Substrate specificity of 3-methyladenine-DNA glycosylase from calf thymus

3-Methyladenine-DNA glycosylase from calf thymus recognizes both 3-methyladenine (3-mAde), 7-methylguanine (7-mGua) and 3-methylguanine (3-mGua) residues in calf thymus DNA; the rate of release of 3-mAde is approximately eightfold higher than that for 7-mGua. The best DNA polymer substrates appeared to be those having an A-type helical conformation such as d(A-T)n and d(G-C)n. The Km values for release of 3-mAde and 7-mGua were approximately the same for the above mentioned two substrates whereas the Vmax for excision of 3-mAde was threefold higher than that of 7-mGua. The rate of hydrolysis of 7-mGua residues in d(G-C)n was similar to that found for the excision of 3-mAde in calf thymus DNA. The polymer d(G)n X d(C)m, which possesses a B-type helical conformation, was a poor substrate and the rate of excision here was approximately the same as with calf thymus DNA having the B-type structure. Polyamines greatly influenced the activity and at low concentrations a 50-100% increase in the release of 7-mGua, but not 3-mAde, was observed. With higher concentrations the rate of excision of both bases decreased sharply. The sequence specificity of the DNA glycosylase on naturally occurring DNA was studied using methylated DNA fragments from the plasmid pUC18. The results revealed that some 3-mAde as well as 7-mGua residues were seldom attacked. These 3-mAde residues were positioned either 5' to another Ade residue or in a stretch of pyrimidines, and the 7-mGua residue 3' to another Gua residue. The 3-mAde residue most frequently recognized was situated 3' to another Ade residue, and in the case of 7-mGua it was the central Gua residue in the sequence -G-G-G-.

A highly conserved endonuclease activity present in Escherichia coli, bovine, and human cells recognizes oxidative DNA damage at sites of pyrimidines

We have compared the sites of nucleotide incision on DNA damaged by oxidizing agents when cleavage is mediated by either Escherichia coli endonuclease III or an endonuclease present in bovine and human cells. E. coli endonuclease III, the bovine endonuclease isolated from calf thymus, and the human endonuclease partially purified from HeLa and CEM-C1 lymphoblastoid cells incised DNA damaged with osmium tetroxide, ionizing radiation, or high doses of UV light at sites of pyrimidines. For each damaging agent studied, regardless of whether the E. coli, bovine, or human endonuclease was used, the same sequence specificity of cleavage was observed. We detected this endonuclease activity in a variety of human fibroblasts derived from normal individuals as well as individuals with the DNA repair deficiency diseases ataxia telangiectasia and xeroderma pigmentosum. The highly conserved nature of such a DNA damage-specific endonuclease suggests that a common pathway exists in bacteria, humans, and other mammals for the reversal of certain types of oxidative DNA damage.

Separation of damage specific DNA endonuclease activities present in calf thymus

A DNA endonuclease activity present in calf thymus specific for incision on DNA damaged by ultraviolet light, osmium tetroxide, potassium permanganate, hydrogen peroxide and acid has been purified from whole cell extracts. The enzymatic activity was heterogeneous both with regard to molecular mass and charge. The molecular mass of the enzyme varied from 25 to 35 kDa, but the different enzymatic species appeared to possess similar activities. The enzymes acted equally well on damage in supercoiled and relaxed forms of DNA. It further had a narrow optimum with regard to salt concentrations, the optimum activity being observed at a concentration of KCl from 40 to 65 mM.

A highly conserved endonuclease activity present in Escherichia coli, bovine, and human cells recognizes oxidative DNA damage at sites of pyrimidines

We have compared the sites of nucleotide incision on DNA damaged by oxidizing agents when cleavage is mediated by either Escherichia coli endonuclease III or an endonuclease present in bovine and human cells. E. coli endonuclease III, the bovine endonuclease isolated from calf thymus, and the human endonuclease partially purified from HeLa and CEM-C1 lymphoblastoid cells incised DNA damaged with osmium tetroxide, ionizing radiation, or high doses of UV light at sites of pyrimidines. For each damaging agent studied, regardless of whether the E. coli, bovine, or human endonuclease was used, the same sequence specificity of cleavage was observed. We detected this endonuclease activity in a variety of human fibroblasts derived from normal individuals as well as individuals with the DNA repair deficiency diseases ataxia telangiectasia and xeroderma pigmentosum. The highly conserved nature of such a DNA damage-specific endonuclease suggests that a common pathway exists in bacteria, humans, and other mammals for the reversal of certain types of oxidative DNA damage.

Substrate specificity of a mammalian DNA repair endonuclease that recognizes oxidative base damage

The substrate specificity of a calf thymus endonuclease on DNA damaged by UV ligh, ionizing radiation, and oxidizing agents was investigated. End-labeled DNA fragments of defined sequence were used as substrates, and the enzyme-generated scission products were analyzed by using DNA sequencing methodologies. The enzyme was shown to incise damaged DNA at pyrimidine sites. The enzyme incised DNA damaged with UV light, ionizing radiation, osmium tetroxide, potassium permanganate, and hydrogen peroxide at cytosine and thymine sites. The substrate specificity of the calf thymus endonuclease was compared to that of Escherichia coli endonuclease III. Similar pyrimidine base damage specificities were found for both enzymes. These results define a highly conserved class of enzymes present in both procaryotes and eucaryotes that may mediate an important role in the repair of oxidative DNA damage.

Purification and properties of a single strand-specific endonuclease from mouse cell mitochondria

A nuclease was purified from mitochondria of the mouse plasmacytoma cell line, MCP-11 which acts on single-stranded DNA endonucleolytically and appears to have no activity upon native DNA. It degrades unordered RNA somewhat more effectively than it does DNA. The enzyme activity and the major detectable polypeptide migrate to a position corresponding to an Mr of 37,400 on denaturing polyacrylamide gels; in its native form the activity has an S value of 4.7, which corresponds to a molecular weight of roughly 73,000. The single-strand DNase activity has a pH optimum near 7.5, requires a divalent cation and is inhibited by EDTA, phosphate, KCl and NaCl. The enzyme is remarkably similar to fungal mitochondrial enzymes whose absence in various mutants correlates with defective DNA repair and recombination. It reacts weakly with antibody to a form of such an enzyme from Neurospora crassa.

Substrate specificity of a mammalian DNA repair endonuclease that recognizes oxidative base damage

The substrate specificity of a calf thymus endonuclease on DNA damaged by UV ligh, ionizing radiation, and oxidizing agents was investigated. End-labeled DNA fragments of defined sequence were used as substrates, and the enzyme-generated scission products were analyzed by using DNA sequencing methodologies. The enzyme was shown to incise damaged DNA at pyrimidine sites. The enzyme incised DNA damaged with UV light, ionizing radiation, osmium tetroxide, potassium permanganate, and hydrogen peroxide at cytosine and thymine sites. The substrate specificity of the calf thymus endonuclease was compared to that of Escherichia coli endonuclease III. Similar pyrimidine base damage specificities were found for both enzymes. These results define a highly conserved class of enzymes present in both procaryotes and eucaryotes that may mediate an important role in the repair of oxidative DNA damage.

Formation of cytosine glycol and 5,6-dihydroxycytosine in deoxyribonucleic acid on treatment with osmium tetroxide

OsO4 selectively forms thymine glycol lesions in DNA. In the past, OsO4-treated DNA has been used as a substrate in studies of DNA repair utilizing base-excision repair enzymes such as DNA glycosylases. There is, however, no information available on the chemical identity of other OsO4-induced base lesions in DNA. A complete knowledge of such DNA lesions may be of importance for repair studies. Using a methodology developed recently for characterization of oxidative base damage in DNA, we provide evidence for the formation of cytosine glycol and 5,6-dihydroxycytosine moieties, in addition to thymine glycol, in DNA on treatment with OsO4. For this purpose, samples of OsO4-treated DNA were hydrolysed with formic acid, then trimethylsilylated and analysed by capillary gas chromatography-mass spectrometry. In addition to thymine glycol, 5-hydroxyuracil (isobarbituric acid), 5-hydroxycytosine and 5,6-dihydroxyuracil (isodialuric acid or dialuric acid) were identified in OsO4-treated DNA. It is suggested that 5-hydroxyuracil was formed by formic acid-induced deamination and dehydration of cytosine glycol, which was the actual oxidation product of the cytosine moiety in DNA. 5-Hydroxycytosine obviously resulted from dehydration of cytosine glycol, and 5,6-dihydroxyuracil from deamination of 5,6-dihydroxycytosine. This scheme was supported by the presence of 5-hydroxyuracil, uracil glycol and 5,6-dihydroxyuracil in OsO4-treated cytosine. Treatment of OsO4-treated cytosine with formic acid caused the complete conversion of uracil glycol into 5-hydroxyuracil. The implications of these findings relative to studies of DNA repair are discussed.

Properties of a DNA repair endonuclease from mouse plasmacytoma cells

The properties of a DNA-repair endonuclease isolated from mouse plasmacytoma cells have been further studied. It acted on ultraviolet-light-irradiated supercoiled DNA, and the requirement for a supercoiled substrate was absolute at ultraviolet light doses below 1.5 kJ m-2. At higher doses relaxed DNA could also serve as a substrate, but the activity on this DNA was due mostly to hydrolysis of ultraviolet-light-induced apurinic/apyrimidinic (AP) sites by the AP-endonuclease activity associated with the enzyme. The latter enzyme activity did not require a supercoiled form of the DNA. The enzyme also introduced nicks in unirradiated d(A-T)n. The nicked ultraviolet-light-irradiated DNA served as a substrate for DNA polymerase I, showing that the nicks contained free 3'-OH ends. Treatment of the nicked ultraviolet-light-irradiated DNA with bacterial alkaline phosphatase followed by T4 polynucleotide kinase, resulted in the phosphorylation of the 5' ends of the nicks, indicating that the nicks possessed a 5'-phosphate group; 5'- and 3'-mononucleotide analyses of the labelled DNA suggested that the enzyme introduced breaks primarily between G and T residues. The enzyme did not act on any specific region on the supercoiled DNA molecule; it produced random nicks in ultraviolet-light-modified phi X 174 replicative form I DNA. Antibodies raised against ultraviolet-light-irradiated DNA inhibited the activity. DNA adducts such as N-acetoxy-2-acetylaminofluorene and psoralen were not recognized by the enzyme. It is suggested that the enzyme has a specificity directed toward helical distortions.

Thymine glycol and thymidine glycol in human and rat urine: a possible assay for oxidative DNA damage

Thymine glycol is a DNA damage product of ionizing radiation and other oxidative mutagens. In an attempt to find a noninvasive assay for oxidative DNA damage in individuals, we have developed an HPLC assay for free thymine glycol and thymidine glycol in urine. Our results indicate that humans excrete about 32 nmol of the two glycols per day. Rats, which have a higher specific metabolic rate and a shorter life span, excrete about 15 times more thymine glycol plus thymidine glycol per kg of body weight than do humans. We present evidence that thymine glycol and thymidine glycol are likely to be derived from repair of oxidized DNA, rather than from alternative sources such as the diet or bacterial flora. This noninvasive assay of DNA oxidation products may allow the direct testing of current theories which relate oxidative metabolism to the processes of aging and cancer in man.

Hydroxymethyluracil DNA glycosylase in mammalian cells

An activity has been purified 350-fold from extracts of mouse plasmacytoma cells that forms 5-hydroxymethyluracil (alpha-hydroxythymine) and apyrimidinic sites with phage SPO1 DNA, which contains this base in place of thymine. This DNA glycosylase presumably functions to eliminate hydroxymethyluracil, a major thymine-derived DNA lesion produced by ionizing radiation and oxidative damage. The enzyme has no cofactor requirement and is active in EDTA. Neither intermediate formation nor hydrolysis of hydroxymethyl-deoxyuridine or hydroxymethyldeoxyuridine monophosphate was detected. The enzyme does not cleave apyrimidinic sites in DNA. It does release uracil from the uracil-containing DNA of phage PBS2, but this activity is less than 2% of the predominant uracil DNA glycosylase activity of the cell, which is separated by phosphocellulose chromatography. The major uracil DNA glycosylase does not release hydroxymethyluracil from SPO1 DNA. The hydroxymethyluracil glycosylase is also separated upon phosphocelluose chromatography from a thymine glycol DNA glycosylase activity that is accompanied by an apyrimidinic endonuclease activity.

Properties of a human lymphoblast AP-endonuclease associated with activity for DNA damaged by ultraviolet light, gamma-rays, or osmium tetroxide

An endonuclease activity for UV-irradiated DNA, gamma-irradiated DNA, and OsO4-treated DNA that was partially purified from human lymphoblasts was found to have associated with it an endonuclease activity for partially depurinated DNA. When this apurinic endonuclease (Endo A) was characterized and compared with the cells' major apurinic endonuclease (Endo B), several notable differences were observed. (1) Endo A bound to oxidized DNA-Sepharose under conditions where Endo B did not. (2) Endo A did not require Mg2+, retaining full activity in 10 mM ethylenediamine-tetraacetic acid, while Endo B showed an absolute requirement for Mg2+. (3) Whereas the nicks made in depurinated DNA by Endo B were efficient priming sites for Escherichia coli polymerase I, those made by Endo A were not. Further characterization of the nicks indicated that Endo A incises depurinated DNA 3' to apurinic sites, leaving 3'-terminal deoxyribose, a poor priming site for DNA synthesis. Endo A action on UV-irradiated DNA produced nicks that resembled those it made in depurinated DNA, suggesting that the UV endonuclease activity acts through an apurinic/apyrimidinic site intermediate.

Urea--DNA glycosylase in mammalian cells

Urea-DNA glycosylase, an enzyme presumed to be active in the repair of DNA damage caused by oxidizing agents, has been identified previously in Escherichia coli. This enzyme has now been shown to be present in cell extracts of calf thymus and human fibroblasts. It catalyzes the release of free urea from a double-stranded polydeoxyribonucleotide containing thymine residues fragmented by KMnO4 and NaOH treatment. The calf thymus enzyme has been 400-fold purified and largely separated from previously identified mammalian DNA glycosylases. It has a molecular weight of about 25 000 and requires no cofactors. The identity of the enzymatically released product as unsubstituted urea has been verified by its susceptibility to urease.

UV-irradiated SV40 minichromosomes as substrates for DNA repair endonucleases

UV-irradiated SV40 minichromosomes have been shown to be a substrate for a purified DNA repair endonuclease. A UV-repair endonuclease activity was also found to be associated with the isolated SV40 minichromosomes themselves. It appeared to have similar properties to the enzymes described from other mammalian sources.

5,6-Saturated thymine lesions in DNA: production by ultraviolet light or hydrogen peroxide

Thymine analogs with saturated 5-6 bonds are important types of DNA damage that are recognized by the DNA N-glycosylase activity of E. coli endonuclease III. Seeking agents which could preferentially form 5,6-hydrated thymine residues in duplex DNA both in vivo and in vitro, we exposed purified duplex DNA to 325- or 313-nm light; however, after such exposure pyrimidine dimers greatly predominated over 5,6-hydrated thymine. Hydrogen peroxide, on the other hand, formed significant numbers of endonuclease III-sensitive sites in vitro which were not apurinic/apyrimidinic lesions and thus were likely to be 5,6-hydrated thymines.

Purification and properties of 3-methyladenine-DNA glycosylase from L-cells

3-Methyladenine-DNA glycosylase from L-cells has been purified approximately 800-fold. The enzyme is present primarily in the nucleus of the cells. The enzymatic reaction was sensitive to changes in the assay conditions and optimum activity was found at pH 6.5 and at 100 mM KCl. Mg2+ did not effect the enzymatic reaction, which also worked in the presence of EDTA. The activity on denatured methylated DNA was 20-40% of that of the native double-stranded form. Sephadex gel filtration of the most purified fraction revealed enzyme species with molecular weights of 68000, 47000 and 27000, which differ from those reported for corresponding enzymes from other organisms. Addition of the product, 3-methyladenine, to the reaction mixture resulted in inhibition of the glycosylase activity of up to 60%. The remaining activity could not be abolished by increasing the concentration of 3-methyladenine.