Uracil-DNA glycosylase activity in chronic lymphoproliferative disorders

The activity of uracil-DNA glycosylase, a repair enzyme for the excision of uracil from DNA, was studied in patients with chronic lymphoproliferative disorders and with malignant plasma cell dyscrasias. The biochemical assay was performed on mononuclear cells, isolated by density gradient centrifugation from peripheral blood, from bone marrow or from both. The activity of the uracil-DNA glycosylase of peripheral blood cells in 8/8 cases of myeloma and in 3/3 cases of Waldenstr?m's macroglobulinemia was in the same range as in 22 non-hematological control patients, i.e. 2.4-25.1 U/mg of protein. Higher activities were found in 9/12 cases of chronic lymphocytic leukemia (CLL), in 2/4 cases of hairy cell leukemia (HCL), in 2/2 cases of chronic T-cell lymphocytosis and in the only case of small cell lymphocytic lymphoma. Follow-up of some CLL and HCL patients revealed that uracil-DNA glycosylase activity was fairly stable during the course of the disease. We conclude that malignant cells in chronic lymphoproliferative disorders are characterized by a normal or even increased capability to repair DNA, as exemplified by uracil-DNA glycosylase in this study.     

Uracil-DNA glycosylase activity in human acute leukemia

The expression of the DNA excision repair enzyme uracil-DNA glycosylase was investigated in bone marrow and peripheral samples from seven patients with acute lymphoblastic leukemia (ALL), from 17 patients with acute non-lymphocytic leukemia (ANLL), and from one patient with chronic granulocytic leukemia (CGL) in blast crisis. In addition, uracil-DNA glycosylase activities were determined in nine human leukemia/lymphoma cell lines. There was a clear correlation between the percentage of blast cells and the enzyme activity when mononuclear cell fractions from patient samples were analysed. The following uracil-DNA glycosylase activities were recorded (mean +/- S.D., number of samples): ALL = 45.6 +/- 14.8 U/mg of protein, N = 10; ANLL = 41.1 +/- 13.8 U/mg of protein, N = 22; CGL (blast crisis) = 44.7 U/mg of protein. The uracil-DNA glycosylase activity in nine human leukemia/lymphoma cell lines ranged from 35.2 to 66.0 U/mg of protein, and no striking differences were observed between the T-ALL, B-ALL, null cell ALL or myeloid lines. Similarly, the various biological features, such as the common ALL surface antigen, the terminal deoxynucleotidyl transferase enzyme, the sub-type of leukemia, chromosomal aberrations, or previous chemotherapy, did not apparently affect the expression of uracil-DNA glycosylase. We propose that the integrity of the genetic information is well protected by uracil-DNA glycosylase in different forms of leukemia, including cases with a low proportion of S-phase blasts, as assessed by flow cytometry in the present work. When compared to the activities in benign hematopoietic progenitor cells, studied previously in this laboratory, no big differences between the benign and malignant hematopoiesis were demonstrated. Hence, it is unlikely that selectivity of chemotherapy towards malignant vs benign hematopoietic growth could be based on the enzyme uracil-DNA glycosylase.     

Cell mediated inhibition of erythropoiesis and megaloblastic anemia in T-cell chronic lymphocytic leukemia

A patient with T-cell chronic lymphocytic leukemia presented with severe megaloblastic anemia with normal serum folic acid and cobalamin concentrations. BFU-E could not be cultured from the patient's peripheral blood unless T-lymphocytes were removed by E-rosette formation. Inhibitory activity by the patient's T-cells was restricted to autologous BFU-E. After cyclic chemotherapy the anemia and megaloblastic changes resolved, peripheral blood BFU-E could be cultured from unfractionated peripheral blood and the T-cell inhibitory activity could no longer be demonstrated. The anemia in this patient is probably due to the neoplastic expansion of a suppressor T-lymphocyte population.     

Incorporation of dUMP into DNA is a major source of spontaneous DNA damage, while excision of uracil is not required for cytotoxicity of fluoropyrimidines in mouse embryonic fibroblasts

Uracil may arise in DNA as a result of deamination of cytosine or through incorporation of dUMP instead of dTMP during replication. We have studied the steady-state levels of uracil in the DNA of primary cells and mouse embryonic fibroblast (MEF) cell lines from mice deficient in the Ung uracil-DNA glycosylase. The results show that the levels of uracil in the DNA of Ung(-/-) cells strongly depend on proliferation, indicating that the uracil residues originate predominantly from misincorporation during replication. Treatment with 5-fluoro-2'-deoxyuridine (5-FdUrd) or 5-fluorouracil (5-FU) gives rise to a dose-dependent increase of uracil in Ung(-/-) MEFs (up to 1.5-fold) but not in wild-type cells. Interestingly, Ung(-/-) MEFs accumulate AP-sites as well as uracil in response to 5-FdUrd but not to 5-FU. This accumulation of repair intermediates suggests a loss of tightly co-ordinated repair in the absence of Ung, and correlates with stronger inhibition of cell proliferation in response to 5-FdUrd, but not to 5-FU, in Ung(-/-) MEFs compared with wild-type cells. However, other cytotoxic effects of these fluoropyrimidines are comparable in both wild-type and Ung-deficient cells, demonstrating that excision of uracil from DNA by the Ung uracil-DNA glycosylase is not a prerequisite for obtaining cytotoxicity.     

Interindividual variation in the activity of O6-methyl guanine-DNA methyltransferase and uracil-DNA glycosylase in human organs

As a step towards understanding the significance of DNA repairenzymes in the protection against genotoxic and carcinogenicagents, we have examined the activity of O⁶-methylguanine-DNAmethyltransferase and uracil-DNA glycosylase in adult humanliver, stomach, small intestine and colon. Liver had on averagea 5- to 8-fold higher activity of O⁶-MeG-DNA methyltransferasethan the other organs and showed about an 8-fold inter-individualvariation. In colon and small intestine an even larger inter-individualvariation was observed (10- and 40-fold, respectively). In twocolon tumors examined the activity of O⁶-MeG-DNA methyl-transferasewas several fold higher than in non-neoplastic colon mucosafrom the same individuals, while uracil-DNA glycosylase activitywas essentially equal in neoplastic and non-neoplastic tissues.O⁶-MeG-DNA methyltransferase activities in two gastric tumorsexamined were not higher than in average non-neoplastic tissue.In general the activity of uracil-DNA glycosylase did not correlatewith the O⁶-MeG-DNA methyltransferase activity. The inter-individualvariation of this enzyme in the activity was only 3-fold inliver and normal stomach, but varied 5.5 and 60-fold in colonand small intestine, respectively. In conslusion, we have foundthat O⁶-MeG-DNA methyltransferase as well as uracil-DNA glycosylaseactivity vary considerably between different tissues as wellas between different individuals. Whether this variation hasa genetic basis or reflects variation in ‘life style’is not known.     

Telomerase activity in myelodysplastic syndromes

Myelodysplastic syndromes are clonal hematopoietic stem cell disorders characterized by ineffective hematopoiesis and peripheral cytopenias. Telomeres are thought to be critical in maintaining normal hematopoiesis. In this study, we assessed telomere dynamics in order to obtain further insight into the pathogenesis of MDS. We studied telomerase activity (TA) in mononuclear cells from peripheral blood (PB) and bone marrow (BM) from patients with myelodysplastic syndrome (MDS; n=24), acute myeloid leukemia (AML; n=14), chronic myeloid leukemia (CML; n=12) and 11 normal controls using a polymerase chain reaction-based telomeric repeat amplification assay. Telomerase activities (mean+/-S.D.) were found as 0.199+/-0.09, 0.414+/-0.55, 0.253+/-0.26 and 0.181+/-0.05 pg/ml in PB mononuclear cells, respectively (P>0.05). Comparison of TA of BM mononuclear cells from 19 MDS patients versus 10 BM samples from normal controls revealed no significant difference (P=0.3). There was no correlation between the levels of TA and clinical and prognostic parameters of the patients with MDS, such as degree of anemia, platelet counts on presentation, gender, presence of organomegaly, bone marrow fibrosis and BM blast percentages. Patients who had higher TA had significantly inferior survival compared with patients who had lower TA (P=0.005). Consistent with previous data, our results suggest that in patients with MDS, telomerase activity might be insufficient to compensate for the telomere shortening. Furthermore, TA might be prognostically important in patients with MDS. Measurements of enzymatic activity in association with telomere length studies may help to understand the prognostic role of telomere dynamics in patients with myelodysplastic syndromes more reliably.     

低增生性骨髓增生异常综合征与再生障碍性贫血鉴别诊断的研究进展

低增生性骨髓增生异常综合征是骨髓增生异常综合征的一种特殊类型,与再生障碍性贫血的临床和实验室表现相似,常给临床鉴别诊断带来一定困难.两者的鉴别诊断包括外周血参数、骨髓细胞形态学、骨髓活组织检查、造血祖细胞培养、细胞遗传学、分子生物学和细胞免疫表型分析等方面,文章就两者的鉴别诊断进行综述.     

dUTPase and uracil-DNA glycosylase are central modulators of antifolate toxicity in Saccharomyces cerevisiae

The thymidylate synthase reaction remains an important target for widely used anticancer agents; however, the clinical utility of these drugs is limited by the occurrence of cellular resistance. Despite the considerable amount of information available regarding mechanisms of drug action, the relative significance of downstream events that result in lethality remains unclear. In this study, we have developed a model system using the budding yeast Saccharomyces cerevisiae to dissect the influence of dUMP misincorporation into DNA as a contributing mechanism of cytotoxicity induced by antifolate agents. The activities of dUTPase and uracil-DNA glycosylase, key enzymes in uracil-DNA metabolism, were diminished or augmented, and the manipulated strains were analyzed for biochemical endpoints of toxicity. Cells overexpressing dUTPase were protected from cytotoxicity by their ability to prevent dUTP pool expansion and were able to recover from an early S-phase checkpoint arrest. In contrast, depletion of dUTPase activity leads to the accumulation of dUTP pools and enhanced sensitivity to antifolates. These cells were also arrested in early S-phase and were unable to complete DNA replication after drug withdrawal, resulting in lethality. Inactivation of uracil base excision repair induced partial resistance to early cytotoxicity (within 10 h); however, lethality ultimately resulted at later time points (12-24 h), presumably because of the detrimental effects of stable uracil misincorporation. Although these cells were able to complete replication with uracil-substituted DNA, they arrested at the G(2)-M phase. This finding may represent a novel mechanism by which the G(2)-M checkpoint is signaled by the presence of uracil-substituted DNA. Together these data provide both genetic and biochemical evidence demonstrating that lethality from antifolates in yeast is primarily dependent on uracil misincorporation into DNA, and that uracil-independent mechanisms associated with dTTP depletion play a minor role. Our findings indicate that the relative expression levels of both dUTPase and uracil-DNA glycosylase can have great influence over the efficacy of thymidylate synthase-directed chemotherapy, thereby enhancing the candidacy of these proteins as prognostic markers and alternative targets for therapeutic development.     

Biosynthesis of the human base excision repair enzyme uracil-DNA glycosylase

The biosynthesis of the human DNA repair enzyme uracil-DNA glycosylase has been characterized by the reaction of in vitro- and in vivo-produced protein with an anti-human placental uracil-DNA glycosylase monoclonal antibody. In vitro synthesis of the DNA repair enzyme was examined after the translation of human placental polyadenylated [poly(A)+] RNA by immunoprecipitation of the [35S]methionine-labeled translation products. As defined by sucrose density analysis, immunoprecipitable in vitro products were translated from 16S poly(A)+ RNA and 11S poly(A)+ RNA. While the products of the 11S poly(A)+ RNA were smaller than purified uracil-DNA glycosylase, the product of the 16 S poly(A)+ RNA had a molecular weight of 37,000, identical to the size previously observed for purified human placental uracil-DNA glycosylase. Immunoblot analysis of human placental cell extracts and of normal human fibroblast cell extracts demonstrated the recognition of one Mr 37,000 protein. Immunoprecipitation of [35S]methionine-labeled normal human cell extracts with the anti-glycosylase monoclonal antibody specifically detected only the Mr 37,000 uracil-DNA glycosylase protein. Pulse-chase analysis demonstrated that the 35S radioactivity in the Mr 37,000 uracil-DNA glycosylase decreased over a 5-h interval. These results show that immunoreactive human uracil-DNA glycosylase protein was synthesized at its enzymatically active molecular weight of 37,000 as the primary translation product of a 16S polyadenylated messenger RNA.     

Abnormal regulation of uracil-DNA glycosylase induction during cell cycle and cell passage in Bloom's syndrome fibroblasts

The uracil-DNA glycosylase activity was compared in cell-freeextracts of normal (NHSF6) and Bloom's syndrome (BS) skin fibroblasts(BS1KA and BS2KA from Japanese patients) at middle culture age.The enzyme activity in the extracts of exponentially growingNHSF6 and BS2KA cells increased linearly with the DNA syntheticactivity, while such a relation was not obvious in BS1KA cells.The thermal stability and the inhibition by the end producturacil of the BS1KA enzyme did not differ from those of theNHSF6 enzyme. Synchronized-cell studies showed the followingcharacteristics. (i)Uracil-DNA glycosylase activity was enhancedin a temporal sequence only during S phase and reached a peaklevel a few hours prior to that of DNA synthesis in NHSF6. (ii)BS2KA cells were normal in the temporal induction sequence butBS1KA cells revealed the delayed peak induction of the enzymeoccurring simultaneously with peak DNA synthesis. (iii) Withprogress of culture passage, the uracil-DNA glycosylase activitybecame highly expressed at G₀ and during G₁ despite a littlechange in the S-phase activity in NHSF6 cells. (iv) Anotherabnormality of BS1KA cells was that such a culture-age-dependentdysregulation occurred earlier during middle passages. The aboveresults (i) and (ii) suggest that the delayed enzyme inductionin BS1KA cells may be related to the observation that BS1KAcells were more sensitive to 5-bromodeoxyuridine-induced cellkilling and to sister chromatid exchange formation than BS2KAcells of the clinically milder subject.     

5-Fluorouracil incorporated into DNA is excised by the Smug1 DNA glycosylase to reduce drug cytotoxicity

5-Fluorouracil (FU) has been widely used for more than four decades in the treatment of a range of common cancers. The fluorine-substituted uracil analogue is converted to several active metabolites but the mechanism of cytotoxicity has remained unclear. In a widely cited but unsubstantiated model, FU is thought to kill cells via the inhibition of thymidylate synthase and increased use of dUTP in place of TTP during DNA replication, with subsequent excision of high levels of uracil causing the fragmentation of newly synthesized DNA. Using gene-targeted cell lines defective in one or both of the two mammalian uracil-DNA glycosylase repair enzymes, we were able to test this model of FU cytotoxicity. Here, we show that incorporation of FU itself into DNA has been previously underestimated and is a predominant cause of cytotoxicity. FU readily becomes incorporated into the DNA of drug-treated cells, and accumulation of FU in the genome, rather than uracil excision, is correlated with FU cytotoxicity in mammalian cells. Furthermore, the Smug1, but not the Ung, uracil-DNA glycosylase excises FU from DNA and protects against cell killing. The data provides a clearer understanding of the action of FU, suggesting predictive biomarkers of drug response and a mechanism for acquired resistance in tumors.     

Uracil misincorporation in human DNA detected using single cell gel electrophoresis

Poor folate status may be important in the aetiology of several epithelial cell malignancies including cancer of the uterine cervix. Folic acid is essential in the synthesis of purine nucleotides and the pyrimidine nucleoside thymidine and it is probable that imbalances in these DNA precursors negatively effect DNA stability and may ultimately lead to malignant transformation. The development of a modified 'comet assay' using the bacterial DNA repair enzyme uracil DNA glycosylase, to detect misincorporated uracil in human DNA is reported here. The effect of perturbing folic acid and deoxyuridine levels on uracil misincorporation in normal human lymphocytes and cultured human tumour cells was investigated using this assay. HeLa cells and peripheral human lymphocytes incubated as agarose-embedded nucleoids, with 1 unit of uracil DNA glycosylase per microg of DNA, contained low levels of uracil in their DNA. Both HeLa cells and stimulated human lymphocytes cultured in folate-deficient medium were growth arrested. Incubating human lymphocytes in folate-deficient medium significantly increased the level of uracil detected compared with control cells. HeLa cells showed an increase in non-specific DNA damage (strand breaks). Deoxyuridine (100 microM) significantly increased the level of uracil detected in the DNA of both folate-deficient and control HeLa cells. It appears that this modified comet assay specifically detects misincorporated uracil in single human cells. It should, therefore, prove valuable in determining the role of folic acid status in DNA instability and cancer.      

Immunocytochemical localization of the base excision repair enzyme uracil DNA glycosylase in quiescent and proliferating normal human cells

The immunocytochemical localization of the base excision repair enzyme uracil DNA glycosylase was examined as a function of cell proliferation. Two nontransformed normal human fibroblast cell strains were analyzed using an anti-human uracil DNA glycosylase monoclonal antibody. In quiescent cells, basal levels of a nonnuclear immunocytochemically reactive glycosylase protein were detected. No nuclear immunofluorescence was observed. In contrast, in proliferating cells, intense immunofluorescence could be detected exclusively in the nuclear or perinuclear regions. As proliferation diminished, basal levels of the nonnuclear immunocytochemically reactive glycosylase were again observed. The subcellular distribution of the glycosylase was examined in parallel by in vitro biochemical assay. In quiescent cells, glycosylase activity was observed in both the nuclear and membrane fractions. A small amount of enzyme activity could be detected in the soluble cytoplasmic fraction. Immunoblot analysis demonstrated a Mr 37,000 glycosylase protein in each subcellular fraction. During cell proliferation, there was an increase in glycosylase activity in each of the subcellular fractions. These results suggest a correlation between the proliferative state of normal human cells and the preferential nuclear or perinuclear localization of an immunocytochemically reactive glycosylase protein. Further, immunofluorescence of the nuclear enzyme may be dependent on defined conformational states of that nuclear glycosylase in the cell cycle.     

Presence and consequence of uracil in preneoplastic DNA from folate/methyl-deficient rats

Uracil can arise in DNA by misincorporation of dUTP into nascent DNA and/or by cytosine deamination in established DNA. Based on recent findings, both pathways appear to be promoted in the methyl-deficient model of hepatocarcinogenesis. A chronic increase in the ratio dUTP:dTTP with folate/methyl deficiency can result in a futile cycle of excision and reiterative uracil misincorporation leading to premutagenic apyrimidinic (AP) sites, DNA strand breaks, DNA fragmentation and apoptotic cell death. The progressive accumulation of unmethylated cytosines with chronic methyl deficiency will increase the potential for cytosine deamination to uracil and further stress uracil mismatch repair mechanisms. Uracil is removed by a highly specific uracil-DNA glycosylase (UDG) leaving an AP site that is subsequently repaired by sequential action of AP endonuclease, 5'-phosphodiesterase, a DNA polymerase and DNA ligase. Since the DNA polymerases cannot distinguish between dUTP and dTTP, an increase in dUTP:dTTP ratio will promote uracil misincorporation during both DNA replication and repair synthesis. The misincorporation of uracil for thymine (5-methyluracil) may constitute a genetically significant form of DNA hypomethylation distinct from cytosine hypomethylation. In the present study a significant increase in the level of uracil in liver DNA as early as 3 weeks after initiation of folate/methyl deficiency was accompanied by parallel increases in DNA strand breaks, AP sites and increased levels of AP endonuclease mRNA. In addition, uracil was also detected within the p53 gene sequence using UDG PCR techniques. Increased levels of uracil in DNA implies that the capacity for uracil base excision repair is exceeded with chronic folate/methyl deficiency. It is possible that enzyme-induced extrahelical bases, AP sites and DNA strand breaks interact to negatively affect the stability of the DNA helix and stress the structural limits of permissible uracil base excision repair activity. Thus substitution of uracil for thymine induces repair- related premutagenic lesions and a novel form of DNA hypomethylation that may relate to tumor promotion in the methyl-deficient model of hepatocarcinogenesis.      

Production of human pluripotent progenitor cell colony stimulating activity (CFU-GEMMCSA) in patients with myelodysplastic syndromes

The present study was aimed at assessing the possible relationship between the T-lymphocyte abnormalities and the stem cell dysfunction in myelodysplastic syndromes (MDS), by investigating the production of specific stimulators of stem cell differentiation in such patients. Conditioned media from peripheral blood mononuclear cells (PBMNC) of MDS patients and healthy controls, prepared with or without phytohaemagglutinin (PHA), were assayed for their capacity to stimulate the in-vitro formation of multilineage colonies (CFU-GEMM) in target marrow cell cultures of healthy donors. Both PHA-induced DNA synthesis and T-cell subpopulation ratios (T4/T8) in patient cells were significantly lower than in controls. However, no impaired production of pluripotent progenitor cell colony stimulating activity (CFU-GEMMCSA) by PHA-stimulated and unstimulated PBMNC, could be found. Normal levels of activity were also produced by isolated T lymphocytes of MDS patients. Autologous serum neither enhanced nor suppressed the production of CFU-GEMMCSA. Our observations demonstrate that PHA-induced production of CFU-GEMMCSA is not directly correlated with DNA synthesis. Furthermore, we have found that both major T-cell subsets, defined by OKT4 and OKT8 monoclonal antibodies, are equally capable of producing CFU-GEMMCSA. The normal production of CFU-GEMMCSA by T cells of MDS patients suggests that this T-cell function is not an etiological factor in the stem cell disorder of myelodysplastic syndromes.     

The clinical, quality of life, and economic consequences of chronic anemia and transfusion support in patients with myelodysplastic syndromes

Most patients with myelodysplastic syndromes (MDS) require transfusions due to chronic anemia. Apart from the acute risks associated with transfusions, chronic anemia and red blood cell (RBC) transfusion dependence impact negatively on survival and quality of life (QoL), and are associated with iron overload, potentially leading to organ damage. QoL studies demonstrate that regular transfusions do not correct the impact of chronic anemia. Furthermore, chronic transfusion support requires substantial resources. Therefore, major goals are to prevent or effectively treat anemia. Indeed, innovative drugs have been shown to be effective in achieving transfusion independence by altering the natural course of MDS.     

Enzymology of the repair of etheno adducts in mammalian cells and in Escherichia coli

Exocyclic adducts are generated in cellular DNA by reaction with epoxides that are formed metabolically from various industrial pollutants and by reaction with activated aldehydes that arise during membrane lipid peroxidation. The etheno (epsilon) derivatives of purine and pyrimidine bases, e.g. 3,N4-ethenocytosine, 1,N6-ethenoadenine, N2,3-ethenoguanine and 1,N2-ethenoguanine, are probably involved in carcinogenesis because they are highly mutagenic and genotoxic. Therefore, the repair processes that eliminate exocyclic adducts from DNA should play a crucial role in maintaining the stability of the genetic information. The DNA glycosylases implicated in the repair of etheno adducts have been identified. Human and Escherichia coli 3-methyladenine-DNA-glycosylases excise 1,N6-ethenoadenine residues. We have identified two homologous proteins present in human cells and E. coli that remove 3,N4-ethenocytosine residues by DNA glycosylase activity. The human enzyme is an activity of the mismatch-specific thymine-DNA glycosylase, while the bacterial enzyme is an activity of the double-stranded uracil-DNA glycosylase, i.e., the homologue of the human enzyme. The fact that 1,N6-ethenoadenine and 3,N4-ethenocytosine are recognized and efficiently excised by DNA glycosylases in vitro suggests that these enzymes may be responsible for the repair of these mutagenic lesions in vivo and may contribute importantly to genetic stability.     

Perturbations of enzymic uracil excision due to guanine modifications in DNA

Phage PBS2 DNA, which contains uracil in place of thymine, was used as substrate for purified Bacillus subtilis uracil:DNA glycosylase. Incubation of this DNA with the ultimate carcinogen N-acetoxy-N-2-acetylaminofluorene resulted in the production of N-(deoxyguanosin-8-yl)acetylaminofluorene. A decreased Vmax resulted from the reaction of the glycosylase with this arylamidated substrate. Addition of a 2-fold excess of control PBS2 DNA following initiation of the reaction with the modified substrate showed delayed dissociation of the enzyme from the arylamidated DNA. This shows that the presence of a carcinogen-modified DNA base can reduce the capacity for uracil excision. Therefore, interference with enzymic release of uracil from DNA may be an indirect mechanism of mutagenesis by carcinogen:DNA adducts.     

Induction of the DNA repair enzymes uracil DNA glycosylase and 3-methyladenine DNA glycosylase in regenerating rat liver

The capacity of eukaryotic cells to modulate the activitiesof DNA repair enzymes during cell proliferation was examined.Using regenerating rat liver as a model system, the specificactivities of the DNA repair enzymes uracil DNA glycosylaseand 3-methyladenine DNA glycosylase were determined at specificintervals after partial hepatectomy. The induction of DNA replicationand the stimulation of DNA polymerase were also measured inorder to relate changes in the potential for DNA repair to thoseobserved for DNA replication. As measured in nuclear extracts,the specific activities of both the uracil DNA glycosylase andthe 3-methyladenine DNA glycosylase were increased in regeneratingrat liver reaching maximal levels 18–24 h after partialhepatectomy. The specific activity of each DNA repair enzymereturned to basal levels by 48 h after the hepatectomy. No increasein either enzyme activity was observed in sham operated controls.The products of the reactions were identified as 3-methyladenineor as uracil by high pressure liquid chromatography or by gelfiltration on Sephadex G-10. The 2–3 fold increases inthe specific activity observed for each nuclear DNA repair enzymewas comparable to the 2.7 fold increase observed for DNA polymeraseactivity. The stimulation of DNA repair enzymes in regeneratingrat liver is a further suggestion that eukaryotic cells activelyregulate excision repair pathways in the defined pattern ofgene expression observed during the eukaryotic cell cycle.