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Biomedical subjects

S Linn

Publications and source records attributed to S Linn.

At least 163 records · Page 9Linked to original sources
PubMed1982

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.

Bacteriophages↗
PubMed1982

On the recognition and cleavage mechanism of Escherichia coli endodeoxyribonuclease V, a possible DNA repair enzyme.

Escherichia coli endodeoxyribonuclease V acts at many sites of damage in duplex DNA, including apurinic/apyrimidinic sites, lesions induced by ultraviolet light which are not pyrimidine dimers, adducts of 7-bromomethylbenz[a]anthracene, and, as demonstrated earlier (Gates, F. T., and Linn, S. (1977a) J. Biol. Chem. 252. 1647-1653), it degrades uracil-containing duplex DNA most efficiently. The cleavage rate increases with increasing substitution of uracil for thymine in T5 DNA, with a replacement of one-eight of thymine generating the apparent maximum cleavage rate. However, the apparent reaction limit with DNA containing 3.8% of thymine replaced by uracil corresponds to cleavage at only 6% of the dUMP residues. Evidently, the enzyme recognizes some peculiarities of abnormal DNA structure, but not simply distortions, since some lesions, including pyrimidine dimers, are not substrates. Endonuclease V generates double strand breaks in a constant ratio to single strand nicks, regardless of the substrate. It degrades DNA processively, completing the digestion of one substrate molecule before proceeding to the next. The enzyme also appears to act cooperatively. Cleavage at methylbenz[a]anthracene adducts is usually or always 5' to the lesion. Endonuclease V seems well suited to act as a DNA repair enzyme, surveying the genome for structural distortions generated by lesions for which specific repair systems might not exist.

DNA Repair↗
PubMed1982

A unified mechanism for the nuclease and unwinding activities of the recBC enzyme of Escherichia coli.

Using a gentle method to prepare complexes of duplex DNA and the recBC enzyme for electron microscopy, structures not seen previously were observed to be associated with the double strand DNA exonuclease activity of the enzyme. These were terminal forms and loop + tail(s) structures. Both individual terminal single-stranded tails and single-stranded regions within duplexes were also observed. Observation of the terminal single-stranded structures present after various reaction conditions and after various potentially disruptive treatments helped to identify those structures which might represent true reaction intermediates. Based on these results and those of previous studies, a modified model for the mechanism of recBC enzyme action on double-stranded, linear DNA is proposed.

DNA, Single-Stranded↗
PubMed1982

Changes in DNA polymerases alpha, beta, and gamma during the replicative life span of cultured human fibroblasts.

DNA polymerases from IMR-90 human diploid fibroblasts at various passage levels and from HeLa cells were purified and fractionated into alpha 1, alpha 2, alpha 3, beta, and gamma species and subspecies, and than the accuracy with which each one copied synthetic template-primers was measured in the presence of Mn2+ or Mg2+. All activities from fibroblasts of later population doubling levels incorporated noncomplementary triphosphates more frequently than did the same polymerase type from earlier population doubling levels. HeLa polymerase activities copied several different templates in the presence of Mn2+ with greater fidelity than enzymes from fibroblasts of population doubling level 27 or greater. The total DNA polymerase activity extracted from IMR-90 cells decreased with increasing population doubling levels. The alpha-polymerase activity generally declined with increasing population doubling levels, while beta-polymerase activity remained relatively constant, except at the very end of the cellular replicative life span. In addition, the amounts of alpha 2 and alpha 3 became progressively lower relative to alpha 1, and a new alpha-type polymerase activity, alpha 0, appeared upon diethylaminoethylcellulose chromatography. HeLa cells also contained three alpha species, though two of them eluted from diethylaminoethylcellulose at higher phosphate concentrations than alpha species from fibroblasts. Postconfluent IMR-90 cells of population doubling level 21 had a decreased level of alpha-polymerase relative to that recovered from rapidly growing cells. This polymerase activity had some chromatographic properties similar to enzyme from late-passage cells. In addition, the alpha-, beta-, and gamma-polymerases from these cells had decreased fidelities relative to those isolated from subconfluent cells.

Cell Division↗
PubMed1982

Delay in conception for former 'pill' users.

Interview data from 3,214 married women having planned pregnancies and singleton deliveries who had no history of fertility treatment and who conceived more than two weeks after cessation of contraception were analyzed. The interval from cessation of contraception to conception was 13 months or greater for 24.8% of prior "pill" users vs 10.6% for former users of all other methods. As compared with former users of other methods, pill users had a notably lower monthly percentage of conceptions for the first three months and somewhat lower percentage from four to ten months. Results were not altered when potential confounding variables were controlled by logistic regression. At least 15 months of unsuccessful trials might be a more appropriate working definition of infertility for previous pill users rather than the 12-month interval generally accepted for this purpose.

Adult↗
PubMed1982

No association between coffee consumption and adverse outcomes of pregnancy.

We analyzed interview and medical-record data of 12,205 non-diabetic, non-asthmatic women to evaluate the relation between coffee consumption and adverse outcomes of pregnancy. Low birth weight and short gestation occurred more often among offspring of women who drank four or more cups of coffee a day and more often among the offspring of smokers. After controlling for smoking, other habits, demographic characteristics, and medical history by standardization and logistic regression, we found no relation between low birth weight or short gestation and heavy coffee consumption. Furthermore, there was no excess of malformations among coffee drinkers. These negative results suggest that coffee consumption has a minimal effect, if any, on the outcome of pregnancy.

Adult↗
PubMed1982

Isolation of an altered form of DNA polymerase I from Escherichia coli cells induced for recA/lexA functions.

A novel form of DNA polymerase I (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, DNA nucleotidyltransferase, EC 2.7.7.7) activity has been isolated from Escherichia coli cells that had been activated for expression of the DNA damage-inducible genes. Induction was by treatment of normal cells or cells carrying the spr-51 and tif-1 mutations with nalidixic acid. This activity, DNA polymerase I, seems to be a form of DNA polymerase I because it is insensitive to N-ethylmaleimide, is inhibited by antibody to DNA polymerase I, and does not appear in a polA1 strain. DNA polymerase I activity sediments through sucrose gradients as a broad peak with s20.w = 6.6--10.5, compared with an s20,w = 4.8--5.5 for DNA polymerase I. The fidelity during polymerization reactions of DNA polymerase I is relatively low with a variety of synthetic templates and deoxynucleoside triphosphates, although the enzyme appears to have a normal level of 3' greater than 5' exonuclease. Polymerase I has properties that might implicate it in some form of mutagenic DNA repair.

DNA Polymerase I↗
PubMed1981

Selective inhibition by harmane of the apurinic apyrimidinic endonuclease activity of phage T4-induced UV endonuclease.

1-Methyl-9H-pyrido-[3,4-b]indole (harmane) inhibits the apurinic/apyrimidinic (AP) endonuclease activity of the UV endonuclease induced by phage T4, whereas it stimulates the pyrimidine dimer-DNA glycosylase activity of that enzyme. E. coli endonuclease IV, E. coli endonuclease VI (the AP endonuclease activity associated with E. coli exonuclease III), and E. coli uracil-DNA glycosylase were not inhibited by harmane. Human fibroblast AP endonucleases I and II also were only slightly inhibited. Therefore, harmane is neither a general inhibitor of AP endonucleases, nor a general inhibitor of Class I AP endonucleases which incise DNA on the 3'-side of AP sites. However, E. coli endonuclease III and its associated dihydroxythymine-DNA glycosylase activity were both inhibited by harmane. This observation suggests that harmane may inhibit only AP endonucleases which have associated glycosylase activities.

Alkaloids↗
PubMed1981

Purification and characterization of an apurinic/apyrimidinic endonuclease from HeLa cells.

An endodeoxyribonuclease from HeLa cells acting on apurinic/apyrimidinic (AP) sites has been purified to apparent homogeneity as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The presence of Triton X-100 was necessary throughout the purification for stabilization and stimulation of activity. The endonuclease has an apparent native molecular weight of 32,000 determined by molecular sieving and an apparent subunit molecular weight of 41,000 as judged by its electrophoretic mobility in SDS-polyacrylamide gels. The activity has an absolute requirement for Mg2+ or Mn2+ and a broad pH optimum between 6.7 and 9.0 with maximal activity near pH 7.5. The enzyme has no detectable exonuclease activity, nor any endonuclease activity on untreated duplex or single-stranded DNA. It is inhibited by adenine, hypoxanthine, adenosine, AMP, ADP-ribose, and NAD+, but it is unaffected by caffeine, the pyrimidine bases, ADP, ATP, or NADH. The use of a variety of damaged DNA substrates provided no indication that the enzyme acts on other than AP sites. The enzyme appears to cleave AP DNA so as to leave deoxyribose-5-phosphate at the 5' terminus and a 3'-OH at the 3' terminus; it also removes deoxyribose-5-phosphate from AP DNA which has deoxyribose at the 3' terminus. Specific antibody has been produced in rabbits which interacts only with a 41,000-dalton protein present in the purified enzyme (presumably the enzyme itself), as well as with partially purified AP endonuclease fractions from human placenta and fibroblasts.

Animals↗
PubMed1981

Efficiency of formation of pyrimidine dimers in SV40 chromatin in vitro.

The efficiency of formation of pyrimidine dimers by 254-nm light was studied in mixtures of SV40 chromatin and DNA extracted from that chromatin. At high doses (beyond 380 J/m2), fewer dimers are formed in chromatin than in DNA for a given dose of radiation. This difference is about 10% as saturation with pyrimidine dimers is approached at 6840 J/m2. Conversely, at biologically repairable doses (up to 40 J/m2, less than 2 dimers/genome), significantly more dimers are produced in the chromatin than in the DNA. A maximum increase of about 50% occurs at doses producing 0.5--20 dimers/genome. With isolated nucleosomes from this chromatin, a maximum increase in dimer formation of 77% was observed. Therefore, the increased dimer formation in the whole chromatin can be wholly accounted for in the nucleosome portion.

Animals↗
PubMed1980

DNA N-glycosylases and UV repair.

Repair of some DNA photoproducts can be mediated by glycosylic bond hydrolysis. Thus, Escherichia coli endonuclease III releases 5,6-hydrated thymines as free bases, while T4 UV endonuclease releases one of two glycosylic bonds holding pyrimidine dimers in DNA. In contrast, uninfected E. coli apparently does not excise pyrimidine dimers via a DNA glycosylase.

DNA↗