PubMed Health⌕ Search

Biomedical subjects

F Johnson

Publications and source records attributed to F Johnson.

At least 91 records · Page 5Linked to original sources
PubMed1995

Influence of the oxidatively damaged adduct 8-oxodeoxyguanosine on the conformation, energetics, and thermodynamic stability of a DNA duplex.

As part of an overall program to characterize the impact of mutagenic lesions on the physiochemical properties of DNA, we report here the results of a comparative spectroscopic and calorimetric study on a family of DNA duplexes both with and without the oxidative lesion 2'-deoxy-7-hydro-8-oxoguanosine (8-oxodG). Specifically, we have studied a family of eight 13-mer duplexes of the form [5'-GCGTAC[G* or G]CATGCG-3'].[3'-CGCATG[C, A, T, or G]GTACGC-5'] in which G* is the 8-oxodG lesion. These eight duplexes, which we designate by the identity of the variable central base pair (e.g., G*C), reflect two subsets: four duplexes in which the modified guanine base is positioned opposite each of the four possible canonical residues (G*C, G*A, G*G, G*T) and the corresponding four "control" duplexes in which the guanine is not modified (GC, GA, GG, GT). The data derived from our spectroscopic and calorimetric measurements on these eight duplexes allow us to evaluate the influence of the 8-oxodG lesion, as well as the base opposite the lesion, on the conformation, the thermal and thermodynamic stability, and the melting thermodynamics of the host DNA duplex. We find that modification of dG to 8-oxodG (G*) does not change the global DNA duplex conformation as judged by circular dichroism spectra. Despite this structural similarity, our data reveal that the dG to dG* modification does influence duplex thermal and thermodynamic properties, some of which depend on the base opposite the lesion. Thus, apparent structural identity does not mean that two duplexes necessarily will exhibit equivalent thermal and/or thermodynamic properties. In general, we find that the thermodynamic effects induced by the lesion (e.g., GC vs G*C) or by mismatched base pairs (e.g., GC vs GG) can result in relatively large changes in enthalpy which are partially or wholly compensated entropically to produce relatively modest changes in free energy. Our data also suggest that the biologically observed differential recognition of 8-oxodG duplexes and the preferential nucleotide insertion opposite 8-oxodG residues cannot be rationalized simply in terms of large thermodynamic differences.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1995

Azido-iodo-phenyl-analogs of 2',5'-dideoxy-adenosine as photoaffinity ligands for adenylyl cyclase.

Azidoiodophenyl-analogs of 2',5'-dideoxyadenosine were synthesized and tested as potential 'P'-site selective affinity probes for adenylyl cyclases. The 3'-substituted analogs included: 1: 3'-[(4-nitrophenyl)-acetyl]-2',5'-dideoxy-adenosine 2: 3'-[(4-nitrophenyl)-butyryl]-2',5'-dideoxyadenosine 3: 3'-[(4-azido-3-iodophenyl)-acetyl]-2',5'-dideoxyadenosine and 4: 3'-[(4-azido-3-iodophenyl)-butyryl]-2',5'-dideoxyadenosine. The azidoiodo-phenyl-analogs inactivated adenylyl cyclase irreversibly and in a light-dependent manner. This was observed with detergent-dispersed enzyme from rat brain, purified native enzyme from bovine brain, and recombinant Type I bovine adenylyl cyclase expressed in membranes from fall army worm ovarian (Sf9) cells. Inactivation of the recombinant enzyme was inversely dependent on ATP concentration and was not completely prevented by 2',5'-dideoxyadenosine. Inhibition kinetics with the recombinant enzyme in the absence of light suggested two sites of inhibition, whereas with the native Type I enzyme inhibition kinetics exhibited a straightforward noncompetitive mechanism. Occupation of either or both sites by ligand protected the enzyme against denaturation by UV-irradiation per se. The data are consistent with inactivation of the recombinant enzyme occurring both through the 'P'-site and the catalytic active site, but suggest that this is a characteristic of the recombinant enzyme and is not dependent on the probes per se. The data suggest the potential for independent interactions of such ligands with different sites on a given enzyme and also with other enzymes containing adenosine or adenine nucleotide binding domains.

Adenosine Triphosphate↗
PubMed1995

Topographic organization of a forebrain pathway involved with vocal learning in zebra finches.

A serial pathway from a thalamic nucleus (DLM; the medial portion of the dorsolateral nucleus of the anterior thalamus) to a cortical region (lMAN; the lateral magnocellular nucleus of the anterior neostriatum) to a motor-cortical region (RA; the robust nucleus of the archistriatum) is necessary for vocal production during song learning in juvenile zebra finches but not for the recitation of a song already learned by adults. To obtain new information about the possible function of the DLM-->lMAN-->RA pathway in vocal learning, we used anterograde and retrograde tract-tracing techniques (pressure injections of DiI and DiA) to map the pattern of axonal connections between these brain regions in adult male zebra finches. Results revealed two topographically organized pathways that traverse the songbird forebrain in parallel. An oval-shaped dorsal/lateral portion of DLM projects solely to the central core of lMAN (lMANcore), whereas a crescent-shaped region, including ventral and medial DLM, projects exclusively to a parvicellular shell that encircles lMANcore (lMANshell). In turn, lMANshell neurons project solely to an arc-shaped region of dorsal archistriatum just lateral to RA (Ad; archistriatum, pars dorsalis), whereas lMANcore neurons project exclusively to RA. We also identified crossed and reciprocal pathways between lMANcore/shell and the lateral portion of the ventral archistriatum, which may contribute to interhemispheric coordination of vocal behavior. A robust topographic organization was observed in the axonal projections from dorsal/lateral-DLM-->lMANcore-->RA and from ventral/medial-DLM-->lMANshell-->Ad, raising the question of what is being mapped within these two forebrain pathways. Because RA projection neurons are organized myotopically with respect to the major vocal (syringeal) muscles (D.S. Vicario, 1991, J. Comp. Neurol. 309:486-494), one possibility is that a mapping of vocal/expiratory musculature is preserved "upstream" within these pathways. Similarly, the presence of song-selective auditory neurons in DLM, lMAN, and RA (A.J. Doupe and M. Konishi, 1991, Soc. Neurosci. Abstr. 18:527) suggests that these pathways might subserve some form of auditory or auditory-motor mapping.

Aging↗
PubMed1995

Design and synthesis of novel inhibitors of HIV-1 reverse transcriptase.

A variety of N1-substituted pyrimido[5,4-f]benzo[1,4]thiazepines, 5, designed as conformationally constrained analogs of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymidine HEPT (1), were synthesized and evaluated for their inhibition of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). The preparation of these compounds was carried out based on a Mannich-type cyclization of 6-[(2-aminophenyl)thio]uracils followed by alkylation at N1 by a one-pot Vorbruggen reaction. The pyrimidobenzothiazepines were developed to give molecules with IC50 values in the micromolar range, as exemplified by [[(2-ethoxyethyl)oxy]methyl]-pyrimido[5,4- f]benzo[1,4]thiazepine, 25, (IC50 = 0.64 microM), the most active compound of this series. The structural and electronic features of this novel class of HIV-1 RT inhibitors are presented and compared with those of HEPT (1), TIBO (2), and nevirapine (3).

Crystallography, X-Ray↗
PubMed1995

A 60-Hz magnetic field increases the incidence of squamous cell carcinomas in mice previously exposed to chemical carcinogens.

The exposure of tumor-bearing SENCAR mice to a magnetic field of 2 mT at a frequency of 60 Hz for 52 weeks, was found to increase the rate of malignant conversion in chemically-induced dorsal skin papillomas. Detailed histopathology revealed the presence of squamous cell papillomas and squamous cell carcinomas in both sham and magnetic field exposed mice at week 52. However, of the nine mice assessed as having squamous cell carcinomas, eight came from the group exposed to magnetic fields, a difference which is statistically significant at P = 0.03.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1995

Chromophore-modified antitumor anthracenediones: synthesis, DNA binding, and cytotoxic activity of 1,4-bis[(aminoalkyl)amino]benzo[g]-phthalazine-5,10-diones.

As part of a program aimed at exploring the effect of the introduction of heteroatoms into the anthracene-9,10-dione chromophore, we have synthesized novel 1,4-bis[(aminoalkyl)amino]-benzo[g]phthalazine-5,10-diones (BPDs) 1 which are related to the antitumor agents ametantrone and mitoxantrone. Derivatives 1 were prepared by chromic acid oxidation of acylated benzo[g]phthalazines 5 followed by acid hydrolysis or by silylation-amination of 5,10-dihydroxybenzo[g]phthalazine-1,4-dione (8). The 1-[(aminoalkyl)amino]-4-amino congeners 2 were isolated in low yields as byproducts from the oxidation of 5. Against a panel of human tumor cell lines, the benzo[g]phthalazine-5,10-diones 1 and 2 exhibited cytotoxic activity comparable or even superior to that of mitoxantrone. In compounds 1, structure-activity relationships different than those operative in the carbocyclic series appeared to emerge. DNA-binding studies with the ametantrone-like compound 1c and its single-armed congener 2c indicated that the introduction of a 2,3-diaza subunit into the anthracene-9,10-dione chromophore reduces the affinity of the drug for DNA in comparison with ametantrone. On the other hand, the number of side-chain groups does not affect binding to a great extent. These findings seem to suggest mechanisms of cell death other than those induced by simple interaction of the 1,4-BPDs 1 and 2 with DNA.

Cell Survival↗
PubMed1995

Differential estrogen accumulation among populations of projection neurons in the higher vocal center of male canaries.

The higher vocal center (HVC) of adult male canaries undergoes a seasonal change in volume that corresponds to seasonal modifications of vocal behavior: HVC is large when birds produce stereotyped song (spring) and is small when birds produce plastic song and add new song syllables into their vocal repertoires (fall). We reported previously that systemic exposure to testosterone (T) produces an increase in the volume of HVC similar to that observed with long-day photoperiods. T-induced growth of HVC occurred regardless of whether the borders of HVC were defined by Nissl-staining, the distribution of androgen-concentrating cells, or the distribution of projection neurons [separate neuronal populations within HVC project to the robust nucleus of the archistriatum (RA) and to Area X of the avian striatum (X)]. In the present study we used steroid autoradiography to determine whether T can influence the distribution of HVC cells that bind estrogen, and we combined estrogen autoradiography with retrograde labeling to determine whether HVC neurons that project to RA versus X differ in their ability to accumulate estrogen. Results showed that T increased the volume of Nissl-defined HVC and although HVC contained a low density of estrogen-concentrating cells, T increased the spatial distribution of these cells to match the Nissl borders of HVC. We also identified a region containing a high density of estrogen-concentrating cells located medial to HVC [we call this region paraHVC (pHVC)], and T also increased the volume of pHVC. pHVC also contained numerous X-projecting neurons, but few if any RA-projecting neurons. Double-labeling analysis revealed that RA-projecting neurons did not accumulate estrogen, a small percentage of X-projecting neurons in HVC accumulated estrogen, and the majority of X-projecting neurons in pHVC showed heavy accumulation of estrogen. The data reported here and in our previous article suggest distinct roles for gonadal steroids within the HVC-pHVC complex: estrogens are concentrated by neurons that project to a striatal region that influences vocal production during song learning (X), whereas androgens are concentrated primarily by neurons that project to a motor region that is involved in vocal production during both song learning and the recitation of already-learned song (RA).

Animals↗
PubMed1995

A randomized, prospective study of adjunctive ceftizoxime in preterm labor.

OBJECTIVE: Our purpose was to determine the effect of ceftizoxime in the prolongation of pregnancies receiving tocolysis for preterm labor. STUDY DESIGN: A prospective, randomized, double-blinded, placebo-controlled trial was conducted in patients with preterm labor between 24 and 35 weeks' gestation. A total of 545 patients with intact membranes and without chorioamnionitis who were receiving magnesium sulfate were screened. Exclusions were for advanced cervical dilatation, penicillin allergy, current antibiotic therapy, and failure to give informed consent. Of these, 117 consecutive, eligible, consenting patients were randomized to receive either 2 gm of ceftizoxime or a placebo every 8 hours. The primary end point was prolongation of gestation. Statistical comparisons were performed by use of unpaired two-tailed t tests and chi 2 analysis. RESULTS: Of the 58 ceftizoxime and 59 placebo patients, there was no difference in the interval to delivery (34.5 +/- 21.1 days vs 34.6 +/- 24.5 days, p = 0.99) and no difference in the rate of delivery before 37 weeks' gestation (60% in the ceftizoxime group vs 58% in the placebo group, p = 0.91). Subanalyses of 61 subjects who received nine or more doses and who were group B streptococci negative, 69 patients at < or = 32 weeks on presentation, and 22 sets of twins all showed no differences in delivery interval or rate of delivery at < 37 weeks. The sample size of this study was sufficient to detect a 9-day difference in prolongation of pregnancy (alpha = 0.05, beta = 0.2). CONCLUSIONS: Ceftizoxime had no effect on interval to delivery or duration of pregnancy in women treated for preterm labor.

Adolescent↗
PubMed1995

The preterm prediction study: significance of vaginal infections. National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network.

OBJECTIVE: Our purpose was to evaluate the association of bacterial vaginosis, trichomonas vaginitis, and monilial vaginitis with spontaneous preterm birth at < 35 weeks 0 days. STUDY DESIGN: A total of 2929 women at 10 centers were studied at 24 and 28 weeks' gestation by Gram stain of vaginal smear, wet mount, and 10% potassium hydroxide preparations to detect vaginal infections. RESULTS: The rates of detected infection at 24 and 28 weeks, respectively, were bacterial vaginosis 23.4% and 19.4%, trichomonas 3.3% and 2.7%, and monilia 21.1% and 19.5%. The occurrence of bacterial vaginosis at 28 weeks was associated with an increased risk of spontaneous preterm birth, odds ratio 1.84 (95% confidence interval 1.15 to 2.95, p < 0.01). Detection of Trichomonas vaginalis (by wet mount) or monilia (by potassium hydroxide preparation) had no significant associations with preterm birth. CONCLUSION: The presence of bacterial vaginosis at 28 weeks' gestation is associated with an increased risk of spontaneous preterm birth.

Adult↗
PubMed1995

Evaluation of a metaphase chromosome finder: potential application to chromosome-based radiation dosimetry.

An automated metaphase chromosome finder is described which combines a microscope, state-of-the-art computer technology and a simple decision-making algorithm. A microscope slide is systematically scanned under computer control and the location of each positive 'signal' placed into memory for later recall and review by a human operator. The software identifies two events, positives (the presence of a 'signal') and negatives (the absence of a 'signal'). The performance of the metaphase finder was evaluated using receiver operating characteristic curve analysis. At the optimum decision threshold, the detection rates for true positives (metaphase spreads) was about 74%, false positives (type I error) about 6%, and false negatives (type II error) about 26%. The overall accuracy, which accounts for differences in the sensitivity of the detector to positive and negative events, was 89.4% (+/- 0.01%; standard error of the mean, n = 8). Potential applications to radiation dosimetry are discussed.

Algorithms↗
PubMed1995

Kerley B lines in AIDS.

Explore the source record for details and available documents.

Acquired Immunodeficiency Syndrome↗
PubMed1994

Mutagenic potency of exocyclic DNA adducts: marked differences between Escherichia coli and simian kidney cells.

A single-stranded shuttle vector containing a single 3,N4-etheno-2'-deoxycytidine (epsilon dC) or 1,N2-(1,3-propano)-2'- deoxyguanosine (PdG) DNA adduct was used to investigate translesional DNA synthesis in Escherichia coli and simian kidney (COS) cells. The presence of either exocyclic adduct was associated with reduced numbers of transformants. In E. coli, this inhibitory effect could be overcome partially by irradiating cells with UV light before transformation. Translesional synthesis past both exocyclic lesions was accompanied by targeted mutations. For PdG, the primary mutagenic events observed in both hosts were PdG-->T transversions; in preirradiated E. coli, PdG-->A transitions were also observed. The targeted mutation frequency for single-stranded DNA that contained PdG was 100% in nonirradiated E. coli, 68% in preirradiated cells, and 8% in COS cells. In contrast, the targeted mutation frequency for single-stranded DNA that contained epsilon dC was 2% in nonirradiated E. coli, 32% in preirradiated cells, and 81% in COS cells. The primary mutations generated by epsilon dC in both E. coli and COS cells were epsilon dC-->A and epsilon dC-->T base substitutions. These observations appear to reflect the variable specificity of DNA replication complexes in incorporating bases opposite certain adducts. We conclude that DNA synthesis past the same DNA adduct can have strikingly different consequences in bacteria and mammalian cells, underscoring the importance of establishing the intrinsic mutagenic potential of DNA adducts in mammalian cells.

Animals↗
PubMed1994

Substrate specificity of Fpg protein. Recognition and cleavage of oxidatively damaged DNA.

The 8-oxoguanine-DNA glycosylase of Escherichia coli, also known as formamidopyrimidine-DNA glycosylase (Fpg protein), has N-glycosylase and AP-lyase activities. This enzyme repairs oxidative DNA damage by efficiently removing formamidopyrimidine lesions and 8-oxoguanine residues from DNA. Defined oligodeoxynucleotides containing various 8-oxopurines were used to examine the substrate specificity of Fpg protein and to establish the role of functional groups in DNA on damage recognition and catalysis. Binding affinities of Fpg protein were established for duplex oligodeoxynucleotides containing 8-oxo-2'-deoxyguanine, 8-oxo-2'-deoxyadenine, 8-oxo-2'-deoxynebularine, 8-oxo-2'-deoxyinosine, abasic sites, and a ring-open adduct of C8-aminofluorene guanine. The C8 keto group of 8-oxodG:dC presents in the major groove and is correlated with tight binding (Kd = 8.9 nM). Binding is much weaker when the C8 keto functional group is in the minor groove, as in 8-oxodG:dA (Kd = 340 nM). Km and Vmax were determined for the cleavage reaction. Specificity constants (Kcat/Km) are consistently higher for oligodeoxynucleotide duplexes containing 8-oxopurines with C6 and C8 keto groups, as in 8-oxodG:dC and 8-oxodI:dC, where Kcat/Km are 9.3 and 18 min-1 nM x 10(-3), respectively. 8-oxodN:dC lacks the C6 keto group; the specificity constant is 0.024 min-1 nM x 10(-3). Taken together, our data suggest that the C8 keto group of 8-oxodeoxyguanine and the carbonyl moiety of formamidopyrimidine enable Fpg protein to recognize and bind duplex DNA containing these modified bases. An enzyme-catalyzed reaction involving the C6 keto group of the substrate leads to removal of these lesions. A mechanism involving protonation at O-6 of 8-oxoguanine is proposed to account for the N-glycosylase activity of this enzyme.

Catalysis↗
PubMed1994

Homology models of two isozymes of manganese peroxidase: prediction of a Mn(II) binding site.

The three-dimensional structures of two isozymes of manganese peroxidase (MnP) have been predicted from homology modeling using lignin peroxidase as a template. Although highly homologous, MnP differs from LiP by the requirement of Mn(II) as an intermediate in its oxidation of substrates. The Mn(II) site is absent in LiP and unique to the MnP family of peroxidases. The model structures were used to identify the unique Mn(II) binding sites, to determine to what extent they were conserved in the two isozymes, and to provide insight into why this site is absent in LiP. For each isozyme of MnP, three candidate Mn(II) binding sites were identified. Energy optimizations of the three possible Mn(II) enzyme complexes allowed the selection of the most favorable Mn(II) binding site as one with the most anionic oxygen moieties best configured to act as ligands for the Mn(II). At the preferred site, the Mn(II) is coordinated to the carboxyl oxygens of Glu-35, Glu-39, and Asp-179, and a propionate group of the heme. The predicted Mn(II) binding site is conserved in both isozymes. Comparison between the residues at this site in MnP and the corresponding residues in LiP shows that two of the three anionic residues in MnP are replaced by neutral residues in LiP, explaining why LiP does not bind Mn(II).

Amino Acid Sequence↗
PubMed1994

Mechanism of electrical enhancement of efficacy of antibiotics in killing biofilm bacteria.

The bioelectric effect, in which electric fields are used to enhance the efficacy of biocides and antibiotics in killing biofilm bacteria, has been shown to reduce the very high concentrations of these antibacterial agents needed to kill biofilm bacteria to levels very close to those needed to kill planktonic (floating) bacteria of the same species. In this report, we show that biofilm bacteria are readily killed by an antibiotic on all areas of the active electrodes and on the surfaces of conductive elements that lie within the electric field but do not themselves function as electrodes. Considerations of electrode geometry indicate that very low (< 100 microA/cm2) current densities may be effective in this electrical enhancement of antibiotic efficacy against biofilm bacteria, and flow experiments indicate that this bioelectric effect does not appear to depend entirely on the possible local electrochemical generation of antibacterial molecules or ions. These data are expected to facilitate the use of the bioelectric effect in the prevention and treatment of device-related bacterial infections that are caused by bacteria that grow in biofilms and thereby frustrate antibiotic chemotherapy.

Anti-Bacterial Agents↗
PubMed1994

Synthetic and oxidative studies on 8-(arylamino)-2'-deoxyguanosine and -guanosine derivatives.

Facile aerial oxidation is a general feature of guanine ribo- and 2'-deoxyribonucleosides that are substituted at the 8-position by an aminoaryl group. In previous work, it had been suggested that two of the major oxidation products are a pair of diastereomers having a spiro structure. These were presumed to be related by a chiral difference at the spiro carbon atom. The pattern of the oxidative process involves a contraction of the pyrimidine ring. It was thought to be analogous to that suggested by other investigators for the oxidation of uric acid, but for which no really definitive evidence had been presented. We have been able now to isolate in a crystalline state one of the diastereomers produced by the aerial oxidation of 8-phenylaminoguanosine under alkaline conditions. Analysis by X-ray diffraction has now confirmed the type of spiro structure promulgated previously. These findings also imply that spiro compounds are likely to be produced during the aerial oxidation of any 8-arylaminoguanine nucleoside or 2'-deoxynucleoside. In addition, this work adds considerable weight to the results of Poje and Sokolic-Maravic who proposed that a spiro intermediate is produced during the aerial oxidation of uric acid (12,13). However, they found this compound to be unstable to base, in contrast to the arylaminoguanine oxidation products. In the course of the above work we showed that the 8-arylamino derivatives of guanosine can be converted by the Barton deoxygenation method to the corresponding 2'-deoxyribonucleosides. This makes available a number of the latter compounds, which are not easily prepared by other methods.

DNA Adducts↗