Search PubMed⌕ Search

Biomedical subjects

L D Williams

Publications and source records attributed to L D Williams.

71 records · Page 4Linked to original sources
PubMed1987

Protonated base pairs explain the ambiguous pairing properties of O6-methylguanine.

The base-pairing interactions of promutagenic O6-methylguanine (O6-MeGua) with cytosine and thymine in deuterated chloroform were investigated by 1H NMR spectroscopy. Nucleosides were derivatized at hydroxyl positions with triisopropylsilyl groups to obtain solubility in nonaqueous solvents and to prevent the ribose hydroxyls from forming hydrogen bonds. We were able to observe hydrogen-bonding interactions between nucleic acid bases in a solvent of low dielectric constant, a condition that approximates the hydrophobic interior of the DNA helix. O6-MeGua was observed to form a hydrogen-bonded mispair with thymine. Whereas O6-MeGua did not form hydrogen bonds with cytosine (via usual, wobble, or unusual tautomeric structures), it did form a 1:1 hydrogen-bonded complex with protonated cytosine. The pairing of unprotonated cytosine in chloroform is thus consistent with the known preference of O6-MeGua for thymine over cytosine in polymerase reactions. In contrast, the pairing of protonated cytosine is consistent with the greater stability of oligonucleotide duplexes containing cytosine.O6-MeGua as compared with thymine.O6-MeGua base pairs [Gaffney, B. L., Markey, L. A. & Jones, R. A. (1984) Biochemistry 23, 5686-5691]. Our observation that cytosine must be protonated in order to pair with O6-MeGua suggests that the cytosine.O6-MeGua base pair in DNA is stabilized by protonation of cytosine. Through this mechanism, methylation at the O6 position of guanine in double-stranded DNA could promote cross-strand deamination of cytosine (or 5-methylcytosine) to produce uracil (or thymine).

Base Composition↗
PubMed1986

Changes in selected cardiorespiratory responses to exercise and in body composition following a 12-week aerobic dance programme.

Cardiorespiratory and body composition changes were evaluated in 25 sedentary females, aged 18 to 30 years, following 12 weeks of aerobic dance training (3 days a week, 45 min a session). Fifteen subjects, from the same population, comprised a control group: they maintained their normal activity and dietary habits over the course of the study. Analysis of variance of the values for selected cardiorespiratory responses revealed that the aerobic dance programme produced training effects in the experimental group. These training effects were indicated by significant improvements in O2 pulse, VE, heart rate and perceived exertion during submaximal exercise. Significant improvements were also noted in VO2 max, maximal O2 pulse, VE max, maximal heart rate and maximal running time on the treadmill. Additionally, increases in lean body mass and body density, together with decreases in percentage body fat and the sum of four skinfold thicknesses were found to be significant for the experimental group. No significant improvements in any of these variables were found for the control group. It was concluded that this 12-week aerobic dance programme was successful in promoting beneficial changes in cardiorespiratory fitness and body composition.

Adolescent↗
PubMed1986

Ultrastructure of ventral membranes of rat hepatocytes spread on type IV collagen.

Rat hepatocytes obtained by means of liver perfusion with collagenase were allowed to spread on type IV collagen coated coverslips for 20 h. Interference reflection microscopy revealed a peripheral ring of dark spots. Carbon replicas of ventral membranes left attached to coverslips after lysis and squirting provided high resolution information on the ultrastructure of the protoplasmic surface. Correlative light and electron microscopy of the same ventral membrane showed that the area of the peripheral 'adhesion annulus' was rich in clathrin-coated structures (sheets, pits and vesicles). In vertical thin sections of hepatocyte monolayers numerous small smooth vesicles were observed piled up below the peripheral portion of the cell. These findings suggest high cytotic activity at the cell periphery during spreading. No bundles of microfilaments were observed in cells after squirting or in sections, but a ring of filaments at the cell periphery could be seen in many cells in whole mount preparations after treatment with Triton X-100. The absence of microfilaments associated with the points of adhesion indicates a cytoskeleton independent adhesion mechanism in hepatocytes during the first 20 h of spreading.

Animals↗
PubMed1984

The organization of frog virus 3 as revealed by freeze-etching.

A variety of freeze-fracture techniques has been employed in this study with the aim of dissecting the frog virus 3 virion and obtaining further information about its architecture. The icosahedral capsid has a skew symmetry with a triangulation number of 133 or 147. The capsomers are closely packed with a center-to-center spacing of 72 A. The inner membrane contains transmembrane proteins which appear as intra-membranous particles on both fracture faces. Rod-like structures (about 100 A in diameter) are present in the virus interior suggesting that the DNA-protein complex is highly organized.

Animals↗
PubMed1982

Ultrastructure of a hyaluronic acid matrix.

Freeze-etch replicas of a hylauronic acid matrix were visualized by electron microscopy. In water a coarse branching fibrillar network of hyaluronic acid aggregates was seen. The high solvent permeability of this matrix suggests that the spaces observed are relatively devoid of unaggregated polymer. Addition of calcium disordered the matrix, resulting in a more dispersed felt of polymer.

Extracellular Space↗
PubMed1981

Ultrastructure of sheep erythrocyte plasma membranes and cytoskeletons bound to solid supports.

Sheep erythrocyte plasma membrane monolayers were formed on positively charged supports by means of controlled lysis and squirting of cells so that the original protoplasmic or inner surfaces (PS) were exposed. The appearance of the surface was studied by transmission electron microscopy of platinum/carbon replicas following freeze-drying of the membranes. After gentle washing with water or dilute buffer, a network of filaments and particles was found to cover the surface. Whole cells bound to positively charged supports were treated with Triton X-100 and hypertonic KCl, which left the protein components of the cytoskeleton on the support. Stereo electron microscopy of Pt/C replicas of these residues showed complex networks of filaments, similar to those seen on the cytoplasmic surfaces of the intact membranes. In both cases the lengths of the filaments correspond to that of the spectrin dimer. Removal of spectrin, actin and other proteins by alkaline treatment led to loss of this network and revealed 15.4-nm particles on the membranes PS. These particles, which were also visible after negative staining, could be removed by treatment with trypsin and were found to correspond to band 4 protein (the equivalent of human erythrocyte band 3 protein). Membranes freeze-fractured following treatment with alkali showed normal intramembranous particles with frequencies similar to those of native membranes. This indicates that band 4 protein spans the sheep erythrocyte membrane and forms a very high proportion of the intramembranous particles. The protoplasmic portions of these particles may be membrane binding sites for the cytoskeleton in sheep erythrocytes.

Animals↗
PubMed1980

Freeze-fracture autoradiography of the red blood cell plasma membrane.

A procedure for electron microscope autoradiography of red blood cell ghosts or their outer membrane leaflets is described: sheep erythrocytes, attached to positively charged mica, were either lysed and air-dried, or freeze-fractured "by hand" and freeze-dried. This was followed by shadowing and carbon replication; coating with photographic emulsion; exposure in darkness at 277 K; developing and fixation; carbon coating; floating off the mica and mounting on grids. Erythrocytes iodinated with 125I using the lactoperoxidase reaction were used as a test object. The efficiency of the procedure was found to be between 3.4% and 5.6%.

Animals↗
PubMed1978

Negative staining of freeze-fractured envelopes of Escherichia coli K12.

Envelope fragments of E. coli K12 have been produced by freeze-fracturing "by hand" and negatively stained after thawing. The outer leaflet of the plasma membrane disintegrated upon thawing whereas the outer leaflet of the outer membrane did not. Negative staining revealed the following structural features on the outer membrane fragments: (i) "grooves" 4-6 nm wide, (ii) spherical particles 6-8 nm in diameter, (iii) "black dots" 3-8 nm in diameter. Treatment of cells with EDTA before freeze-fracturing caused dilation of grooves into holes eventually leading to fragmentation of the outer membrane. A mutant strain deficient in two outer membrane proteins fractured exclusively through the outer membrane. The outer leaflets so obtained disintegrated upon thawing similarly as observed for the outer leaflet of the plasma membrane.

Cell Membrane↗
PubMed1977

Freeze-fracturing of monolayers (capillary layers) of cell, membranes and viruses: some technical considerations.

A novel hinged device for freeze-fracturing of cell monolayer in the Balzers freeze-etch unit is described. It is economical on biological material and enables oriented adsorption of sheet-like membrane fragments. For freeze-fracturing 'by hand' a monolayer is formed on a positively charged piecie of mica (with polylysine) and this is covered with another piece of mica, thin brass plate of filter paper. Such a sandwich is frozen in liquid nitrogen and fractured by means of forceps. Several modifications of this technique as well as practical examples are described. Among possible application are: negative staining of intramembranous protein particles; chemical or physical analyses of single membrane leaflets; identification of protein complexes by immunoelectron microscopy, etc.

Adenoviridae↗