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D J Thomas

Publications and source records attributed to D J Thomas.

At least 91 records · Page 5Linked to original sources
PubMed1996

Benefits and risks of hormone/estrogen replacement.

Menopause is a natural event, yet almost 25% of postmenopausal women are taking some form of hormone replacement (HRT). Women can expect to live one-third of their lives postmenopausal. There is a plethora of conflicting literature about estrogen, and women faced with the decision whether or not to take HRT are confused. This article outlines the risks and benefits of hormone/estrogen replacement based on a comprehensive review of the current literature so that health care providers can give their patients the best information for making informed decisions about HRT.

Adult↗
PubMed1995

Time dependence of accumulation and binding of inorganic and organic arsenic species in rabbit erythrocytes.

The uptake by rabbit erythrocytes of 0.4 mM arsenate, As(V), monomethylarsinate, MMA(V) and dimethylarsonate, DMA(V) were compared over 24 h. In membrane-free hemolysate, the distribution of As between proteins (10 kDa) and ultrafiltrate was determined by ultrafiltration and arsenic species in the ultrafiltrate were identified by thin layer chromatography methods. 1H spin-echo Fourier transform NMR was used to follow the binding of these arsenic species to glutathione (GSH). 31P-NMR was used to observe their effects on high-energy adenine nucleotide levels (ATP, ADP). These results demonstrate that As(III) readily accumulates in cells, reaches a quasi-plateau at 78% of the total As in the incubation after 1 h and 88% of the total As after 24 h. On average, 20% of the total erythrocyte As(III) burden is associated with the protein fraction, particularly with hemoglobin (Hb). About 68% of the erythrocyte As(III) burden is bound to GSH. As(III) has no effect on ATP levels during a 5-h incubation. By comparison, As(V) enters erythrocytes more slowly (53% of the total As after 5 h). Erythrocytes take up 81% of the As(V) in the reaction system after a 24 h incubation. Of the total As burden in As(V)-exposed erythrocytes, 22% was associated with the proteins (10 kDa) and possibly reduced to As(III) and 59% was in the ultrafiltrate (8% as As(III) and 51% as As(V)). This finding indicates that, over a 24 h incubation period, the reduction of As(V) to As(III) may account for 30% of the total As in rabbit erythrocytes. As(V) present in the erythrocytes enters the phosphate pool and depletes ATP. In comparison, about 65% of the total MMA(V) or about 44% of the total DMA(V) in the incubation system is taken up by rabbit erythrocytes during a 24 h incubation. Neither organoAs species perturbed the Hb signals observed by spin-echo Fourier transform NMR and the binding to GSH was minimal. Unlike As(V), MMA(V) and DMA(V) do not perturb phosphate metabolism, showing that, despite their pentavalent oxidation state, these arsenic species are not analogs for phosphate.

Adenosine Diphosphate↗
PubMed1995

Factors affecting flow cytometric detection of apoptotic nuclei by DNA analysis.

Apoptotic thymocyte nuclei normally appear on a flow cytometric DNA histogram as a subdiploid peak. We observed that addition of a specific RNase A preparation to the detergent-based lysing buffer increased the fluorescence of toxicant-induced apoptotic nuclei to the level of untreated diploid nuclei. The chelating agent EDTA partially inhibited the RNase effect, suggesting contaminating divalent cations may have been involved. Moreover, spectrofluorometric analysis revealed that addition of RNase or divalent cations decreased the amount of DNA present in the lysate. This suggested that the upscale fluorescence shift was due to a decrease in the ability of the lysing buffer to extract DNA, possibly as a result of cation-induced chromatin condensation, rather than increased accessibility of fluorochrome binding sites due to apoptotic degeneration. Moreover, during a 16-h culture, we observed a similar, but time-dependent, upscale shift in the fluorescence of thymocytes undergoing apoptosis either spontaneously or as a result of exposure to 1 microM tributyltin methoxide (TBT), 2% ethanol, 2% methanol, or 1 microM dexamethasone phosphate (DEX). This commonality of effect suggests that a similar magnitude of chromatin reorganization occurs in apoptotic cells in prolonged culture regardless of the method of apoptotic induction. These findings should alert investigators to potential inaccuracies in the flow cytometric quantitation of apoptosis in in vitro systems employing prolonged toxicant exposures or complex lysing cocktails that may contain active contaminants.

Animals↗
PubMed1995

NFATc3, a lymphoid-specific NFATc family member that is calcium-regulated and exhibits distinct DNA binding specificity.

Signals transduced by the T cell antigen receptor (TCR) regulate developmental transitions in the thymus and also mediate the immunologic activation of mature, peripheral T cells. In both cases TCR stimulation leads to the assembly of the NFAT transcription complex as a result of the calcium-dependent nuclear translocation of cytosolic subunits, NFATc, and the Ras/protein kinase C-dependent induction of a nuclear subunit, NFATn. To further understand the diverse roles of antigen receptor signaling throughout T cell development, we have identified a new NFATc family member, NFATc3, that is expressed at highest levels in the thymus. NFATc3 is the product of a gene on murine chromosome 8 that is not linked to the other NFATc genes. NFATc3, like other NFATc family members, contains a conserved rel similarity domain, and also defines a region conserved among NFATc family members, the SP repeat region, characterized by the repeated motif SPxxSPxxSPrxsxx (D/E)(D/E)swl. NFATc3 activates NFAT site-dependent transcription when overexpressed, yet exhibits a pattern of DNA site specificity distinct from other NFATc proteins. Additionally, thymic NFATc3 undergoes modifications in response to agents that mimic T cell receptor signaling, including a decrease in apparent molecular mass upon elevation of intracellular calcium that is inhibited by the immunosuppressant FK506. Given the preferential expression of NFATc3 in the thymus, NFATc family members may regulate distinct subsets of genes during T cell development.

Amino Acid Sequence↗
PubMed1995

Identification of methylated metabolites of inorganic arsenic by thin-layer chromatography.

TLC on cellulose plates was used to identify methylated products of inorganic arsenic metabolism (monomethylarsonate and dimethylarsinate) in biological samples. Two solvent systems were tested: methanol-ammonium hydroxide (8:2) and isopropanol-acetic acid-water (10:1:2.5). The latter solvent system produced the most satisfactory separation of radiolabelled methylated arsenic compounds in aqueous solution, in rat liver cytosol incubated with carrier-free or 1 microM [73As]arsenite and in urine of mice given carrier-free [73As]arsenate or 5 mg of [73As]arsenate/kg per os. Oxidation of samples by hydrogen peroxide improved the separation and quantitation of monomethylarsonate in both biological matrices.

Animals↗
PubMed1995

Chaos in protein folding: a simple one-dimensional model.

A highly simplified one-dimensional model of protein folding has been developed in which the driving force is the attempt to achieve uniform internal density. The polypeptide chain is described as point-like beads on a polynomial curve, and the local density of beads is represented using wavefunctions. Despite the physical and mathematical simplicity of the model, it nonetheless displays chaotic behaviour, and analogies can be drawn to the molten-globular state, the transition state and the correctly folded state. The results support a more random view of protein folding than is often seen in the literature.

Algorithms↗
PubMed1995

In vitro inhibition of glutathione reductase by arsenotriglutathione.

Arsenotriglutathione, a product of the reaction of arsenate or arsenite with glutathione, is a mixed-type inhibitor (Ki = 0.34 mM) of the in vitro reduction of glutathione disulfide by purified yeast glutathione reductase. Notably, arsenotriglutathione was a 10-fold more potent inhibitor than either arsenite or glutathione. The inhibition of glutathione reductase by arsenotriglutathione was partly reversed by the addition of meso-2,3-dimercaptosuccinic acid (DMSA). However, high concentrations of DMSA also inhibited the reduction of glutathione disulfide by the yeast enzyme (IC50 of 7 mM with 0.1 mM glutathione disulfide). Ultrafiltration of the enzyme-arsenotriglutathione complex recovered about 74% of the original (non-inhibited) activity, suggesting that the inhibition of glutathione reductase by arsenotriglutathione had both reversible and irreversible components. The relatively high potency of arsenotriglutathione as an inhibitor of glutathione reductase may alter the reduction of glutathione disulfide and affect the availability of glutathione that is required for the reduction of arsenate to arsenite and for the formation of the arsenotriglutathione complex.

Animals↗
PubMed1995

Racial differences in the incidence of gout. The role of hypertension.

OBJECTIVE: To estimate the incidence of and examine risk factors for the development of gout in black and white male physicians. METHODS: Data from 2 cohorts of former medical students, 352 black men in the Meharry Cohort Study and 571 white men in the Johns Hopkins Precursors Study, were analyzed. Cases of gout were identified by self-report. Baseline variables and incident hypertension were examined as risk factors for the development of gout in both cohorts. RESULTS: The incidence of gout was 3.11 and 1.82 per 1,000 person-years in the black men and the white men, respectively (P < 0.05); the cumulative incidence was 10.9% and 5.8%, respectively (P = 0.04). The relative risk (RR) for gout among the black men was 1.69 (95% confidence interval [95% CI] 1.02-2.80). This excess risk persisted after adjustment for baseline systolic blood pressure (adjusted RR = 1.96 [95% CI 1.14-3.38]). Incident hypertension was independently associated with the development of gout in univariate analysis (RR = 3.78 [95% CI 2.18-6.58]); when this variable was included as a time-dependent covariate in a Cox model, the excess risk for gout in black men was reduced and no longer significant (adjusted RR = 1.30 [95% CI 0.77-2.19]). CONCLUSION: The approximately 2-fold excess risk for gout among black men is explained, in part, by a greater risk of incident hypertension.

Adult↗
PubMed1995

Comparative in vitro methylation of trivalent and pentavalent arsenicals.

The time course and extent of methylation of 1 microM arsenite (iAsIII), arsenate (iAsV), methylarsenite (MeAsIII), methylarsenate (MeAsV), and MeAsIII-diglutathione complex (MeAsIII(GS)2) were examined in an in vitro assay system that contained rat liver cytosol. Precursor arsenicals and methylated metabolites were analyzed by thin-layer chromatography (TLC) or by hydride generation-atomic absorption spectrophotomoetry (HG-AAS). More than 90% of iAsIII was converted to a dimethylated species (Me2As) during a 90-min incubation at 37 degrees C; the amount of monomethylated metabolite was maximal at 15 min. In contrast, only 40% of iAsV was dimethylated during a 90-min incubation. Comparison of the yields of methylated species in the whole in vitro assay system as determined by HG-AAS and in an ultrafiltrate prepared from the in vitro assay system as determined by TLC indicated that nearly 70% of the dimethylated metabolite (possibly Me2AsIII) that was produced during a 90-min incubation was bound to proteins (> 10 kDa). The percentage of protein-bound arsenic in the assay system incubated at 0 degree C with trivalent arsenicals was three-to fivefold greater than the binding of corresponding pentavalent species. This indicated that both iAsIII and trivalent organoarsenicals interact avidly with proteins. Both MeAsIII prepared by metabisulfite-thiosulfate reduction of MeAsV and a MeAsIII(GS)2 were quantitatively converted to Me2As during 90-min incubation. In contrast, only 3% of MeAsV was dimethylated during this interval. These results suggest that trivalent arsenicals are preferred substrates for methylation reactions and that the reduction of As from pentavalent to trivalent states may be a critical step in the control of the rate of metabolism of As.

Animals↗
PubMed1995

The folding and stability of titin immunoglobulin-like modules, with implications for the mechanism of elasticity.

Titin (first known as connectin) is a vast modular protein found in vertebrate striated muscle. It is thought to assist myofibrillogenesis and to provide a passive elastic restoring force that helps to keep the thick filaments properly centered in the sarcomere. We show that representative titin modules do indeed fold independently, and report their stabilities (i.e., delta G of unfolding and melting temperature) as measured by circular dichroism, fluorescence, and nuclear magnetic resonance spectroscopies. We find that there is a region-dependent variation in stability, although we find no evidence to support a proposed elastic mechanism based on a molten-globular-like equilibrium folding intermediate, nor do our calculations support any mechanism based on the configurational entropy of the molecule itself; instead we suggest a model based on hydrophobic hinge regions that would not be strongly dependent on the precise folding pattern of the chain.

Animals↗
PubMed1995

Biomechanics of skull fracture.

This study was conducted to determine the biomechanics of the human head under quasistatic and dynamic loads. Twelve unembalmed intact human cadaver heads were tested to failure using an electrohydraulic testing device. Quasistatic loading was done at a rate of 2.5 mm/s. Impact loading tests were conducted at a rate of 7.1 to 8.0 m/s. Vertex, parietal, temporal, frontal, and occipital regions were selected as the loading sites. Pathological alterations were determined by pretest and posttest radiography, close-up computed tomography (CT) images, macroscopic evaluation, and defleshing techniques. Biomechanical force-deflection response, stiffness, and energy-absorbing characteristics were obtained. Results indicated the skull to have nonlinear structural response. The failure loads, deflections, stiffness, and energies ranged from 4.5 to 14.1 kN, 3.4 to 16.6 mm, 467 to 5867 N/mm, and 14.1 to 68.5 J, respectively. The overall mean values of these parameters for quasistatic and dynamic loads were 6.4 kN (+/- 1.1), 12.0 mm (+/- 1.6), 812 N/mm (+/- 139), 33.5 J (+/- 8.5), and 11.9 kN (+/-0.9), 5.8 mm (+/- 1.0), 4023 N/mm (+/- 541), 28.0 J (+/- 5.1), respectively. It should be emphasized that these values do not account for the individual variations in the anatomical locations on the cranium of the specimens. While the X-rays and CT scans identified the fracture, the precise direction and location of the impact on the skull were not apparent in these images. Fracture widths were consistently wider at sites remote from the loading region. Consequently, based on retrospective images, it may not be appropriate to extrapolate the anatomical region that sustained the impact forces. The quantified biomechanical response parameters will assist in the development and validation of finite element models of head injury.

Aged↗
PubMed1995

Cloning and chromosomal localization of the human and murine genes for the T-cell transcription factors NFATc and NFATp.

The nuclear factor of activated T cells (NFAT) is a transcription factor complex involved in the activation of cytokines and cell surface molecules associated with coordinating the actions of different cells required for an immune response. Two different genes have recently been cloned that encode proteins capable of functioning as the pre-existing (p) and cytosolic (c) component of the NFAT transcription complex, NFATc of human and NFATp of murine origin (Northrop et al., 1994; McCaffrey et al., 1993b). We report here the partial cDNA cloning of the murine homolog of NFATc and the human homolog of NFATp, and the chromosomal localization of both genes in both species to conserved syntenic regions. Through the use of mapping panels of human x Chinese hamster and mouse x rodent cells hybrids, the NFATc genes were mapped to human and mouse chromosomes 18. By analyzing a chromosome 18 radiation hybrid panel, the human NFATc gene was localized to the q terminus, closely linked to STS marker D18S497. The murine Nfatc gene was sublocalized to chromosome band 18E4 by FISH. The NFATp genes were mapped by somatic cell hybrid analysis to human chromosome 20 and mouse chromosome 2. Human NFATp was assigned to chromosome region 20q13.2-->q13.3 by FISH. Based on the conserved syntenic region on human chromosome 20 and mouse chromosome 2, murine Nfatp is predicted to reside in the vicinity of a mutant locus wasted. Homozygous wst/wst mice display a phenotype reminiscent of severe combined immune deficiency or ataxia telangiectasia, disorders that could therefore be considered candidates for NFATp mutations.

Amino Acid Sequence↗
PubMed1995

Genetic modification and selection of microorganisms for growth on Mars.

Genetic engineering has often been suggested as a mechanism for improving the survival prospects of terrestrial microoganisms when seeded on Mars. The survival characteristics that these pioneer microorganisms could be endowed with and a variety of mechanisms by which this can be achieved are discussed, together with an overview of some of the potential hurdles that must be overcome. Also, a number of biologically useful properties for these microorganisms are presented that could facilitate the initial human colonisation and ultimately the planetary engineering of Mars.

Bacteria, Anaerobic↗
PubMed1994

NF-AT components define a family of transcription factors targeted in T-cell activation.

The NF-AT transcription complex is required for the expression of a group of proteins that collectively coordinate the immune response. Here we purify two proteins encoded by separate genes that represent the pre-existing (p) and cytosolic (c) components of NF-AT. Expression of the full-length complementary DNA encoding NF-ATc activates the interleukin (IL-2) promoter in non-T lymphocytes, whereas a dominant negative of NF-ATc specifically blocks activation of the IL-2 promoter in T lymphocytes, indicating that NF-ATc is required for IL-2 gene expression. NF-ATc RNA expression is largely restricted to lymphoid tissues and is induced upon T-cell activation. The other protein, NF-ATp, is highly homologous to NF-ATc over a limited domain which shows similarity to the Dorsal/Rel family, but has a wider tissue distribution. Agents that increase intracellular Ca2+ or activate protein kinase C independently modify NF-ATc, indicating that distinct signalling pathways converge on NF-ATc to regulate its function.

Amino Acid Sequence↗
PubMed1994

Tributyltin and dexamethasone induce apoptosis in rat thymocytes by mutually antagonistic mechanisms.

The nuclei of apoptotic thymocytes can be identified by flow cytometry as a subpopulation exhibiting reduced DNA content. We observed that rat thymocyte cultures exposed to 1.0-2.5 microM tri-n-butyltin methoxide (TBT) exhibited a rapid time- and concentration-dependent induction of apoptosis, with > 85% of cells exhibiting reduced DNA content within 1 hr after exposure to 2.0-2.5 microM TBT. In contrast, exposure to 1.0 microM dexamethasone phosphate (DEX) resulted in a gradual time-dependent increase to approximately 45% induction of apoptosis by 6 hr versus approximately 15% spontaneous induction in controls. However, simultaneous exposure to TBT and DEX resulted in a decreased response: although TBT concentrations between 0.1 and 0.5 microM did not induce apoptosis, they reduced the ability of DEX to initiate apoptosis; while at TBT concentrations > or = 1.0 microM, simultaneous exposure to DEX substantially decreased the extent of TBT-induced apoptosis and cytotoxicity. Furthermore, while treatment with the protein synthesis inhibitor cycloheximide or the protein kinase C inhibitor H-7 completely blocked DEX-induced apoptosis, neither significantly reduced induction of apoptosis by TBT. Taken together, the toxicant-specific differences in the timing and extent of apoptotic induction and the dissimilar responses to CHX and H-7 suggest that TBT and DEX initiate endonuclease-mediated apoptotic cell death through different mechanisms. Moreover, the ability of each agent to retard the action of the other suggests that these mechanisms are directly or indirectly antagonistic.

Analysis of Variance↗
PubMed1994

Reduction and binding of arsenate and dimethylarsinate by glutathione: a magnetic resonance study.

By observing the chemical shifts of the proton and carbon-13 nuclei of reduced glutathione, the interactions of arsenate, arsenite and dimethylarsinate with this tripeptide have been characterized. These spectral studies show the reduction and complexation of arsenic to be a two-step process. Initially, the oxidation of 2 mol of glutathione reduces arsenate to arsenite. Then, 3 mol of glutathione are consumed in the formation of a glutathione-arsenite complex. Similar experiments with arsenite identified a (glutathione)3-arsenite complex; however, no oxidized glutathione was detected. The arsenite binding site in the glutathione-arsenite complex is the cysteinyl sulfhydryl. The glutathione-arsenite complex is stable over the pH range from 1.5 to 7.0-7.5. At higher pH, dissociation occurs releasing reduced glutathione. For a glutathione to dimethylarsinate ratio of 3, oxidized glutathione is also coupled with a reduction to trivalent dimethylarsinous acid, prior to the formation of a 1:1 glutathione-dimethylarsinite complex. The role of reduced glutathione in the metabolism of arsenic is consistent with the previously described effects of this agent on the organismic toxicity of arsenic.

Arsenates↗