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Biomedical subjects

D J Thomas

Publications and source records attributed to D J Thomas.

At least 19 recordsLinked to original sources
PubMed1992

Concepts in protein folding.

Certain concepts and misconceptions in the field of protein folding are discussed from the viewpoint of a theoretical physicist. It is argued that there can be no protein folding code and that perceived correlations between sequence or composition and three-dimensional structure are more likely to be an artefact of a limited database than a real result. Attempts at using molecular dynamics algorithms are also likely to produce artefactual results because results depend critically on the unknown hamiltonian energy function. Correct calculations of configurational entropy are thought to be the most likely next step in understanding how and why proteins fold.

Protein Conformation↗
PubMed1992

Arctic marine ecosystem contamination.

The current state of knowledge of levels, spatial and temporal trends of contaminants in the Arctic marine ecosystem varies greatly among pollutants and among environmental compartments. Levels of polychlorinated biphenyls (PCBs), organochlorine (OC) pesticides and some heavy metals such as mercury and lead, in Arctic marine mammals and fish are relatively well documented because of the need for comparisons with biota in more polluted environments and interest in the contamination of native diets. Levels of heavy metals, alkanes, polyaromatic hydrocarbons (PAH) and OCs in the Arctic Ocean are comparable to uncontaminated ocean waters in the mid-latitudes. But concentrations of alpha- and gamma-hexachlorocyclohexane (HCHs) are higher in northern waters far removed from local sources, possibly because lower water temperature reduces transfer to the atmosphere. Bioaccumulation of OCs and heavy metals in Arctic marine food chains begins with epontic ice algae or phytoplankton in surface waters. Polychlorinated camphenes (PCC), PCBs, DDT- and chlordane-related compounds are the major OCs in marine fish, mammals and seabirds. Mean concentrations of most PCBs and OC pesticides in ringed seal (Phoca hispida) and polar bear (Ursus maritimus) populations in the Canadian Arctic are quite similar indicating a uniform geographic distribution of contamination, although alpha-HCH showed a distinct latitudinal gradient in bears due to higher levels in zones influenced by continental runoff. Ringed seals from Spitzbergen have higher levels of PCBs, total DDT and polychlorinated dioxins/furans (PCDD/PCDFs). In contrast to other OCs, PCDD/PCDFs in Canadian Arctic ringed seals and polar bears were higher in the east/central Arctic than at more southerly locations. Remarkably high cadmium levels are found in kidney and liver of narwhal (Monodons monoceros) from western Baffin Bay (mean of 63.5 micrograms g-1) and western Greenland waters (median of 39.5 micrograms g-1). Mercury concentrations in muscle of ringed seal and cetaceans frequently exceed 0.5 microgram g-1 especially in older animals. Cadmium concentrations in polar bear liver increased from west to east, while mercury levels were higher in ringed seals from the western Canadian Arctic, which suggests that natural sources of these metals predominate. Studies of temporal trends in OCs in ringed seals and seabirds in the Canadian Arctic indicate PCB and DDT levels declined significantly from the early 1970s to the 1980s. There is a lack of temporal trend data for other OC pesticides as well as for heavy metals and hydrocarbons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Arctic terrestrial ecosystem contamination.

Limited data have been collected on the presence of contaminants in the Arctic terrestrial ecosystem, with the exception of radioactive fallout from atmospheric weapons testing. Although southern and temperate biological systems have largely cleansed themselves of radioactive fallout deposited during the 1950s and 1960s, Arctic environments have not. Lichens accumulate radioactivity more than many other plants because of their large surface area and long life span; the presence and persistence of radioisotopes in the Arctic is of concern because of the lichen----reindeer----human ecosystem. Effective biological half-life of cesium 137 is reckoned to be substantially less than its physical half-life. The database on organochlorines in Canadian Arctic terrestrial mammals and birds is very limited, but indications are that the air/plant/animal contaminant pathway is the major route of these compounds into the terrestrial food chain. For terrestrial herbivores, the most abundant organochlorine is usually hexachlorobenzene followed by hexachlorocyclohexane isomers. PCB accumulation favours the hexachlorobiphenyl, pentachlorobiphenyl and heptachlorobiphenyl homologous series. The concentrations of the various classes of organochlorine compounds are substantially lower in terrestrial herbivore tissues than in marine mammal tissues. PCBs and DDT are the most abundant residues in peregrine falcons (a terrestrial carnivore) reaching average levels of 9.2 and 10.4 micrograms.g-1, respectively, more than 10 times higher than other organochlorines and higher than in marine mammals, including the polar bear. Contaminants from local sources include metals from mining activities, hydrocarbons and waste drilling fluids from oil and gas exploration and production, wastes from DEW line sites, naturally occurring radionuclides associated with uranium mineralization, and smoke containing SO2 and H2SO4 aerosol from the Smoking Hills at Cape Bathurst, N.W.T.

Animals↗
PubMed1992

Presence and implications of chemical contaminants in the freshwaters of the Canadian Arctic.

Hydrocarbons, stable organochlorines, metals and radionuclides are widespread in the freshwaters of the Canadian Arctic. Petroleum-associated hydrocarbon sources include natural seepage, wastes and effluents from exploration, production and refining at Norman Wells and spills. Hydrocarbons also originate from combustion of carbon-based fuels, generally at lower latitudes and then reach the Arctic with air movements. Organochlorine compounds also move throughout the hemisphere by aerial pathways and have become distributed widely in Arctic fish. The organochlorine at highest concentration in Arctic freshwater is alpha-HCH, while those generally at highest concentrations in the fish are toxaphene, PCBs and chlordane. Metals are ubiquitous in Arctic freshwaters, with inputs of several metals by precipitation superimposed on natural geologic backgrounds. Mercury is found in muscle of fish from Arctic freshwaters at concentrations up to about 0.5 ppm. Radionuclides are also widespread at levels below those acceptable in food, with some local elevations near former mines. The implications of these contaminants for the northern ecosystems and the people dependent upon them are still not clear. Preliminary studies of inducible enzymes in fish suggest that the thresholds for biological damage have not been reached.

Animals↗
PubMed1992

Regulation of prostaglandin H synthase activity in dog urothelial cells.

TPA regulation of prostaglandin H synthase activity in primary and subcultured dog urothelial cells was investigated. Previous studies have demonstrated an early (0-2 hr) increase in PGE2 synthesis mediated by TPA which is dependent upon release of endogenous arachidonic acid by a phospholipase-mediated pathway. In this study, prostaglandin H synthase activity was assessed directly with microsomes and indirectly after addition of exogenous arachidonic acid at a maximum effective concentration (100 microM) to media. PGE2 synthesis, measured by radioimmunoassay, served as an index of prostaglandin H synthase activity. After a 24-hr incubation with 0.1 microM TPA or 1.0 microM A23187, arachidonic acid elicited significantly more PGE2 synthesis in agonist-treated cells than it did in control cells in primary culture. Microsomes from 24-hr TPA-treated cells exhibited significantly more prostaglandin H synthase activity than did those from control cells. In addition, the PGE2 content of overnight media was approximately 10-fold greater in TPA-treated cells than in control cells. The late (24 hr) response was more sensitive to lower concentrations of TPA than was the earlier (0-2 hr) response. TPA at 0.1 microM was a maximum effective dose for both responses. The 24-hr response was blocked by cycloheximide and staurosporine, inhibitors of protein synthesis and protein kinase C, respectively. Pretreatment of cells with aspirin, an irreversible inhibitor of prostaglandin H synthase, prior to addition of TPA did not prevent the late TPA-mediated increase in PGE2 synthesis. Subcultured cells exhibited both an early and a late TPA response. Only the early response was inhibited by aspirin pretreatment. Results suggest that the late response with TPA is caused by de novo synthesis of prostaglandin H synthase. Thus, primary and subcultured dog urothelial cells possess two distinct mechanisms for regulating signal transduction by arachidonic acid metabolism. This study provides a basis for assessing these mechanisms of signal transduction in urothelial cell lines and transformed cells.

Alkaloids↗
PubMed1992

Altered regulation of arachidonic acid metabolism by SV40 immortalized human urothelial cells.

Regulation of arachidonic acid metabolism was investigated in an SV40 immortalized, non-tumorigenic human urothelial cell line (SV-HUC). This cell line is being used to evaluate the multistage carcinogenic process. Media from confluent cultures were analyzed for radioimmunoassayable prostaglandin E2 (PGE2). A variety of agonists, including 12-O-tetradecanoylphorbol-13-acetate (TPA) and A23187 were tested and did not increase PGE2 synthesis within 2 h of addition. This was not due to the lack of prostaglandin H synthase activity because addition of exogenous arachidonic acid increased PGE2 synthesis. Cultures prelabeled overnight with [3H]arachidonic acid failed to increase the release of radioactivity following agonist addition. Thus, the lack of an early response in SV-HUC appears to be due to decreased release of endogenous arachidonic acid by phospholipase(s). After a 24 h incubation with 0.1 microM TPA or 1.0 microM A23187, the addition of arachidonic acid elicited significantly more PGE2 synthesis in agonist-treated cells than it did in control cells. Microsomes from 24 h TPA-treated cells produced approximately 15-fold more PGE2 than did those from control cells. In addition, the PGE2 content of overnight media was significantly greater in TPA-treated cells than in control cells. The 24 h agonist response was blocked by cycloheximide and staurosporine--inhibitors of protein synthesis and protein kinase C respectively. Pretreatment of cells with aspirin, an irreversible inhibitor of prostaglandin H synthase, prior to addition of TPA did not prevent the late 24 h TPA-mediated increase in PGE2 synthesis. Results suggest that this late effect of TPA is due to de novo synthesis of prostaglandin H synthase. Thus, SV-HUC has lost the early but retains the late response to agonists.

Arachidonic Acid↗
PubMed1992

Rheology and strokes.

Further evidence is presented that rheological abnormalities antedate stroke and are not just a consequence of stroke. Whether these abnormalities should be corrected in the acute phase after stroke is still uncertain. Until more data are available haemodilution is not to be recommended in most cases of acute stroke.

Animals↗
PubMed1992

Acute urinary retention secondary to carcinoma of the prostate. Is initial channel TURP beneficial?

Over a 2-year period patients presenting with acute urinary retention secondary to locally advanced prostate carcinoma (stage T3/T4) were randomized to one of two treatments. Ten patients underwent channel transurethral resection of prostate (TURP) and bilateral orchidectomy, 12 patients underwent bilateral orchidectomy alone. Treatment by channel TURP and bilateral orchidectomy was complicated by difficulties in voiding in four patients, one requiring a further TURP. Ten of the 12 patients were voiding well one month following bilateral orchidectomy alone. Only two patients in this group required TURP. In patients with acute urinary retention secondary to prostate carcinoma, in whom hormonal manipulation is thought appropriate due to bulk of local tumour or metastatic disease, channel TURP may confer extra morbidity and therefore be held in reserve for those patients unable to void after hormonal manipulation.

Acute Disease↗
PubMed1991

A simplified mechanical model of proteins tested on the globin fold.

It has been shown that the distribution of presently known protein loop lengths is consistent with even the simplest available theory of rubber-like elasticity, and with the idea that such loops generate an entropically derived end-to-end tension. It has also been asserted that the molten globule phase, just like the native form, must be mechanically stable, and that a simple demonstration of the potential for mechanical stability would be a powerful test in predictions of new protein folds. This paper amplifies this suggestion by explicit calculation of a familiar but non-trivial test case: sperm-whale myoglobin. The method used is to describe the protein molecule in terms of a highly simplified mechanical model bearing some resemblance to a pre-stressed mechanism. The alpha-helices are treated as rigid rods and the loops are treated as elastic strings. The entropic tensions exerted by the loops are imposed on the mechanism using an approximation proposed earlier. The helices are then held to generate frictionless reaction forces at their mutual points of contact. These contact forces are calculated to null out maximally the effects of the loop tensions, and hence stabilize the molecule. It is shown that the crystallographically determined structure of myoglobin has a significantly higher mechanical stability on this model than does any of a previously published set of combinatorially generated predictions. Amongst the predictions alone, the best is also the one with the highest stability. It is anticipated that this result could be of general importance in sorting or filtering out bad predictions. A further exciting feature of the model is that it offers a natural explanation for the strong conservation of the C2 proline and the invariably long unconserved sequence from the end of the C helix to the start of the E helix in the globins and phycocyanins.

Animals↗
PubMed1991

Cyclic AMP response in SV40 immortalized human bladder cells.

Forskolin-mediated increase in cyclic AMP and subsequent activation of protein kinase A were evaluated in SV40-immortalized human urothelial cells. This cell line is being used to evaluate the multistep carcinogenic process. Forskolin elicited a time- and dose-dependent increase in cyclic AMP. Increases in intracellular cyclic AMP preceded media increases in cyclic nucleotide. Large increases in intracellular cyclic AMP occurred within 5 min of forskolin addition. The lowest effective concentration of forskolin was between 0.1 and 1.0 microM. Cyclic AMP increases as large as 20- to 100-fold were observed in cells and media following forskolin addition. A 60 min preincubation with 12-O-tetradecanoylphorbol-13-acetate (TPA) did not reduce the magnitude of the forskolin increase in cyclic AMP. TPA has been shown to affect cyclic AMP metabolism in many types of cells including primary and secondary cultures of urothelial cells. In the latter, preincubation with TPA reduces the magnitude of the forskolin increase. A 4.2-fold increase in protein kinase A activity was observed within 0.5 min of forskolin addition, while only small increases in cyclic AMP (1.6-fold) were detected within 1 min. Much more cyclic AMP is synthesized than is needed to maximally activate protein kinase A. While results demonstrate a forskolin-responsive cyclic AMP system, this system does not appear to be regulated by TPA. Lack of regulation of this second messenger system by TPA may be part of the immortalization process.

Cell Line, Transformed↗
PubMed1990

Effect of 5-azacytidine on metallothionein inducibility and sensitivity to lethality of cadmium in rat osteosarcoma (ROS 17/2.8) cells.

ROS 17/2.8 cells, a cloned rat osteoblastic osteosarcoma cell line, were found to be extremely sensitive to the lethal effects of cadmium and to synthesize little, if any, metallothionein in response to cadmium exposure. Culture of cells for 24 h in the presence of 1 microM 5-azacytidine, a cytidine analog, increased the inducibility of metallothionein by cadmium and significantly reduced (P less than 0.001) cytotoxicity. Anion exchange chromatographic analysis of cadmium binding to low molecular mass cytotoxicity. Anion exchange chromatographic analysis of cadmium binding to low molecular mass cytosolic proteins showed that cells treated with cadmium and 5-azacytidine expressed at least 2 isoforms of metallothionein. One isoform of metallothionein with a low affinity for cadmium was constitutively expressed by these cells. The association of poor inducibility of metallothionein by cadmium with extreme sensitivity of cells to cadmium emphasizes the role of this protein in the cellular response to this toxic metal. The modulation of metallothionein inducibility and sensitivity to cadmium by 5-azacytidine treatment suggest that metallothionein gene structure and regulation are altered in ROS 17/2.8 cells.

Animals↗