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J D Bell

Publications and source records attributed to J D Bell.

At least 109 records · Page 6Linked to original sources
PubMed1993

Quantification of the interaction of lysolecithin with phosphatidylcholine vesicles using bovine serum albumin: relevance to the activation of phospholipase A2.

The activity of soluble phospholipase A2 to hydrolyze phosphatidylcholine vesicles increases abruptly after a lag time of several minutes. The onset of this apparent activation event probably results from the accumulation of a threshold mole fraction of the hydrolysis products (lysolecithin and fatty acid) in the bilayer. One important observation relevant to the mechanism of this activation process is the biphasic dependence of the lag time on vesicle concentration. To test whether this dependence can be attributed entirely to the strength of partitioning of the lysolecithin into the phosphatidylcholine bilayer, we estimated the apparent partition coefficient of lysophospholipid in the membrane of phosphatidylcholine vesicles. Based on competition between bovine serum albumin and the vesicles for the lysophospholipid, we estimated the partition coefficient to be about 5.10(-7) for palmitoyl lipids at 39 degrees C and about 9.10(-7) for myristoyl lipids at 22 degrees C. These values were able to rationalize the behavior of the lag time with dipalmitoylphosphatidylcholine vesicles, but they were unable to predict the behavior with dimyristoylphosphatidylcholine. Therefore, it appears that the complete dependence of the lag phase on vesicle concentration must be explained by additional means such as the possible contribution of nascent fatty acid or previously proposed kinetic activation mechanisms.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1993

The proton NMR spectrum in acute EAE: the significance of the change in the Cho:Cr ratio.

We have studied by nuclear magnetic resonance spectroscopy the evolution of the proton spectral changes in acute experimental allergic encephalomyelitis. We found an in vivo elevation in the ratio between the peaks assigned to "choline containing compounds" (Cho) and creatine plus phosphocreatine (Cr). This was associated with an increase in choline, betaine, and phosphorylcholine (PC) as well as a reduction in N-acetylaspartate (NAA), aspartate, N-acetylaspartatylglutamate and inositol in vitro. Histological examination revealed inflammation with no evidence of demyelination or neuronal loss. We conclude that the increase in the ratio of Cho:Cr was due to an increase in the concentrations of PC, betaine, and choline in association with inflammation, and not as others have suggested, with demyelination. The reported reduction in NAA may be due to dysfunction of neurones rather than their loss.

Animals↗
PubMed1993

In vivo fluorine-19 magnetic resonance spectroscopy of cerebral halothane in postoperative patients: preliminary results.

This study reports the use of 19F MRS to study halothane in the brain of eight patients recovering from halothane anesthesia of short duration. Resonances attributable to halothane were observed up to 90 min after withdrawal of the anesthetic agent. The signal-to-noise ratio for an unlocalized spectrum acquired using a 6 cm surface coil was typically 20 with data collection times of 2 min. In seven patients a single resonance was seen with a mean (+/- SD) chemical shift of +43.3 (+/- 1.8) ppm, referenced to NaF at 0 ppm. This resonance exhibited a T1 value of between 0.5 and 1 s, and a T2* (estimated from the linewidth of the resonance) between 3.5 and 10 ms. In one patient two resonances were observed with chemical shifts of +38 and +41 ppm. Because we cannot exclude the possibility that this was due to field inhomogeneity, the significance of the last finding is uncertain. However, phantom studies show that the chemical shift of halothane in different environments (such as water, olive oil, methanol, and lecithin) can vary to an extent that accounts for the two resonances seen in our patient. These results demonstrate the feasibility of in vivo 19F MRS studies of fluorinated volatile agents in humans. The potential for clinical 19F MRS of fluorinated anesthetics is discussed.

Adult↗
PubMed1993

Altered phosphorylation status, phospholipid metabolism and gluconeogenesis in the host liver of rats with prostate cancer: a 31P magnetic resonance spectroscopy study.

31P magnetic resonance spectroscopy (MRS) in vivo and in vitro was used to study modulation of host liver (HL) metabolism in rats bearing the MAT-LyLu variant of the Dunning prostate tumour. Animals were inoculated either with 10(6) or 10(7) MAT-LyLu cells, or with saline to serve as controls. Carcass weight in tumour-bearing (TB) animals decreased despite similar food and water intake in both groups. Absence of metastatic tumour cells from HL of all TB animals was confirmed by histological examination. Twenty-one days after inoculation, 31P MRS showed a 2.5-fold increase in [Pi]/[ATP] ratios in HL in vivo (P < 0.001) which was confirmed by 31P MRS of liver extracts in vitro (P < 0.005). Phosphodiester to ATP ratios were significantly increased (P < 0.05) in HL in vivo, but absolute PDE levels were similar in both groups. Phosphomonoester to ATP ratios did not change, although absolute phosphomonoester levels in HL were reduced by -41% (not significant). In HL extracts in vitro, sharp reductions in the levels of glucose-6-phosphate (P < 0.05), fructose-6-phosphate (P = 0.05), phosphocholine (P < 0.001), glycerophosphocholine (P < 0.001), and glycerophosphoethanolamine (P < 0.001) were observed. Electron microscopy revealed increased amounts and altered distribution of rough endoplasmic reticulum in HL. These findings show that experimental prostate cancer significantly affects hepatic phosphorylation status, phospholipid metabolism, and gluconeogenesis in the host animal, and demonstrate the value of combined MRS in vivo and in vitro in monitoring HL metabolism in cancer.

Animals↗
PubMed1993

Detection of aluminium(III) binding to citrate in human blood plasma by proton nuclear magnetic resonance spectroscopy.

Reactions of Al3+ (50-500 mumol l-1) with intact blood plasma and its low relative molecular mass ultrafiltrate (< 5 kDa) have been studied by proton nuclear magnetic resonance spectroscopy. Binding to citrate was detected and was reversed by addition of desferrioxamine. The use of combined exponential and sine-bell functions for the resolution enhancement of spectra of plasma is illustrated.

Aluminum↗
PubMed1992

Reversibility of the activation of soluble phospholipase A2 on lipid bilayers: implications for the activation mechanism.

The time-courses of hydrolysis of large vesicles of dipalmitoylphosphatidylcholine were compared using four species of phospholipase A2 (Agkistrodon piscivorus piscivorus, Crotalus adamanteus and Naja naja venoms and porcine pancreatic). In all four cases, the hydrolysis rate suddenly increases 10 to 100-fold at the time (tau) when a specific mole fraction of reaction products has accumulated. The intrinsic fluorescence emission of the three venom enzymes also increases suddenly at time tau. Both the activation and the fluorescence change are reversible with a half-time of about 50 s for the activity and 2 to 6 s for the fluorescence. These reversal rates and the vesicle concentration dependence of tau are considered for monomer and dimer enzyme activation models. Apparently, at least three states of the enzyme exist beyond the initial unbound state: (1) inactive and bound, (2) inactive with high fluorescence and (3) active. The dimer model already contains the necessary number of states but requires that the activation rate be much lower than the reversal rate to account for the vesicle concentration dependence of tau. Success of the monomer model requires an enzyme state additional to those proposed previously. Although these results do not exclude either the monomer or dimer models conclusively, they do impose important constraints on each model.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Molecular details of the activation of soluble phospholipase A2 on lipid bilayers. Comparison of computer simulations with experimental results.

The initial rate of hydrolysis of large unilamellar vesicles of dipalmitoylphosphatidylcholine by phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus is small and elevates gradually until it suddenly increases by a factor of 10 to 1000 depending on the experimental conditions. This abrupt onset of high enzyme activity appears to be correlated to a specific mole fraction of reaction product at which point a cooperative compositional phase transition in the bilayer occurs. Five models that describe the activation process in terms of its being coupled to the putative product-induced lipid transition are presented. These models include one in which the lipid structure enhances the affinity of enzyme binding to the bilayer surface, two in which the equilibrium position between an active and an inactive form of the enzyme-substrate complex is altered, and two in which the rate of a quasi-irreversible spontaneous activation process is increased. Whether the active form of the enzyme is a monomer or dimer is also considered in the last two pairs of models. Computer simulations of time courses for the different models show how a set of four experimental observables distinguishes qualitatively among them. Comparison of the experimental behavior with the computer-simulated behavior of the observables for each model indicates that activation of phospholipase A2 on the lipid surface involves formation of an enzyme dimer which spontaneously converts to an active form. The active enzyme persists in the active state as it exchanges between vesicles. This model of activation is similar to that proposed previously for activation of porcine pancreatic phospholipase A2.

Computer Simulation↗
PubMed1992

Membrane structure, toxins and phospholipase A2 activity.

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Animals↗
PubMed1992

Phosphorus-31 magnetic resonance spectroscopy of the human liver using chemical shift imaging techniques.

Phosphorus-31 magnetic resonance spectroscopy of the human liver was undertaken in 28 healthy adult individuals and in 49 patients with liver disease of varying aetiology. Data localised to the liver were obtained using chemical shift imaging techniques. The mean (+/- 1 S.D.) of the peak area ratio phosphomonoesters (PME)/phosphodiesters (PDE) in healthy adult individuals, from spectra obtained with pulse angle 45 degrees and repetition time 1 s, was 0.24 +/- 0.07. The intra-examination variability of this ratio was 20%, the intra-subject variability 27% and the inter-subject variability 32%. An increase in the PME/PDE was observed in the 31P hepatic MR spectrum from primary or secondary tumours in all 17 patients studied, which invariably represented an increase in PME/ATP and, in some cases, a reduction in PDE/ATP. The spectra did not show aetiological characteristics. A non-specific elevation in PME/PDE was also observed in the 31P hepatic MR spectra of 10 (40%) of 25 patients studied who had diffuse liver diseases, such as cirrhosis and infiltrating malignancies. The spectral pattern did not distinguish between diseases of varying aetiologies, but there was a linear correlation between increasing PME/PDE and a reduction in plasma albumin concentrations (p = 0.03). In three patients with hepatic malignancy and abnormal hepatic 31P-MRS, marked spectral changes were observed after successful treatment to debulk the tumour. Only minor changes were observed in the abnormal spectrum of a fourth patient in whom treatment was unsuccessful. Hepatic 31P-MR spectroscopy may prove useful for monitoring disease processes and treatment effects in well characterised patient populations.

Adenocarcinoma↗
PubMed1992

Effect of L-alanine infusion on 31P nuclear magnetic resonance spectra of normal human liver: towards biochemical pathology in vivo.

1. 31P n.m.r. spectroscopy in vivo was used to study the effect of L-alanine infusion on the concentrations of gluconeogenic intermediates in normal human liver. Studies were performed in six healthy male subjects (34-44 years, fasted overnight) using a chemical shift imaging pulse sequence on a whole-body n.m.r. system operating at 1.6T. Hepatic 31P n.m.r. spectra were obtained from 10 min before to 70 min after intravenous administration of 0.70 (n = 2), 1.40 (n = 3) or 2.80 (n = 5) nmol of L-alanine/kg body weight over 4.5 min. Concentrations of phosphomonoesters, Pi and phosphodiesters relative to ATP were calculated from peak areas in the n.m.r. spectra, using the beta-ATP peak as a reference. 2. Dose-dependent spectral changes were observed for [phosphomonoesters]/[ATP] and [Pi]/[ATP]. At the highest dose given, maximal changes in [phosphomonoesters]/[ATP] (mean +/- SEM: 98 +/- 12%, P < 0.005) and [Pi]/[ATP] (-33 +/- 3%, P < 0.001) were observed approximately 45 min after the L-alanine infusion. [Phosphodiesters]/[ATP] showed a maximal increase of 24 +/- 6% (P < 0.05), which was independent of the L-alanine dose. Hepatic ATP levels and pH did not change. 3. To identify the metabolites responsible for the changes observed in vivo, male Wistar rats were infused with 11.2 mmol of L-alanine/kg body weight. After 15 min, livers were freeze-clamped and were extracted according to standard procedures. In vitro, 31P n.m.r. spectra obtained at 8.4 or 11.7 T revealed sharp increases in the concentrations of 3-phosphoglycerate and phosphoenolpyruvate after L-alanine infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1991

Metabolites in the developing rat liver--a proton nuclear magnetic resonance spectroscopic study.

We have used 1H-NMR spectroscopy in vitro to investigate metabolite changes in the rat liver in the first 21 days of life. The principle findings are firstly that betaine, a metabolite of choline, was relatively low (1-2 mumol/g) on days 1-7, then rose sharply to 5-6 mumol/g by day 19, whereas approximately reciprocal changes occurred in taurine levels. Secondly the lactate levels were remarkably low (0.1-0.8 mumol/g) on days 1-7. Changes in two other choline derivatives, phosphocholine (PC) and glycerophosphorylcholine (GPC) are also reported. The results are discussed in the context of the origin of these metabolites in the neonatal period, their levels in the adult (180 day-old) rat and the significance of the measured changes in metabolite levels during liver development.

Animals↗
PubMed1991

The medical profession and changing attitudes towards advertising and competition.

This paper examines the attitudes of general medical practitioners towards competition and advertising and the changes that have occurred between 1985 and 1988. The data was derived from a self completion questionnaire, 1500 of which were evenly distributed among the members of five professions; doctors, dentists, lawyers, accountants and veterinarians. General practitioners are now favourably disposed towards advertising by the profession as a whole in an effort to increase awareness of medical services (70% in favour in 1988 compared to only 53% in 1985), but the perceived need for increased business efficiency has lessened (70% in 1988 compared to 78% in 1985). Collegiality continues to be the dominant ideology but this position has weakened slightly. In 1988 only 65% of general practitioners regarded other members of the profession as colleagues rather than competitors, compared to 73% in 1985.

Advertising↗
PubMed1991

Nuclear magnetic resonance studies of blood plasma and urine from subjects with chronic renal failure: identification of trimethylamine-N-oxide.

We have used 1H-, 13C- and 14N-NMR spectroscopy to investigate the constituents of plasma and urine in 16 patients with chronic renal failure (CRF). Resonances not previously observed in spectra of plasma from healthy volunteers were seen in CRF plasma, including those for trimethylamine-N-oxide (TMAO) and dimethylamine (DMA). A possible analogy with the plasma of elasmobranch fishes, in which TMAO stabilizes proteins in the presence of very high urea concentrations, is noted. The intensity of the TMAO resonance for CRF subjects was correlated with the plasma concentration of urea (R = 0.55) and creatinine (R = 0.74), suggesting that the presence of TMAO is closely related to the degree of renal failure. When normal subjects ate a meal of TMAO-containing fish, TMAO appeared rapidly in the plasma and in the urine. Thus TMAO is efficiently cleared by the healthy kidney. Differences in the interaction of lactate with plasma proteins were detected by NMR, suggesting that uraemia impairs their transport roles.

Adult↗
PubMed1991

Effect of aging and diet on proton NMR spectra of rat urine.

500 MHz 1H NMR studies of rat urine are used to assess metabolic changes resulting from aging (0.5-20 months) and changes in diet (casein versus chow). Aging rats fed chow diets decrease their excretion of citrate and 2-oxoglutarate, while the output of taurine and creatinine increases. Only young rats (1 month or less postweaning) excrete significant amounts of betaine and trimethylamine-N-oxide. Rats fed casein diets for 1 month postweaning do not excrete 2-oxoglutarate and excrete lower levels of hippurate, succinate, and citrate compared to rats fed chow diets. They also excrete N-methylnicotinamide. These high resolution proton NMR studies provide metabolic profiles which are not readily available by other techniques.

Aging↗
PubMed1991

Approaches to editing, assignment and interpretation of proton spectra.

Clinical 1H spectroscopy of the brain is complemented by parallel analyses of biopsy specimens and by studies of animal models of disease. 1H spectroscopy has been carried out on perchloric acid extracts of biopsy specimens from patients with intracranial tumours. The data suggest that clinical spectroscopy may be useful in the identification and grading of these tumours. In addition, the spectra from extracts derived from normal white matter add weight to the possibility that acetyl-containing compounds other than N-acetylaspartate may make a significant contribution to the signal at 2.0 ppm in vivo. Edited 1H spectra of brain metabolites in rats with acute liver failure demonstrate an elevation of glutamine and of lactate, suggesting a role for 1H spectroscopy in clinical investigations of metabolic encephalopathies. However, the observation and resolution of signals from glutamate and glutamine is more difficult at the lower fields that are available for clinical spectroscopy. Finally, some studies of patients with inborn errors of metabolism are described. It is shown that in a disorder of oxidative metabolism, brain lactate can be detected without the need for complex spectral editing techniques. Investigations of the metabolic abnormalities associated with Canavan's disease have shed further light on a possible role for N-acetylaspartate.

Animals↗
PubMed1991

Thermodynamics of phospholipase A2-ligand interactions.

Future investigations into the role of the structure of phospholipid substrates and the interrelationships between substrate, calcium, and enzyme conformation in the activation process are clearly needed. Enzyme dimerization in the activation of phospholipase A2 has been indicated, and a complex equilibrium between calcium, substrate, and monomer and dimer enzyme apparently exists. The incorporation of proton binding further complicates the scheme, and one is quickly faced with obtaining a large number of equilibrium constants in order to describe the system explicitly. Nevertheless, similarly complex systems have been well characterized using thermodynamic approaches such as those described herein. An excellent example is the complex equilibrium involving the protonation of the histidine residues and the binding of a mononucleotide to ribonuclease A. Achieving a complete thermodynamic description of that system allowed the investigators to make strong mechanistic statements about models for the catalytic mechanism of ribonuclease A. Since phospholipase A2 is available for study at the same level of detail, one can anticipate a similar degree of quantitative detail regarding the important interactions of this enzyme to be forthcoming.

Animals↗
PubMed1991

Activation of phospholipase A2 on lipid bilayers.

So far, three phospholipases A2 that display activation kinetics during the time course of hydrolysis of DPPC LUV have been found to undergo a fluorescence change coincident with the activation: the monomer (AppD49) and the dimer enzymes from A. piscivorus piscivorus and the dimer enzyme from C. atrox. The porcine pancreatic enzyme produces similar time courses of hydrolysis but does not display a concurrent fluorescence change. It is assumed that other phospholipases A2 will behave similarly in terms of the hydrolysis reaction. Which enzymes respond with a similar change in intrinsic fluorescence during the time course may well depend on the position of tryptophan residues and the amino acid sequence. Even though a given phospholipase A2 may not change its fluorescent properties on activation, the simultaneous monitoring of the hydrolysis reaction and the fluorescence of probes of the bilayer structure can be done with any phospholipase A2. A variety of probes exist which are sensitive to slightly different membrane properties and could be used as described here for TMA-DPH. For example, 1,3-dipyrenylpropane is sensitive to the apparent microviscosity of the bilayer is terms of the ability of molecules to translationally diffuse in the membrane. 6-Palmitoyl-2-[[2-(trimethylammonio)ethyl]methylamino]naphthalene chloride is sensitive to the ability of a molecule to rotate in the bilayer and displays large changes in its steady-state fluorescence as the anisotropy of the bilayer changes. 6-Propionyl-2-(dimethylamino)naphthalene is sensitive to the polarity and degree of hydration of its environment. Finally, a compound titled NK-529 has recently been introduced that apparently monitors the lateral phase separation of fatty acids in the bilayer. The fact that activation of phospholipase A2 can be monitored during the time course of hydrolysis of DPPCLUV makes this system an excellent choice for studying the mechanisms of activation and possible effects of various activators and inhibitors. The experimental system described here provides a way to determine whether such regulators exert their effects through alterations of the properties of the membrane and/or the enzyme. Importantly, this system allows one to seek temporal correlations of the various events in the process.

1,2-Dipalmitoylphosphatidylcholine↗