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Biomedical subjects

C G Morgan

Publications and source records attributed to C G Morgan.

32 records · Page 2Linked to original sources
PubMed1987

Light-induced fusion of liposomes with release of trapped marker dye is sensitised by photochromic phospholipid.

Liposomes have been prepared from dipalmitoylphosphatidylcholine containing small amounts of a synthetic photochromic phospholipid, 'Bis-Azo PC'. In the dark, these are stable at room temperature, and contents do not significantly leak over weeks. Photoisomerisation results in immediate release of trapped marker, and in liposome fusion to form larger structures. Fusion has been detected using a fluorescence polarisation assay, and confirmed by electron microscopy. In mixtures, fusion occurs between 'photochromic' liposomes and those of pure lipid. Bis-Azo PC contains two photochromic acyl chains; analogues bearing a single photochromic chain appear to have little effect on bilayer permeability after isomerisation. Photo-induced leakage and liposome fusion suggest possible applications for localised drug delivery as an adjunct to phototherapy. The ability to non-invasively trigger fusion processes should be useful in fundamental studies of membrane interactions. We believe this to be the first report of photo-induced fusion to date.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1986

Fusogenic activity of delta-haemolysin from Staphylococcus aureus in phospholipid vesicles in the liquid-crystalline phase.

A study has been conducted of the interaction of the lytic toxin delta-haemolysin with vesicles of phospholipid, using electron microscopy, fluorescence depolarisation and excimer fluorescence. The peptide is shown to be a fusogen towards phosphatidylcholine vesicles in fluid phases. In the presence of gel phase lipid, fusion between fluid and gel phases is not seen. Fluid phase lipid vesicles are fused together to form large multilamellar structures, and initial vesicle size does not appear to be important since small unilamellar vesicles and large unilamellar vesicles are similarly affected. Fusogenic activity of delta-haemolysin is compared to that of melittin. The former is a progressive fusogen for fluid phase lipid, while the latter causes vesicle fusion in a manner related to occurrence of a lipid phase transition.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1986

Lytic effects of melittin and delta-haemolysin from Staphylococcus aureus on vesicles of dipalmitoylphosphatidylcholine.

The effects of the lytic peptides, melittin and delta-haemolysin, are compared in vesicles of gel-phase dipalmitoylphosphatidylcholine (DPPC), using calcein as trapped marker. At low concentration, both toxins cause vesicles to lose contents in 5 mM phosphate buffer near neutral pH, with melittin being the more active. As phosphate concentration is increased, the kinetics of melittin-induced leakage change from a slow, sustained loss to a rapid 'burst' of leakage when melittin is present mainly as tetramer in solution, under conditions where it is reported to lose haemolytic activity towards erythrocytes. At low phosphate concentration, the leakage induced by delta-haemolysin is preceded by a lag phase, though fluorescence measurements show that binding of toxin is rapid. At higher phosphate concentration, the toxin binds rapidly to vesicles, but causes no leakage of entrapped calcein. Steady-state fluorescence spectra show no obvious differences in tryptophan emission for delta-haemolysin bound to lipid in high- or low-phosphate buffer. Spin-label fluorescence-quenching studies show that the single tryptophan residue of delta-haemolysin is buried within the lipid bilayer at all phosphate concentrations used. In gel-phase DPPC, delta-haemolysin shows no tendency to cause vesicle aggregation over several hours, as judged by light scattering, though a slow non-linear effect is seen above the lipid phase transition temperature. These effects are contrasted with those of melittin under similar conditions.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1983

Melittin induces fusion of unilamellar phospholipid vesicles.

Melittin, the soluble lipophilic peptide of bee venom, causes fusion of phospholipid vesicles when vesicle suspensions are heated or cooled through their thermal phase transition. Fusion was detected using a new photochemical method (Morgan, C.G., Hudson, B. and Wolber, P. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 26-30) which monitors lipid mixing. Electron microscopy and gel filtration confirmed that most of the lipid formed large vesicular structures. Fluorescence experiments with a water-soluble, membrane-impermeable complex of terbium (Wilschut, J. and Papahadjopoulos, D. (1979) Nature 281, 690-692) demonstrate that these ionic contents are released during fusion. The large structures formed by melittin-induced fusion are impermeable to these ions and are resistant to further fusion. This is in contrast to the behavior observed for the cationic detergent cetyltrimethylammonium bromide (CETAB). The large size of the vesicles formed, the extreme speed of the fusion event and the appearance of electron microscope images of the vesicles prior to fusion suggest that the mechanism of the fusion process includes a preaggregation step.

Bee Venoms↗
PubMed1983

The use of fluorescence energy transfer to distinguish between poly(ethylene glycol)-induced aggregation and fusion of phospholipid vesicles.

Two fluorescence energy transfer assays for phospholipid vesicle-vesicle fusion have been developed, one of which is also sensitive to vesicle aggregation. Using a combination of these assays it was possible to distinguish between vesicle aggregation and fusion as induced by poly(ethylene glycol) PEG 8000. The chromophores used were 1-(4'-carboxyethyl)-6-diphenyl-trans-1,3,5-hexatriene as fluorescent 'donor' and 1-(4'-carboxyethyl)-6-(4"-nitro)diphenyl-trans-1,3,5-hexatriene as 'acceptor'. These acids were appropriately esterified giving fluorescent phospholipid and triacylglycerol analogues. At 20 degrees C poly(ethylene glycol) 8000 (PEG 8000) caused aggregation of L-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles without extensive fusion up to a concentration of about 35% (w/w). Fusion occurred above this poly(ethylene glycol) concentration. The triacylglycerol probes showed different behaviour from the phospholipids: while not exchangeable through solution in the absence of fusogen, they appeared to redistribute between bilayers under aggregating conditions. DPPC vesicles aggregated with less than 35% poly(ethylene glycol) could not be disaggregated by dilution, as monitored by the phospholipid probes. However, DPPC vesicles containing approx. 5% phosphatidylserine which had been aggregated by poly(ethylene glycol) could be disaggregated by either dilution or sonication. Phospholipid vesicles aggregated by low concentrations of poly(ethylene glycol) appear to fuse to multilamellar structures on heating above the lipid phase transition temperature.

Cell Fusion↗
PubMed1982

The use of a phospholipid analogue of diphenyl-1,3,5-hexatriene to study melittin-induced fusion of small unilamellar phospholipid vesicles.

A phospholipid analogue incorporating the diphenyl-1,3,5-hexatriene (DPH) chromophore has been synthesized. The compound has been shown to have similar fluorescence properties to DPH itself but, unlike DPH, is unable to exchange freely through solution when incorporated as probe in a subset of phospholipid vesicles of given composition. The non-exchangeability of this probe has been exploited to study the fusion of phospholipid vesicles to form larger structures. The peptide melittin was used to initiate fusion, and it was shown that vesicles which had been induced to fuse by heating in the presence of melittin would not fuse with subsequently added vesicles.

Bee Venoms↗
PubMed1980

Photochemical dimerization of parinaric acid in lipid bilayers.

Parinaric acid (9,11,13,15-octadecatetraenoic acid), a conjugated tetraene fatty acid, undergoes a second-order photochemical reaction in phospholipid bilayers. The reaction results in the loss of the characteristic absorption of this chromophore and the development of new absorption demonstrating the presence of a triene chromophore. The progress of this reaction is easily monitored by measurement of the decrease in the fluorescence intensity from a uniformly illuminated sample. The reaction rate measured in this way is sensitive to the thermal phase transition of the bilayer and to the presence of cholesterol. The relationship of the second-order rate constant to the lipid diffusion coefficient is discussed. This relationship differs from that previously used for the analysis of similar photochemical processes.

Journal Article↗
PubMed1977

Positron lifetimes in phospholipid dispersions.

Positron lifetimes have been determined in phospholipid dispersions. In fluid phosphatidylcholines, a lifetime of 3.3 ns is found, and a lifetime of 2.8 ns is found for frozen phosphatidylcholines. In dispersions where fluid and frozen phases coexist due to lateral phase separation, an intermediate lifetime is found.

Membrane Lipids↗
PubMed1976

Measurement and interpretation of fluorescence polarisations in phospholipid dispersions.

An instrument that measures the temperature dependence of fluorescence polarisation and intensity directly and continuously is described. The behaviour of four fluorescent probes bound to a number of well characterised model systems was then examined. The motional properties of the probes were determined from the polarisation and intensity data and were found to be sensitive to the crystalline-liquid crystalline phase transitions in phospholipid vesicles of dimyristoly and dipalmitoly phosphatidylcholine. Binary mixture of dilauroyl and dipalmitoyl phosphatidylcholine show lateral phase separation and in this system the probes parition preferentially into the more 'fluid' phase. In systems that have been reported to contain 'short range order' or 'liquid clustering', such as dioleoyl phosphatidylcholine and liquid paraffin, the motion of the probes was found to have anomalous Arrhenius behaviour consistent with the idea that homogeneous phases were not being sampled. The significance of these findings for the interpretation of the behaviour of fluorescent probes bound to natural membranes is discussed.

Computers, Analog↗
PubMed1976

Photobleaching. A novel fluorescence method for diffusion studies in lipid system.

(1) The fluorescent molecular 12(9-anthroyloxy)-stearic acid dimerises on irradiation with light of 366 nm wavelength. (2) The dimer is nonfluorescent and can be reconverted to the parent compound by irradiation at 254 nm. (3) Kinetic analysis suggests that the dimerisation proceeds by a diffusion-limited second order mechanism in many solvents. (4) Anomalously high rates seen in other systems can be attributed to localised high concentration regions (clusters) of the fluorescent molecule. (5) The analysis has been extended to oriented lipid bilayers and the results suggest that below the gel-liquid crystalline transition temperature the 12(9-anthroyloxy)-stearic acid is excluded by the lipid matrix and forms regions of localised high concentration. (6) In fluid lipid the results suggest an isotropic distribution of the probe. Calculated diffusion coefficients correspond to those found by other techniques.

Diffusion↗
PubMed1975

Detection of Lactobacillus acidophilus in feces of humans, pigs, and chickens.

Lactobacilli in fecal material from humans, pigs, and chickens were enumerated on lactobacillus selective agar (LBS). In all samples, higher numbers of lac-tobacilli were detected when plates were incubated in a system flushed with CO2 rather than in air. Much higher numbers of bacteria from human feces were detected when the LBS agar plates were incubated anaerobically in a hydrogen-carbon dioxide atmosphere (GasPak) than when incubated in CO2. The bacteria from human feces isolated on LBS agar incubated anaerobically were predominately bifidobacteria. Cultures from all three sources isolated on LBS agar incubated under CO2 were lactobacilli, including Lactobacillus acidophilus. Differences were observed in biochemical characteristics of some of the L. acidophilus isolated from all three sources. Guanine plus cytosine base ratios of deoxyribonucleic acid isolated from L. acidophilus cultures from humans were lower, in most cases, than those from pigs and chickens.

Agar↗