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Biomedical subjects

D L Melchior

Publications and source records attributed to D L Melchior.

32 records · Page 2Linked to original sources
PubMed1983

Alterations in the organization of phosphatidylcholine/cholesterol bilayers by tetrahydrocannabinol.

The interactions of delta 9-tetrahydrocannabinol (THC) with various phosphatidylcholines (PCs) was studied in model membranes by differential scanning calorimetry. THC present in PC bilayers above a certain concentration complexed stoichiometrically with phospholipids containing both saturated and unsaturated fatty acids. When the bilayer PCs were sufficiently dissimilar for phase separation to occur, THC preferentially associated with the lower melting point lipid. The presence of cholesterol below 20 mol% in dipalmitoylphosphatidylcholine bilayers enhanced THC X PC complex formation. Above 20 mol% cholesterol, there was no indication of THC X dipalmitoylphosphatidylcholine complex formation. This is in agreement with a phase rearrangement occurring in PC bilayers at concentrations of cholesterol of approximately 20 mol%. These studies suggest several possible mechanisms for the modulation of membrane activities by hydrophobic drugs such as THC.

Animals↗
PubMed1983

The preparation and characterization of liposomes containing X-ray contrast agents.

Unilamellar phospholipid vesicles loaded with the water-soluble, ionic X-ray contrast agent diatrizoate (Hypaque, Renografin) were manufactured by reverse-phase evaporation for use as organ-enhancing agents in X-ray computed tomography. Encapsulation efficiency was determined as a function of various diatrizoate concentrations in vesicles of varying lipid composition. Loss of encapsulated diatrizoate over 24 h was examined in vesicles composed of several egg phosphatidylcholine/cholesterol ratios. Size estimates for loaded vesicles were obtained by negative-stain electron microscopy, Millipore filtration and light microscopy. Intravenous in vivo injection of loaded vesicles in the rat resulted in significant enhancement of both spleen and liver on subsequent scans. Vesicles were similarly prepared with the water-soluble, nonionic agent metrizamide (Amipaque). Encapsulation efficiency was determined, and in vivo behavior was observed.

Animals↗
PubMed1983

Asymmetric or symmetric? Cytosolic modulation of human erythrocyte hexose transfer.

(1) The Michaelis-Menten parameters for hexose transfer in erythrocytes, erythrocyte ghosts and inside-out vesicles at 20 degrees C were determined using the light scattering method of Sen and Widdas ((1962) J. Physiol. 160, 392-403). (2) The external Km for infinite-cis exit of D-glucose in cells and ghosts is 3.6 +/- 0.5 mM. (3) Dilution of cellular solute (up to X 90 dilution) by lysing and resealing cells in varying volumes of lysate is without effect on the Vm for net D-glucose exit. The Km for net exit, however, falls from 32.4 +/- 3.7 mM in intact cells to 12.9 +/- 2.3 mM in ghosts. This effect is reversible. (4) Infinite-cis net D-glucose uptake measurements in cells and ghosts reveal the presence of a low Km, high affinity internal site of 5.9 +/- 0.8 mM. The Vm for net glucose entry increases from 23.2 +/- 3.7 mmol/1 per min in intact cells to 55.4 +/- 6.3 mmol/l per min in ghosts. (5) The external Km for infinite-cis D-glucose exit in inside-out vesicles is 6.8 +/- 2.7 mM. The kinetics of zero-trans D-glucose exit from inside-out vesicles are changed markedly when cellular solute (obtained by lysis of intact cells) is applied to either surface of inside-out vesicles. When solute is present externally, the Km and Vmax for zero-trans exit are decreased by up to 10-fold. When solute is present at the interior of inside-out vesicles, Vmax for zero-trans exit is reduced; Km for exit is unaffected. In the nominal absence of cell solute, transfer is symmetric in inside-out vesicles. The orientation of transporter in the bilayer is unaffected by the vesiculation procedure. (6) External application of cellular solute to ghosts reduces Vmax for D-glucose exit but is without effect on the external Km for infinite-cis exit. (7) The inhibitory potency of cell lysate on hexose transfer is lost following dialysis indicating that the factors responsible for transfer modulation are low molecular weight species. (8) We consider the hexose transfer in human erythrocytes is intrinsically symmetric and that asymmetry of transfer is conferred by interaction of the system with low molecular weight cytosolic factors.

Biological Transport, Active↗
PubMed1982

The physical state of quick-frozen membranes and lipids.

Lipid bilayers and biomembranes produce nearly identical calorimeter scans regardless of whether they are slowly cooled under near-equilibrium conditions or rapidly frozen at rates used in freeze-fracture electron microscopy. Except for the melting of ice at 273 K, for both cooling regimens no significant thermal events occur from 100 K to the usual gel to liquid crystal transition. The gel to liquid crystal transition itself is somewhat altered by rapid cooling when bilayers contain mixed lipid species. Combined with X-ray diffraction studies, the results indicate that quickly frozen bilayers are crystalline, but that the crystalline domains are quite small or otherwise disordered. In contrast to the behavior of lipids in bilayers, hexagonal-phase calcium cardiolipin easily forms a glass upon cooling.

Animals↗
PubMed1981

The organization of cholesterol esters in membranes of Mycoplasma capricolum.

The organization of cholesterol esters in Mycoplasma capricolum membranes was studied by differential scanning calorimetry. Cells grown in the presence of horse serum incorporated large amounts of cholesterol esters into their membranes. The cholesterolester-containing membranes after incubation at low temperature showed an endotherm characteristic of a cholesterol ester crystalline leads to isotropic liquid transition that was identical in membranes both before and after thermal protein denaturation. This transition was not observed in membranes of cells grown in medium in which the horse serum was replaced by bovine albumin, fatty acids and unesterified cholesterol unless cholesterol esters were added to the growth medium. In membrane preparations obtained both from cells grown in horse serum and from cells grown with bovine albumin plus cholesterol and fatty acids, the free cholesterol content was sufficient to eliminate the bilayer order/disorder transition observed in isolated membrane phospholipids. Our studies indicate that the majority of cholesterol esters in M. capricolum membranes is not present in attached serum lipoprotein particles, nor is intimately associated with membrane protein, but exists as relatively large cholesterol ester droplets or pockets tightly associated with the membrane. The cholesterol esters in these pockets appear relatively pure, although the presence of small amounts of other membrane components is likely.

Calorimetry, Differential Scanning↗
PubMed1980

Dilatometry of dipalmitoyllecithin-cholesterol bilayers.

The interactions of cholesterol and dipalmitoyl-phosphatidylcholine in bilayers were investigated by differential scanning dilatometry and related techniques. Dipalmitoyl-phosphatidylcholine bilayers ranging from 0 to 50 mol % cholesterol were studied over a temperature range of 0-50 degrees C. These investigations allowed construction of a three-dimensional surface with dimensions of mole fraction of cholesterol, temperature, and apparent partial specific volume. Much of the phenomenology reported for dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine-cholesterol bilayers appears and can be interrelated on this surface. In addition to the thermotropic events associated with the system, two cholesterol-induced events at 17.5-20 and 29 mol % cholesterol are particularly in evidence.

Cholesterol↗
PubMed1977

Control of fatty acid composition of Acholeplasma laidlawii membranes.

The temperature-dependent pattern of incorporation of palmitate and oleate from the growth medium into Acholeplasma laidlawii membrane lipids correlates with the physical state of the membrane defined by calorimetry. Both the pattern and the state can be changed at will by changing the fatty acid composition of the membrane lipids. The ratio of palmitate to oleate incorporated is independent of temperature when the membrane bilayer is below its transition and fully ordered, but becomes temperature dependent upon the onset of the transition and continues to be temperature dependent when the membrane is above its transition and fully fluid. This behavior is mimicked by the physical binding of palmitate and oleate to bilayers of extracted membrane lipids and to bilayers of lecithin. Selective binding by membranes may provide a means for controlling lipid fatty acid composition without invoking an enzymatic mechanism.

Acholeplasma laidlawii↗
PubMedDate not supplied

Physical state of membrane lipids of Mycoplasma capricolum.

The physical state of the lipids in Mycoplasma capricolum membranes was studied by differential scanning calorimetry. Cells grown in the presence of horse serum incorporated large amounts of cholesterol esters into their membranes. After incubation at a low temperature, the cholesterol ester-containing membranes showed an endotherm characteristic of a cholesterol ester transition from a crystalline state to an isotropic liquid that was identical in membranes both before and after thermal protein denaturation. This transition was not observed in membranes of cells grown in medium in which the horse serum was replaced by bovine albumin, fatty acids, and unesterified cholesterol unless cholesterol esters were added to the growth medium. In membrane preparations obtained from both horse serum-grown cells and from cells grown with bovine albumin plus cholesterol and fatty acids, the free cholesterol content was sufficient to eliminate the bilayer order/disorder transition observed in isolated membrane phospholipids. Our calorimetric studies indicate that the majority of cholesterol esters in M. capricolum membranes is not present in attached serum lipoprotein particles nor is it intimately associated with membrane protein but exists as relatively large droplets of cholesterol ester or as pockets in the membrane. The cholesterol esters in these pockets exist in a liquid-like state at growth temperature and appear to be relatively pure, although the presence of small amounts of other membrane components, especially glycerides, is likely. The existence of a low-temperature endotherm in membrane attributable to glycerides suggests there may be glyceride-rich regions in the membranes.

Animals↗