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 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Equilibrium surface pressure-area isotherms of dipalmitoyllecithin monolayers were measured on substrates containing various concentrations of the surfactant, cetrimonium (hexadecyltrimethylammonium) bromide. From these isotherms, the saturation adsorptions of surfactant for various surface lecithin concentrations were calculated. Plotting of these adsorptions against the inverse of the area per lecithin molecule, as required for the "accessible area" theory, revealed two linear segments, corresponding to penetrating at high and at low monolayer areas. At both high and low areas, the adsorption into the accessible areas of the surface was similar to adsorption at a monolayer-free surface. The effective cross-sectional area of the monolayer molecule in the low area region was equal to the collapse area; in the high area region, it was equal to an area corresponding to the co-area, as calculated from the Amagat equation. The change in cross-sectional area corresponded to the transition in the monolayer from a liquid condensed state to a liquid expanded state.
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PMID:Equilibrium penetration of monolayers IV: dipalmitoyllecithin-centrimonium bromide system. 58 Apr 39

Equilibrium surface pressure-area isotherms for the penetration of cholesterol monolayers by the surfactant cetrimonium bromide are presented. From these isotherms, the saturation adsorptions of surfactant for various surface concentrations of cholesterol were calculated. Plots of adsorption against the inverse of the area per cholesterol molecule revealed two linear revealed two linear regions, corresponding to penetration at high and low monolayer areas. At high monolayer areas, surfactant adsorption into accessible areas of the surface was similar to adsorption at a monolayer-free surface. In this region, packing of cholesterol and surfactant molecules on the surface lowered the effective cross-sectional area of the cholesterol molecule. However, at low monolayer areas, adsorption was determined by the size of the surfactant ion and the effective area of the cholesterol molecule was equal to the collapse area of a pure cholesterol monolayer.
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PMID:Equilibrium penetration of monolayers VI: cholesterol-cetrimonium bromide system. 67 Dec 36

A delta pH 6 proton (internal pH 7.5, external pH 1.5) was imposed across the bilayer of egg phosphatidylcholine (PC) vesicles. The presence of the gradient was verified by the use of agents that predictably collapsed it. These agents included a detergent (Triton X-100), a pore-forming Na-H+ exchanger (nigericin), and weak acids capable of shuttling protons across the membrane (acetic and acetylsalicylic acids). The magnitude of the proton gradient and the rate of pH gradient collapse were determined by measuring pyranine fluorescence by use of an isosbestic point technique and calculating intravesicular pH from a calibration curve. Acid flux into the vesicles in the presence of chloride was measured directly by a simpler one-wavelength method. p-Xylene-bis-pyridinium bromide (DPX), a pyranine fluorescence quencher, was used to verify that the fluorescence signal originated from within the vesicles and that the observed rates of acidification were not an artifact due to pyranine leakage. Acid flux was found to be dependent on the ionic composition of the buffer. The presence of chloride ion in the external compartment caused a dramatic increase in the rate of acidification. It was demonstrated that the kinetics of acid flux into the vesicle were clearly controlled by the chloride-dependent mechanism at a relatively low chloride concentration of 2.5 mM. The model presented here is a simple and sensitive method for investigating the factors that influence acidification rates across membranes when acidification is driven by extremely large proton gradients, in the presence of chloride; conditions similar to those found in the stomach.
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PMID:Vesicle acidification driven by a millionfold proton gradient: a model for acid influx through gastric cell membranes. 131 Feb 22

An in vitro test procedure capable of discriminating effectively between intact and membrane-damaged cells has been developed. This procedure utilizes fluorescein diacetate and ethidium bromide as fluorescent probes. The properties of the probes and the collapse in the selective cytoplasmic membrane permeability barrier of the damaged cells ensure the principal feature of the test procedure, that functional cells fluoresce bright green, but membrane-damaged cells fluoresce bright red. Investigations with natural rubber, silicone and acrylic polymers confirmed the suitability of the procedure to distinguish between materials on the basis of cytotoxicity.
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PMID:Biocompatibility assessment: application of fluorescent probe response (FPR) technique. 179 47

Elevation and hardening of the fertilization envelope (FE) occur within 15 min following insemination of the sea urchin egg. When chloride ions were replaced in the media with various anion substitutes, including methyl sulfonate, nitrates, bromide, and isethionate, the fertilization envelope failed to harden and collapsed back to the surface of the egg of Lytechinus variegatus, L. pictus, and Strongylocentrotus purpuratus. At the light microscopy level, the collapse of the envelope was accompanied by a decrease in birefringence, compared with controls. When examined with electron microscopy, the FEs of eggs inseminated in reduced Cl- solutions failed to transform from an amorphous layer into the more robust laminar structure observed around eggs incubated in normal sea water. Furthermore, in the case of S. purpuratus, the I-T transformation of the FE did not occur. When transfer of the inseminated eggs from the Cl- -deficient sea water to normal sea water was carried out before 10 min elapsed, the envelope did not collapse, and the birefringence of the envelope was similar to that of controls. Partial envelope collapse was also observed in a dose-dependent manner, varying with the concentration of the Cl- in the sea water solution. The results suggest that lack of Cl- in the media may interfere with proper fertilization envelope assembly. Possible mechanisms, including proper incorporation of the cortical granule exudate into the nascent envelope structure, are discussed.
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PMID:Fertilization envelope assembly in sea urchin eggs inseminated in Cl- deficient sea water: I. Morphological effects. 322 26

The ATP-dependent proton uptake by chromaffin granule membranes, lysosomes, and synaptosomes was examined. In synaptosomes the reaction was absolutely dependent on the presence of chloride, while in chromaffin granules chloride had a profound effect and in lysosomes only a minor effect. The presence of chloride markedly increases the rate of collapse of delta pH by carbonyl cyanide p-trifluoromethoxyphenylhydrazone in all three organelles. Ascorbate with phenazine methosulfate uncoupled the ATP-dependent proton uptake by chromaffin granules, but had no effect on lysosomes and synaptosomes. Proton uptake by submitochondrial particles was about 50-fold more sensitive to dicyclohexylcarbodiimide than the proton uptake by chromaffin granule membranes. Chromaffin granule membranes were treated with 2 M sodium bromide to inactivate the mitochondrial ATPase. The treatment caused a complete inhibition of the ATP-dependent proton uptake. Solubilization of these membranes by sodium cholate, followed by reconstitution by cholate dilution revealed the ATP-dependent proton uptake of the system. It is concluded that the genuine ATPase enzyme of chromaffin granules is a proton translocator.
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PMID:ATP-driven proton fluxes across membranes of secretory organelles. 661 37

Incubation of a strain of Escherichia coli K12 with 25 mM-methyl methanesulphonate (MMS) for 1 h changed the sedimentation coefficient of the nucleoids from 1600S to 850S. When isolated nucleoids were treated with MMS under identical conditions in vitro there was no change in the sedimentation coefficient. Alkaline sucrose-gradient centrifugation of DNA from cells treated with 25 mM-MMS for 1 h indicated that there were approximately 100 breaks plus apurinic sites per chromosome. Titration with ethidium bromide of nucleoids from MMS-treated cells showed that almost all supercoiling had been lost, suggesting that the breaks plus apurinic sites consisted mostly of breaks. Further experiments showed that the apurinic sites were probably created by non-enzymic depurination and that little non-enzymic strand breakage had occurred. The depurinated sites thus created could then serve as substrates for the apurinic-specific endonucleases of the cell, with the result that strand breakage occurred. MMS treatment did not cause any changes in the DNA:RNA ratio of the nucleoids. Removal of MMS followed by a period of incubation resulted in a decrease in the number of breaks plus apurinic sites and an increase in the sedimentation coefficient of the nucleoids. After 2 h incubation in MMS-free medium the sedimentation coefficient of the nucleoids from MMS-treated cells was the same as that of the control; the supercoiling was also partially restored. The effect of MMS on two MMS-sensitive mutants of E. coli, one a polA and the other a recA mutant, was also studied. In both cases MMS caused complete collapse of the nucleoid structure.
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PMID:Effect of methyl methanesulphonate on the nucleoid structure of Escherichia coli. 703 62

We have shown previously that mutations in nuclear genes, termed MGI, for mitochondrial genome integrity, can convert the petite-negative yeast Kluyveromyces lactis into a petite-positive form with the ability to produce mitochondrial genome deletion mutants (Chen and Clark-Walker, Genetics, 133, 517-525, 1993). Here we describe that two genes, MGI2 and MGI5, encode the alpha- and gamma-subunits of mitochondrial F1-ATPase. Specific mutations, Phe443-->Ser and Ala333-->Val in MGI2, and Thr275-->Ala in MGI5, confer on cells the ability to produce petite mutants spontaneously with deletions in mitochondrial (mt) DNA and the capacity to lose their mitochondrial genomes upon treatment with ethidium bromide. Structural integrity of the F1 complex seems to be needed for expression of the Mgi- phenotype as null mutations in MGI2 and MGI5 remove the ability to form mtDNA deletions. It is suggested that mgi mutations allow petites to survive because an aberrant F1 complex prevents collapse of the mitochondrial inner membrane potential that normally occurs on loss of mtDNA-encoded F0 subunits.
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PMID:Specific mutations in alpha- and gamma-subunits of F1-ATPase affect mitochondrial genome integrity in the petite-negative yeast Kluyveromyces lactis. 762 39

The labial gland of Manduca sexta is a valuable system to study the mechanisms of programmed cell death since the death of the gland is nearly synchronous and, except for the anterior duct, involves all of the tissue. The gland degenerates in 5 days during pupation. Our previous work documents a drop in total protein synthesis as the gland degenerates. To evaluate potential causes of this altered protein synthesis, we monitored several parameters of metabolism in dying cells: levels of adenosine triphosphate to estimate the energy resources of the gland; reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide to assess mitochondrial respiration; levels of acid phosphatase to assay lysosomal enzyme activity; and concentrations of cyclic nucleotides and inositol triphosphate to monitor signaling. While protein synthesis fell precipitously on day 0, total adenosine triphosphate and mitochondrial respiration were unchanged until the cells underwent massive collapse on day 3. Lysosomal acid phosphatase increased during early metamorphosis, and ultimately the bulk of the cytoplasm was destroyed in autophagic vacuoles. Changes in the concentrations of second messengers were modest and late. The relationships between the metabolism and the collapse of the labial gland are under investigation.
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PMID:Metabolic events during programmed cell death in insect labial glands. 765 33

DNA molecules collapse into compact structures in the presence of multivalent cations. To probe the possible importance of supercoiling and conformational effects, pUC18 plasmids (2686 bp) were modified by inserting 12-bp and 20-bp alternating d(CG)n sequences, which are capable of converting to a left-handed Z-conformation under appropriate conditions, into the polycloning region. Condensation was induced by rapid addition of hexaammine cobalt(III) [Co(NH3)6(3+)] and monitored by laser light scattering and electron microscopy. Light scattering shows that plasmids with longer d(CG)n inserts condense more extensively at natural superhelical densities. Electron microscopy indicates that the morphological distribution of condensed d(CG)n-containing plasmids changes as a function of Co(NH3)6(3+) concentration. At lower Co(NH3)6(3+) concentration, the proportion of rods is higher, and at higher Co-(NH3)6(3+) concentration, most of the condensates have the form of toroids. In addition, the inner radii of the toroids are much smaller relative to condensed pUC18 under the same conditions. Enzymatic analysis and chemical probing show that the d(CG)n inserts in naturally supercoiled plasmids have extensively converted from B-form to Z-form in the presence of Co(NH3)6(3+) at the upper range of concentrations under which condensation occurs. To determine whether the enhanced condensation of d(CG)n-containing plasmids results from the change of superhelical density due to the B-Z transition, we treated wild-type pUC18 molecules with topoisomerase I and varying amounts of ethidium bromide to generate a range of supercoil densities. Light scattering indicates that supercoiling did not affect the condensation process.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Condensation of plasmids enhanced by Z-DNA conformation of d(CG)n inserts. 789 47


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