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Biomedical subjects

R Bittman

Publications and source records attributed to R Bittman.

At least 127 records · Page 7Linked to original sources
PubMed1978

Cholesterol distribution and movement in the Mycoplasma gallisepticum cell membrane.

The time course and extent of transfer of [14C]-cholesterol from resting Mycoplasma gallisepticum cells or membrane preparations to high-density lipoproteins were studied. More than 90% of the total cholesterol in isolated, unsealed membrane preparations was exchanged in a single kinetic process. In intact cells, however, cholesterol exists in two different environments. Cholesterol in one environment, representing approximately 50% of the total unesterified cholesterol, is readily exchanged with the cholesterol of high-density lipoproteins, with a half-time of about 4 h at 37 degrees C. The rate of exchange of [14C]cholesterol from the other environment was exceedingly slow, with a half-time of about 18 days. The fraction of the total cholesterol in the readily exchangeable cholesterol pool in intact cells increased somewhat upon aging of the culture. Electron spin resonance spectra of nitroxide-labeled stearic acids incorporated into membranes of M. gallisepticum cells indicated increased rigidity at the late exponential phase of growth. These results suggest that cholesterol is present in approximately equal concentrations on both surfaces of the M. gallisepticum membrane and that in resting cells the rate of movement of cholesterol molecules from the inner to outer halves of the lipid bilayer is exceedingly slow or nonexistent.

Cell Membrane↗
PubMed1978

Sterol-polyene antibiotic complexation: probe of membrane structure.

Polyene antibiotics are useful tools for studying the role of sterols in biological membranes. The interaction of polyene antibiotics with membrane-bound sterols in artificial membrane systems, prokaryotic and eukaryotic cells, and lipid-containing viruses is reviewed. The pentaene macrolide, filipin, is shown to serve as a probe of phosphatidylcholine-sterol interaction and of the localization of cholesterol in the membrane of mycoplasmas.

Amphotericin B↗
PubMed1977

Kinetics of association of amphotericin B with vesicles.

Amphotericin B associates with vesicles prepared from phosphatidylcholines. The influence of lipid composition on the initial rate of amphotericin B association with vesicles was examined using stopped-flow kinetic measurements. A relationship was found between the tightness of packing of phosphatidylcholine molecules in the vesicles and the initial rate of amphotericin B association. Shortening of the fatty acyl chain length of saturated phosphatidylcholines and increasing the number of double bonds in the fatty acyl chains of unsaturated phosphatidylcholines enhance the initial rate, whereas addition of cholesterol of the bilayers reduces the rate. The initial rate of association with phosphatidylcholine-sterol vesicles follows the order, thiocholesterol greater than androst-5-en-3beta-ol greater than epicholesterol greater than ergosterol greater than cholesterol and is thus inversely related to the order of phospholipid-sterol affinity, as revealed by permeability, surface area, and magnetic resonance measurements. These data suggest that the initial rate of amphotericin B uptake into vesicles depends on competition between lipid-lipid and amphotericin-lipid interactions.

Amphotericin B↗
PubMed1977

Binding properties of neutral diamide ligands for alkaline-earth cations.

The complexation of a series of aromatic and alicyclic N,N,N',N'-tetra-n-propyl amides of 1,2-ethylenedioxydiacetic acids with group IIA metal-ion bromides in anhydrous methanol was investigated by ultraviolet absorption spectroscopy. These synthetic ligands were previously found to show selectivity toward divalent over monovalent cations with respect to extraction of ions into bulk organic phase (Borowitz, I.J., Lin, W-O., Wun, T-C., Bittman, R., Weiss, L., Diakiw, V., and Borowitz, G.B. (1977), Tetrahedron, in press). At low concentrations, ligands bearing benzene and naphthalene rings form 1:1 ligand to divalent cation complexes with each of the alkaline-earth metals, but ligands in the cyclohexyl series are stoichiometrically bound to cations in more than one type of complex. Binding isotherms obtained by Scatchard analysis and by the method of continuous variation revealed ligand to divalent ion mole ratios of 2:1, 3:2, and 4:3 for binding of N,N,N',N'-tetra-n-propyl-cis-1,2-cyclohexanedioxydiacetamide with Ca2+, Sr2+, and Ba2+, respectively. In contrast, Scatchard analysis of ultraviolet spectral changes showed that a 1:1 complex is formed between this ligand and Na+ with an apparent association constant of 56 +/- 2M-1; the constant for binding with K+ was smaller (11 M-1). The order of apparent association equilibrium constants for complexation of group IIA cations with this series of neutral ligands was Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+; for example, for N,N,N',N'-tetra-n-propyl-1,2-phenylenedioxydiacetamide the apparent binding constants at 25 degrees C were 7.33 +/- 0.25 X 10(4) M-1 for Ca2+, 1.23 +/- 0.03 X 10(4) for Sr2+, 4.42 +/- 0.09 X 10(3) for Ba2+, and 4.04 +/- 0.24 X 10(2) for Mg2+. The divalent cation binding properties of these synthetic diamide ligands are discussed in relation to those of other synthetic ligands and of two naturally occurring ligands.

Azo Compounds↗
PubMed1977

Ionophorous properties of neutral diamide ligands toward calcium.

The ability of a series of aromatic and alicyclic analogues of 1,2-ethylenedioxydiacetic acids bearing N,N,N',N'-tetra-n-propyl amide or N-methyl-N-carbethoxypentyl amide linkages to enhance the rate of 45Ca2+ efflux from vesicles was studied. The ligands were less potent in enhancing membrane permeability to Ca2+ than A23187 and X537A. Lipid-soluble anions markedly increased the rate and extent of Ca2+ transport mediated by these neutral ligands. The abilities of these synthetic diamide ligands and naturally occurring ionophores to transport Ca2+ across bilayer membranes were sensitive to the lipid composition of the vesicle. The mechanism of Ca2+ transport mediated by this series of synthetic ligands is discussed.

Azo Compounds↗
PubMed1977

Effects of filipin on the structure and biological activity of enveloped viruses.

The interaction of the polyene antibiotic filipin with membrane-bound cholesterol in vesicular stomatitis (VS), influenza, and Rauscher leukemia virions was studied. Exposure of virions to filipin resulted in a series of depressions and ridges in the envelope of VS virions, with a periodicity of 15 to 20 nm perpendicular to the long axis of the particle; similar morphological alterations were observed in negatively stained preparations, in thin-sectioned virions, and in protease-treated virions that lack surface glycoproteins. This morphological effect was specific for filipin, since the envelopes of VS virions that had been treated with another polyene antibiotic, amphotericin B, exhibited markedly different morphology. Morphological alterations induced by filipin in influenza and Rauscher leukemia virions differed from those seen in VS virions. The infectivity of filipin-treated VS virions was reduced up to 500-fold, whereas influenza virions were resistant to filipin treatment. Incorporation of filipin into the virions was demonstrated, and no release of either lipids or proteins from virions was detected after filipin treatment. A stoichiometry of approximately 1 mol of bound filipin per mol of cholesterol was found in both intact and protease-treated VS virions. The equilibrium dissociation constant for filipin-cholesterol interaction was approximately 74-fold larger in intact than in protease-treated VS virions. The initial rate of association of filipin with cholesterol in intact virions was slower than that in protease-treated particles. The fluidity of lipids in VS viral membranes, as probed by a stearic acid derivative spin label, was markedly reduced when either intact or protease-treated virions were treated with filipin.

Amphotericin B↗
PubMed1976

Permeability properties of the membrane of vesicular stomatitis virions.

Observations of the light-scattering properties of several enveloped viruses indicate that virions (vesicular stomatitis, SV5 and influenza), in common with other membrane systems, are osmotically active, responding to NaCl gradients by swelling in hypo-osmolar solutions and shrinking in hyperosmolar solutions. The permeability barrier responsible for this osmotic response in vesicular stomatitis virions was modified both by protease treatment to remove the viral glycoprotein and by treatment with the polyene antibiotic filipin, an agent known to interact with cholesterol in liposomes and membranes. Filipin altered the kinetic and equilibrium permeability behavior of virions but the extent of leakage of osmotic shocking agent was less than that in lecithin/cholesterol and lecithin/ergosterol liposomes and in ergosterol-containing ciliary membranes. Negative-staining electron microscopy revealed that filipin treatment caused structural changes in the viral membrane. Intact virions exhibited appreciably larger responses to osmotic change than did protease-treated virus particles. Thus, the osmotic barrier in intact vesicular stomatitis virions may not be exclusively lipid in nature.

Kinetics↗
PubMed1976

Synthesis of DL-2,3-diacyloxypropylphosphonyl-cholines from DL-2,3-diacyloxyiodopropanes.

The chemical synthesis of racemic diacyloxypropylphosphonylcholines having octanoyl, myristoyl, oleoyl and stearoyl groups is described. The route involved reaction of dioactanoyloxy-dimyristoyloxy-dioleoyloxy-, and distearoyloxpropyliodide with tris (trimethylsilyl) phosphite to yield the corresponding bis (trimethylsilyl) phosphonate. Removal of the trimethylsilyl groups by neutral aqueous hydrolysis gave the free diacylpropylphosphonic acids, which, when treated with choline toluenesulfonate, yielded the desired dioctanoyloxy-, dimyristoyloxy-, dioleoyloxy-, and distearoyloxypropylphosphonylcholines. The paper also describes the synthesis of 2-octadecyleicosylphosphorylcholine.

Chromatography, Thin Layer↗
PubMed1975

Stopped-flow kinetic studies of actinomycin binding to DNAs.

Stopped-flow kinetic studies of the association of actinomycins with narural and synthetic DNA duplexes are presented. The actinomycins examined were D (C1), D lactam (in which the pentapeptide rings are closed by lactam instead of lactone linkages), X2, XObeta, and actinomine. The DNAs used included claf-thymus DNA, PM2, DNA, and two synthetic d(A-T)-lide copolymers containing 2,6-diaminopurine (DAP) in place of adenine residues, poly[d(DAP-T)]-poly[d(DAP-T)] and poly[d(DAP-A-T]-poly[d(DAP-A-T)]. Apparent equilibrium constants indicate that the DAP-containing polynucleotides bind actinomycin strongly. Comples formation of actinomycins D, D lactam, X2 and XObeta with these DNAs can be deconvoluted into five rate processes. These steps do not necessarily proceed to completion. The rates of two of these steps display a firstorder dependence on DNA concentration. The large negative entropies of activation of these steps suggest a high degree of restriction to freedom of motion on the respective transition states. The rates of the remaining three steps are independent of DNA concentration. Kinetic parameters of actinimycin binding to DNAs are presented and suggestions are made about some of the molecular evente believed to be responsible for the appearance of the five rate processes. For example, for DNA, poly[d(DAP-A-T)], and poly[d(DAP-T)], the observed order of apparent second-order rate constants, normalized to the concentration of actinomycin binding sites, suggests that binding of the antibiotic occurs most rapidly at binding sites (G-C of d DAT-T) near d(A-T) base pairs, where weakening of the double-helical conformation requires the least energy. Results obtained from studies of actinomycin D binding to heat-denatured poly[d(DAP-A-T)] and of actinomine and actinomycin D lactam binding to DNA suggest that the slow rate processes are related to an actinomycyl-pentapeptide-induced unwinding of the sugar-phosphate backbone of DNA accompanying insertion of the cyclic peptides into DNA.

Animals↗