DIFFERENTIAL DIALYSIS

Titration calorimetric studies to elucidate the specificity of the interactions of polymyxin B with lipopolysaccharides and lipid A

Lipopolysaccharide (LPS), the major cell wall constituent of Gram-negative bacteria, evokes a multitude of biological effects in mammals including pyrogenicity and toxic shock syndrome. Polymyxin B (PmB), a polycationic cyclic peptide, is known to neutralize most of its activities. The nature of the interaction of PmB with LPS and lipid A was investigated by isothermal titration calorimetry. PmB binds to LPS as well as lipid A stoichiometrically and non-co-operatively with micromolar affinity. These interactions are driven primarily by a favourable change in entropy (delta S) and are endothermic in nature. These positive changes in enthalpies decrease with increasing temperature, yielding a heat capacity change, delta Cp, of -2385 J.mol-1.degree-1 for PmB-LPS interactions while the binding of PmB to lipid A displays a delta Cp of -2259 J.mol-1.degree-1. The negative heat capacity changes provide strong evidence for the role of hydrophobic interactions as the driving force for the association of PmB with LPS and lipid A. A correlation of the energetics of these interactions with analyses of the molecular models of PmB suggests that a cluster of solvent-exposed non-polar amino acid side-chains that line one surface of the molecule, together with a ring of positively charged residues on its other surface, are responsible for its strong and stoichiometric binding to LPS.

Flow microcalorimetric assay of antibiotics--I. Polymyxin B sulphate and its combinations with neomycin sulphate and zinc bacitracin on interaction with Bordetella bronchiseptica (NCTC 8344)

A flow microcalorimetric assay for polymyxin B sulphate has been developed which has a better reproducibility (relative standard deviation less than 3%) and sensitivity (0.35 micrograms ml-1) than conventional microbiological assays, and requires an assay time of ca. 4.5 h. The combinations with zinc bacitracin, with neomycin sulphate, and with both zinc bacitracin and neomycin sulphate indicate antagonism between these antibiotics upon interaction with Bordetella bronchiseptica (NCTC 8344). The combinations of all three antibiotics assayed were: (1) equimolar proportions; and (2) those proportions present in the commercial preparation TrisepR (ICI, Macclesfield, UK).

NMR study of the interactions of polymyxin B, gramicidin S, and valinomycin with dimyristoyllecithin bilayers

The interactions of three polypeptide antibiotics (polymyxin B, gramicidin S, and valinomycin) with artificial lecithin membranes were studied by nuclear magnetic resonance (NMR). Combination of 31P and 2H NMR allowed observation of perturbations of the bilayer membrane structure induced by each of the antibiotics in the regions of the polar headgroups and acyl side chains of the phospholipids. The comparative study of the effects of these membrane-active antibiotics and the lipid bilayer structure demonstrated distinct types of antibiotic-membrane interactions in each case. Thus, the results showed the absence of interaction of polymyxin B with the dimyristoyllecithin membranes. In contrast, gramicidin S exhibited strong interaction with the lipid above the gel to liquid-crystalline phase transition temperature: disordering of the acyl side chains was evident. Increasing the concentration of gramicidin S led to disintegration of the bilayer membrane structure. At a molar ratio of 1:16 of gramicidin S to lecithin, the results are consistent with coexistence of gel and liquid-crystalline phases of the phospholipids near the phase transition temperature. Valinomycin decreased the phase transition temperature of the lipids and increased the order parameters of the lipid side chains. Such behavior is consistent with penetration of the valinomycin molecule into the interior of the lipid bilayers.

The suitability of a membrane diffusion growth chamber for studying bacterial interaction

Rates of diffusion of nutrients and metabolites through 0.1 micron pore diameter polycarbonate membranes are so low that the use of membrane-separated systems to study bacterial interaction seems to have little application. Effective nutrient availability throughout the system is very dependent on membrane area/medium volume ratio. An attempt to demonstrate a known interaction in the membrane diffusion chamber was not successful.

Role of diffusion potential on the flow of angiotensin II, bradykinin and related molecules through cellophane membranes

The diffusion of electrically charged peptides (angiotensin II, bradykinin and [Suc1]angiotensin II) across tight cellophane membranes, obtained by different degrees of acetylation, shows a kinetic behaviour which was interpreted in the literature as indicative of the existence of different molecular conformations presenting slow interconversion velocities and different permeabilities across the membrane. A diffusion potential (delta psi) was found to be present across the membrane along diffusion experiments performed in low ionic strength. Upon annihilation of delta psi by chemical voltage clamping (by equally increasing the ionic strength on both bathing solutions) the diffusion rate was decreased and the flow followed first order kinetics, indicating a major role of delta psi in the process. As the ionic strength increase could also affect molecular conformation, the role of delta psi on the diffusion of those molecules was tested by fitting flux and delta psi experimental results by an integrated form of Nernst-Planck flux equation. It is concluded that the deviation from first order diffusion kinetics, observed in low ionic strength, is solely due to the diffusion potential, and not to the existence of more than one molecular conformation in aqueous solution. This study was extended to amino acids and other related charged molecules.

Preparative scale thin-film dialysis apparatus

A thin-film dialysis apparatus capable of processing large volumes of material is described. Dialysis casing, of defined molecular weight cutoff, is stretched on a compact frame producing a thin-film configuration. The dialysis unit is rotated by a variable speed motor with the direction of rotation periodically alternated by an electronic reversing control. This device utilizes the entire membrane area, eliminates solution stratification, and minimizes viscosity effects; membrane clogging is minimized should gelation or precipitation occur. The solutions are in contact only with the dialysis membrane; the construction materials are Teflon and polymethylmethacrylate (Plexiglas). At a load factor of 0.5 ml/cm of dialysis casing the half-time for solute removal, t1/2, for 8 M urea was 7 +/- 1 min; over 99% of the denaturant was removed in 1.5 h. The following operational factors are discussed: load capacity, rotation rate, membrane, ratio of internal to external solutions and solute contributions.

Hindered diffusion in microporous membranes with known pore geometry

The hindrance effect on the aqueous diffusion rate of solutes within membrane pores of molecular size has been accurately determined. Mica membranes, 3 to 5 micrometers thick, were prepared with uniform, straight pores from 90 to 600 angstroms in diameter. With these membranes a direct estimation was possible of the interaction between pore size and molecular diffusion rates. There were no uncertainties due to wide pore size distributions or nonuniform tortuous channels as in previously used model microporous materials such as dialysis tubing or gels. Aqueous diffusion rates through these mica membranes were measured for a series of compounds with molecular diameters from 5.2 to 43 angstroms and were corrected for "liquid film resistances" adjacent to the membrane-solution interface to obtain estimates of molecular diffusivities within the micropores of the membrane. Definite evidence is presented showing that, even when molecular size is a small fraction of pore size, diffusion rates decrease markedly. The apparent reduction in solute diffusivity in the microporous membrane can be quantitatively estimated by means of the Renkin equation for hindered diffusion.

Regulation of the secondary antibody response in vitro. Enhancement by actinomycin D and inhibition by a macromolecular product of stimulated lymph node cultures

Two opposite effects of actinomycin D on antibody synthesis have been studied in organ cultures of rabbit lymph node fragments. These cultures were prepared from previously primed rabbits and stimulated with antigen(s) on day 0 to yield a secondary response, whose inductive phase extended to about day 9 and whose productive phase may last for several months in the serum-free medium described here. Concentrations of actinomycin D above 0.01 microM (0.012 microg/ml) produce inhibition of antibody synthesis during both phases of the response. However, antibody synthesis is about 10 times more sensitive to inhibition by this drug when it is added during the inductive phase than during the productive phase. During the latter phase, synthesis is more rapidly terminated as the drug level approaches 10 /microM (12.5 microg/ml). At this level the 50% synthesis time is about 2.8 hr, which is identical with that found when 5-10 microM puromycin is added to the medium of parallel cultures. Transient enhancement of antibody synthesis is frequently produced by a brief exposure to low levels of actinomycin D (generally less than 0.01 microM). Enhancement appears in precise temporal association with actinomycin pulses added for 2 days or less only between days 6 and 16. This apparent enhancement of antibody synthesis resembles the increased enzyme synthesis described by Garren et al. (6) and led to a search for an antibody-inhibitory material (AIM) whose synthesis might be stopped preferentially by low levels of the drug. Stimulated lymph node cultures produce between days 6 and 15 a nondialyzable material which inhibits antibody synthesis during the productive phase of heterologous antigen-antibody culture systems. Just as enhancement with low levels of actinomycin D appears within 2 hr after the drug has been added to cultures, so inhibition occurs within 4 hr of adding AIM to cultures during their productive phase. These observations suggest that AIM is analogous to the translational "repressor" postulated by Garren et al. (6). AIM has relevance in two areas of immunology: (a) it may be the explanation for many examples of antigenic competition, and (b) it may represent a normal control mechanism for the productive phase.

Dialysis culture of microorganisms: design, theory, and results

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Semipermeable aqueous microcapsules. I. Preparation and properties

Methods have been developed for depositing thin stable semipermeable polymer membranes around aqueous microdroplets (mean diameter down to 5 μ or less) by either interfacial polymerization or interfacial coacervation. The enclosed aqueous phase may contain enzymes or other proteins, cell fragments, or intact cells. Examples of methods for preparing such microcapsules are given in detail, and some of their properties are described.

ALTERATION OF THE ACETYLATION OF SULFONAMIDES BY PROTEIN BINDING, SULFINPYRAZONE, AND SURAMIN

The following observations resulted from the studies: (1) bovine albumin interfered with the acetylation of sulfamethoxypyridazine. Apparently this was due to the binding of the sulfonamide to the protein since a similar effect was not observed with bovine globulin to which this sulfonamide was not bound. Furthermore, neither protein affected the acetylation of sulfanilamide whose binding to albumin was much less than that of sulfamethoxypyridazine; (2) in the presence of albumin, but not globulin, 2–25 times more of sulfinpyrazone and suramin, respectively, was required to inhibit the acetylation of sulfanilamide to the same extent as in the absence of the protein; (3) there appeared to be no consistent correlation among a number of sulfonamides between their binding to protein, "lipid solubility," pKa, and acetylation.