Conformational transition of insulin induced by n-alkyltrimethylammonium bromides in aqueous solution

A surfactant-induced conformational transition of bovine insulin has been detected by difference spectroscopy for a homologous series of n-alkytrimethylammonium bromides, chain length C10-C16 at pH 10.0, 25 degrees C. The transition was followed as a function of surfactant concentration by absorbance measurements at 275 nm and the data were analysed to obtain the Gibbs energy of the transition in water (delta Gw degree) and in a hydrophobic environment (delta Ghc degree) for saturated protein-surfactant complexes. A value of delta Gw degree of -11.8 +/- 1.8 kJ mol-1 was found independent of n-alkyl chain length, which is similar to the value found for the n-alkylsulfate-induced transition in a previous study (-14.6 +/- 3.0 kJ mol-1). The values of delta Ghc degree were in the range approximately -88 to -100 kJ mol-1 for chain lengths from C10 to C16. The values of delta Ghc degree vs. chain length for both the n-alkyltrimethylammonium bromides and the n-alkylsulfates lie on the same curve, demonstrating that delta Ghc degree is independent of the nature of the surfactant head group.

Encapsulation of vancomycin and gentamicin within cationic liposomes for inhibition of growth of Staphylococcus epidermidis

Liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol) and dimethyldioctadecylammonium bromide (DDAB). The cationic vesicles adsorb to biofilms of the skin-associated bacteria Staphylococcus epidermidis, which have a negative charge. Encapsulation of the antibacterial drug vancomycin into such liposomes enhanced its activity relative to the free agent. The effectiveness of the preparation was dependent on the fluidity of the liposomal membrane and on the level of drug entrapment within the aqueous core of the vesicles. The aminoglycoside antibiotic gentamicin was also encapsulated within similar liposomes but was less effective, possibly due to its slow passage through the membrane. The liposomal vancomycin preparation has potential medical use in treating bacterial infections of foreign body biomedical devices (e.g. catheters), with either topical or intravenous administration.

Evaluation of the Biolog system for the identification of Bacillus anthracis

The potential of the Biolog system for the identification of Bacillus anthracis was evaluated. In-house generated databases allowed the correct identification of 19 of 20 isolates of B. anthracis within 24 h. Five strains of the closely related B. cereus/thuringiensis group were misidentified as B. anthracis. For this reason the test could only serve as a primary screen with further testing being required to confirm identity. In addition 20% of all the strains of bacilli examined during the study gave unreadable reaction profiles due to false-positive reactions.

The surface properties of phospholipid liposome systems and their characterisation

The field of liposome (vesicle) research has expanded considerably over the last 30 years. In physical chemical terms liposomes have many of the characteristics of colloidal particles and their stability is determined in part by the classical surface forces. It is now possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics. The surfaces of liposomes can be modified by the choice of bilayer lipid as well as by the incorporation and covalent linkage of proteins (e.g. antibodies and sugar binding proteins [lectins]), glycoproteins and synthetic polymers. Much of the impetus for liposome design has come from their potential value as drug delivery systems. The development of technologies for the production of such a range of liposome systems has presented interesting problems in the characterisation of their properties. The review addresses the progress that has been made in characterising the surfaces of different types of liposomes with specific reference to their electrophoretic properties and their interpretation and the physical interactions between liposomal bilayers.

The characterization of liposomal glucose oxidase electrodes for the measurement of glucose

Many biosensors have been described for the measurement of glucose in order to monitor diabetic patients. Glucose oxidase has been used commonly in the construction of glucose sensors but the performance of this enzyme is limited by enzyme saturation kinetics, which restrict the measurement of clinically relevant glucose concentrations (0 to 25 mM). Diffusion limiting membranes have been described that result in the exposure of the enzyme to lower concentrations of glucose than are present in the bulk test solution. Recently a liposomal enzyme electrode was reported whereby glucose oxidase was encapsulated within liposomes so that the lipid bilayer was the diffusion limiting membrane. It was shown that the electrode response was defined by the lipid constituents of the liposome, and that a linear response to glucose could be achieved up to 40 mM. This paper describes research undertaken to improve the methods of production of a liposomal enzyme electrode. Improved immobilization of liposomes is demonstrated with the use of poly-L-lysine solution. The variation in electrode response with respect to the amount of glucose oxidase liposomally encapsulated is reported. The new method allows a greater number of sensors to be produced from a single batch of liposomes. Studies also show the biofouling effects of the lipid constituents of ruptured liposomes on the response of the electrode to glucose over time.

The targeting of phospholipid liposomes to bacteria

Phospholipid liposomes have been prepared from phospholipid mixtures including dipalmitoylphosphatidylcholine/phosphatidylinositol (DPPC/PI) and DPPC/dipalmitoylphosphatidylglycerol (DPPC/DPPG) mixtures and targeted to adsorbed biofilms of the skin-associated bacteria Staphylococcus epidermidis and Proteus vulgaris and the oral bacterium Streptococcus sanguis. The effects of time, liposome concentration and density of bacteria in the biofilm have been studied in detail for Staphylococcus epidermidis. The targeting (as assessed by the apparent monolayer coverage of the biofilms by liposomes) to the biofilms was found to be sensitive to the mol% of PI and DPPG in the liposomes and optimum levels of PI were found for targeting to each bacterium. The use of PI and DPPG-containing liposomes for the delivery of the bactericide, Triclosan, to biofilms of Staphylococcus epidermidis was studied as a function of the amount of Triclosan carried by the liposomes. All the liposome systems tested inhibited the growth of bacteria from the biofilms after brief (2 min) exposure to Triclosan-carrying liposomes. At low Triclosan levels bacterial growth inhibition by Triclosan-carrying liposomes exceeded that by an equivalent level of free Triclosan. After short periods (min) of exposure of biofilms to Triclosan-carrying liposomes the bactericide was shown to preferentially concentrate in the biofilms relative to its liposomal lipid carrier. The results suggest that phospholipid liposomes with appropriately chosen lipid composition have potential for the targeting and delivery of bactericide to bacteria.

Polyhydroxy-mediated interactions between liposomes and bacterial biofilms

A theoretical model has been developed for the interaction of the surface polymers of the bacterial glycocalyx with liposomes incorporating lipids with polyhydroxy headgroups such as phosphatidylinositol (PI). The theory is based on a lattice model and equations are derived for the potential energy of interaction between the surfaces of a bacterium and a liposome as a function of their separation. It is shown that a relatively small energy of interaction, less than that of a single hydrogen bond, between the polyhydroxyl headgroup of the liposomal lipid and bacterium surface polymer residues could give rise to a potential energy of interaction in excess of the classical double layer repulsive force and attractive dispersion force interactions. The most important prediction of the theory is that the potential energy of interaction goes through a minimum as a function of the polyhydroxy lipid (PI) concentration in the liposomal surface, thus predicting an optimal liposomal composition for adsorption of liposome to bacterium. This result is in concordance with the adsorption of dipalmitoylphosphatidylcholine-PI liposomes to a range of biofilms of oral and skin-associated bacteria on solid supports, where optimum levels of PI for adsorption have been found. The theory demonstrates that subtle changes in the composition of liposomal and bacterial surfaces involving relatively small interaction energies can markedly influence the nature of their interactions.

The use of phospholipid liposomes for targeting to oral and skin-associated bacteria

Phospholipid (dipalmitoylphosphatidylcholine (DPPC) plus phosphatidylinositol (PI)) proteoliposomes with surface bound lectins (succinylated concanavalin A (s con A) and wheat germ agglutinin (WGA)) have been prepared covering a range of size and surface density of lectin. Negatively charged phospholipid liposomes from DPPC-PI mixtures covering a range of PI mole % and positively charged liposomes from DPPC-cholesterol-stearylamine (SA) mixtures covering a range of SA mole % have been prepared. The targeting of the liposomes and proteoliposomes to a range of oral and skin-associated been prepared. The targeting of the liposomes and proteoliposomes to a range of oral and skin-associated bacterial biofilms has been investigated. The oral bacteria Streptococcus mutans and gordonii and the skin-associated bacterium Coryneform hofmanni can be targeted with s con A bearing proteoliposomes while the skin associated bacterium Staphylococcus epidermidis can be targeted with WGA bearing proteoliposomes. Both oral and skin-associated bacteria can be targeted with positively charged liposomes although the extents of adsorption to the biofilm are low except for Staphylococcus epidermidis. In the case of negatively charged liposomes targeting is critically dependent on the PI content of the liposomes and for all the bacteria studied optimum levels PI for targeting have been found. The adsorption of the oral bacterium Streptococcus gordonii to immobilised monolayers having the optimum PI level for adsorption has been studied by total internal reflection microscopy (TIRM). Both the phospholipid and proteoliposomes have been used to deliver the bactericide Triclosan to biofilms. All the systems studied inhibited bacterial growth to varying degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of glucose oxidase with ionic surfactants: a microcalorimetric study

The enthalpies of interaction of glucose oxidase at 25 degrees C with a homologous series of n-alkyltrimethylammonium bromides (TABs) at pH 10 and a homologous series of n-alkylsulfates at pH 3.2 have been measured by microcalorimetry. For the n-dodecyl member of each series, DTAB and sodium n-dodecylsulfate (SDS), the binding of the surfactants to glucose oxidase as measured by equilibrium dialysis has been used in combination with the enthalpy data to obtain the Gibbs energy (delta Gv), enthalpy (delta Hv) and entropy (delta Sv) of binding per surfactant molecule as a function of the number of surfactant molecules bound (v). The thermodynamic parameters for the glucose oxidase interaction with DTAB at pH 10 and SDS at pH 3.2 are very similar and show that the interactions are entropically driven. The observed enthalpies of interaction of glucose oxidase with the homologous n-alkylsulfates have been analysed in terms of the interactions between the anionic surfactant head group and cationic sites on the protein, hydrophobic binding and the thermal contributions arising from protein unfolding. At surfactant concentrations of 0.5 c.m.c., the enthalpy of unfolding of glucose oxidase is estimated to be 3610 +/- 560 kJ mol-1.

Interaction between sodium n-undecyl sulfate and insulin

The binding of sodium n-undecyl sulfate with bovine insulin was studied at pH 3.2 and 10 by equilibrium dialysis at 25 degrees C. The binding data have been used to determine the Gibbs energies of interaction using the theoretical model of the Wyman binding potential. The curves of Gibbs energies as a function of the number of bound ligands (v) tend to limiting values of around -14 kJ mol-1 at high values of v. The enthalpies of interaction have been measured directly by microcalorimetry showing an increase of exothermicity at lower pH. The results have been compared with similar data for the interaction of anionic surfactants with insulin.

The targeting of immunoliposomes to tumour cells (A431) and the effects of encapsulated methotrexate

Immunoliposomes have been prepared from lipid mixtures of dipalmitoylphosphatidylcholine, wheat germ phosphatidylinositol and a reactive lipid (the m-maleimidobenzoyl-N-hydroxysuccinimide derivative of dipalmitoylphosphatidylethanolamine) which was conjugated to the N-succinimidyl-S-acetylthioacetate (SATA) derivative of a monoclonal antibody (H17E2) raised to human placental alkaline phosphatase (PLAP). The immunoliposomes were prepared by the extrusion technique (VETs) and by reverse phase evaporation (REVs) and were found to effectively target to immobilised PLAP and to PLAP or PLAP-like enzyme on the surface of a tumour cell line (A431) using an ELISA and autoradiography. The extent of binding to immobilised PLAP was a function of immunoliposomal lipid concentration, the weight-average number of antibody molecules per liposome (Pw) and the liposome size. The effectiveness of methotrexate-loaded immunoliposomes in inhibiting the proliferation of A431 cells was investigated relative to equivalent levels of the free drug. The immunoliposomes prepared by the extrusion technique (VETs) inhibited growth of A431 cells but had no effect on the growth of a normal human fibroblastic cell line. Immunoliposomes prepared by reverse phase evaporation (REVs) were less effective in inhibiting A431 cell proliferation. The immunoliposomes probably enter the tumour cells largely by receptor-mediated endocytosis although other mechanisms of uptake cannot be excluded.

Targeting and delivery of bactericide to adsorbed oral bacteria by use of proteoliposomes

Proteoliposomes having surface-bound succinylated concanavalin A (s-conA) have been prepared from a range of phospholipid mixtures by sonication (SUV) and reverse phase evaporation (REV) covering a range of size (weight-average diameter (dw)) from approx. 35 to 310 nm and weight-average number of protein molecules per liposomes (Pw) from approx. 50 to 3000. The targeting of the proteoliposomes to adsorbed biofilms of the bacteria Streptococcus sanguis and Streptococcus mutans has been assessed from the extent of inhibition of an enzyme-linked immunosorbent assay (ELISA) for bacterial cell surface antigens. The surface-bound lectin enhances targeting relative to 'naked' liposomes of comparable concentration by factors of 2-50 depending on the liposomal lipid composition and Pw. The effect of the bactericide Triclosan on the thermal properties and permeability characteristics of liposomes has been studied. At and above a molar ratio of Triclosan to lipid of 0.6, Triclosan eliminates the gel to liquid-crystalline phase transition in dipalmitoylphosphatidylcholine (DPPC) containing liposomes and increases the bilayer permeability of both liposomes and proteoliposomes to D-glucose. The proteoliposomes have been used to deliver Triclosan to S. sanguis biofilms and the inhibition of growth of the bacteria after treatment with liposomally delivered Triclosan has been determined using a microtitre plate re-growth assay and compared with growth inhibition by 'free' Triclosan. It is shown that for short exposure times (1 to 2 min) proteoliposomally delivered Triclosan is a more effective growth inhibitor than free Triclosan. The results are discussed in terms of the targeting, retention and subsequent release of Triclosan into the bacterial biofilms.

Freeze-thaw effects on the detection of blood group substances in detergent extracts by enzyme-linked immunosorbent assays (ELISA)

The sensitivity of an enzyme-linked immunosorbent assay (ELISA) for human blood group antigens extracted from blood stains with the surfactant, n-octyl-beta-D-glucopyranoside (OBG), at concentrations below the critical micelle concentration can be increased by the introduction of a single freeze-thaw step. The ELISA signals increase from 3- to 4-fold for OBG extracts of 80 nl bloodstains. The ELISA signal enhancement occurs irrespective of the age of the bloodstains, at least for bloodstains up to 1 year old. The origin of the effect has been investigated and its is demonstrated that the freeze-thawing cycle increases the extent of adsorption of the blood group determinants in OBG-solubilized complexes onto microtitre plates. Gel filtration has been used to analyse the composition of OBG extracts of bloodstains in terms of the carriers of the blood group substances, protein and phospholipid in fresh and freeze-thawed extracts. It was found that freeze-thawing alters the distribution of blood group active material in the lipid-protein OBG complexes leading to a greater proportion of blood group active material in higher molecular weight complexes. The freeze-thaw effect is eliminated on the addition of a cryoprotectant, such as glycerol, and the factors which contribute to changes in the microstructure of OBG extracts on freeze-thawing are discussed.

The effects of surfactants on the permeability of isolated perfused fish gills to urea

1. The diffusional transfer capacity of [14C]urea in isolated perfused trout (Oncorhynchus mykiss) gills in the presence of sodium n-dodecylsulphate (SDS), n-dodecyltrimethylammonium bromide (DTAB) and p-t-octylbenzene oxyethylene10 (Triton X-100) has been measured over a range of surfactant concentrations. 2. Urea has been shown to be transported transcellularly through the respiratory cells of the gill secondary lamellae by passive diffusion. Each surfactant was found to markedly increase the rate of diffusion and the diffusional transfer capacity reached a steady-state at a particular surfactant concentration. 3. The steady state flux was increased by surfactant in the sequence DTAB greater than SDS greater than Triton X-100 and the surfactant concentrations in terms of the critical micelle concentration (CMC) at which the diffusional transfer capacities reached limiting values were 0.92 x CMC (SDS), 0.53 x CMC (DTAB) and 2.5 x CMC (Triton X-100). 4. Compared to interactions between isolated epithelial cells and the surfactants, the rates at which the surfactants changed the urea flux were slow, suggesting that the mucus layer plays a significant role in protecting the epithelial cells of the secondary lamellae from disruption. 5. Relative to the other surfactants, DTAB had the most marked effect on both the rate of flux change and on the magnitude of the change, at concentrations which are low relative to the CMC, suggesting a more specific interaction with the negatively charged mucus layer consistent with the toxic effects of quaternary ammonium compounds on aquatic organisms.

The thermal behaviour of phosphatidylcholine-glycophorin monolayers in relation to monolayer and bilayer internal pressure

A thermodynamic relationship which allows calculation of the internal pressure (Pi) of a monolayer has been derived, viz: Pi = T(delta pi/delta T)A-pi. Surface pressure (pi)-area (A) isotherms were determined for dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC) and a mixture of the latter with 0.28 mol% glycophorin, an intrinsic membrane protein. The isotherms (10 degrees C-42 degrees C) were used to obtain values of (delta pi/delta T)A. Under conditions in which a monolayer is believed to most closely model a bilayer membrane (i.e. 37 degrees C and A = 0.6 nm2/molecule) internal pressures were: DOPC = 0.32 N m-1; DPPC = 0.13 N m-1 and glycophorin/DPPC = 0.36 N m-1. The results do not support some measurements on amphiphile solubility in natural membranes which had been interpreted as evidence of a large increase in internal pressure, due to the intrinsic membrane proteins.

Bacillus anthracis but not always anthrax

Gram-positive bacilli isolated during epidemiological investigations which, on the basis of conventional tests, resemble Bacillus anthracis but which fail to produce the capsule or to induce anthrax in test animals have long been dismissed in clinical and veterinary laboratories as B. cereus or simply as unidentified Bacillus spp. and thereupon discarded as inconsequential. In this study, the application of newly available DNA probe, polymerase chain reaction and specific toxin antigen detection technology has revealed that a proportion of such strains are B. anthracis which lack the plasmid carrying the capsule gene (pXO2). While these techniques cannot, of course, be used to confirm the identities of strains resembling B. anthracis but which also lack the plasmid carrying the toxin genes (pXO1), the likelihood that these also are bona fide B. anthracis becomes more acceptable. (As yet no naturally occurring pXO1-/2+ strains have been found.) At this point, the significance of the presence of such avirulent forms of B. anthracis in specimens can only be a subject for speculation, but the possibility that they may be indicators of virulent parents somewhere in the system being examined must be considered.

A liposomal enzyme electrode for measuring glucose

Enzyme electrodes have been described for measuring glucose but have been limited by the saturation kinetics of the glucose oxidase not allowing clinically relevant glucose concentrations to be measured (0-25 mM). One way of alleviating this problem is to use diffusion-controlled membranes which result in the enzyme experiencing a smaller substrate concentration than that of the bulk solution. As an extension of this concept we have encapsulated glucose oxidase in liposomes whereby the lipid bilayer wall provides the diffusion-limiting membrane as well as providing a biocompatible layer which is of particular relevance when blood glucose is to be measured. Linear ranges were found to embrace the required glucose concentrations and moreover by using liposomes prepared from different lipids, e.g., dimyristoyl (14:0) phosphatidylcholine (DMPC), dipalmitoyl (16:0) phosphatidylcholine (DPPC) and distearoyl (18:0) phosphatidylcholine (DSPC), the electrode response was shown to depend on the bilayer permeabilities in relation to the lipid phase transition temperatures and as a consequence the linear ranges were duly altered.