Lipid solutions

LIPID SOLUTIONS. Fat solutions (emulsions) should be handled with care to decrease the risk of separation or breaking out of the oil. Separation can be identified by yellowish streaking or the accumulation of yellowish droplets in the emulsion. Fat solutions are administered to adults at a rate no greater than 1 to 2 mL/min. 

When liposomes are prepared from a molecular mixture of lipid components it is important that all lipids be homogeneously dissolved in an organic solvent in order to obteiin bilayers with evenly distributed lipids after hydration. For example, the solubilities of phosphatidylcholine and cholesterol in chloroform are similar their solubility in benzene differs. Upon removal of benzene from the lipid solution an inhomogeneous lipid film is formed on the glass wall and... 

The formation of nitrosamines in aprotic solvents has applicability to many practical lipophilic systems including foods (particularly bacon), cigarette smoke, cosmetics, and some drugs. The very rapid kinetics of nitrosation reactions in lipid solution indicates that the lipid phase of emulsions or analogous multiphase systems can act as "catalyst" to facilitate nitrosation reactions that may be far slower in purely aqueous media (41, 53, 54). This is apparently true in some cosmetic emulsion systems and may have important applicability to nitrosation reactions in vivo, particularly in the GI tract. In these multiphase systems, the pH of the aqueous phase may be poor for nitrosation in aqueous media (e.g., neutral or alkaline pH) because of the very small concentration of HONO or that can exist at these pH ranges. 

Sugano, K., Hamada, H., Machida, M., Ushio, H. High throughput prediction of oral absorption improvement of the composition of the lipid solution used in parallel artifidal membrane permeation assay. J. Biomol. Screen. 2001, 6,189-196. 

We have recently observed in our laboratory that water washes of undamaged leaves in a number of plants contained sterols and other lipids in sufficiently high concentration comparable with concentrations used in typical laboratory bioassays. These aqueous lipid solutions are frequently accompanied by long-chain (C-12 to C-18) fatty acids. We therefore suggest that micelle formation between the lipids and fatty acids may occur. By this mechanism the lipid solubility in the aqueous medium is significantly enhanced, thus allowing the release of otherwise water-insoluble plant constituents into the environment. Presently, experiments are in progress in our laboratory to provide further evidence for the "micelle-mechanism" of allelopathlc lipids. 

Emulsifiers. Natural lecithin is one of the most widely used emulsifiers because it is metabolized in the body. However, type I allergic reaction to soybean lecithin emulsified in lipid solutions has been observed [195], Among the synthetic emulsifying agents, block copolymers of polyoxyethylene-polyoxypropylene (poloxamer) have attracted increasing interest for parenteral emulsions. Other examples of emulsifiers commonly found in parenteral formulations are given in Table 9 [190]. 

Sugano et al. [561,562] explored the lipid model containing several different phospholipids, closely resembling the mixture found in reconstituted brush border lipids [433,566] and demonstrated dramatically improved property predictions. The best-performing lipid composition consisted of a 3% wt/vol lipid solution in 1,7-octadiene (lipid consisting of 33% wt/wt cholesterol, 27% PC, 27% PE, 7% PS, 7% PI). The donor and acceptor compartments were adjusted in the pH interval between 5.0 and 7.4 [562]. With such a mixture, membrane retention is expected to be extensive when lipophilic drugs are assayed. The use of 1,7-octadiene in the assay was noted to require special safety precautions. 

An important extension of lipid-solute interaction components [20] to membrane partitioning is provided by solute molecular structure. Spacing between polar and nonpolar regions (Fig. 8) within a solute molecule may result in significant distortion of the KpDm product across the membrane polar headgroup/lipid core interface [21], Such interactions may be responsible for deviations from projected transport predictions based on simple partitioning theory translating to deviations from predicted absorption kinetics [1],... 

Recently in our group, model membrane permeation barriers have been constructed with concentrated phospholipid solutions, 10-74% wt/vol soy lecithin (approximate %w/w lipid composition 24% PC, 18% PE, 12% PI cf. Table 3.1) in dodecane, supported on high-porosity, hydrophobic microfilters. This newly formulated lipid has a net negative charge at pH 7.4, which further increases above pH 8, as the ethanolamine groups deionize. Also tested were 10% wt/vol egg lecithin lipid solutions in dodecane (approximate composition 73% PC, 11% PE,... 

Fig. 9 Surface modification of cells with ssDNA-PEG-lipid. (a) Real-time monitoring of PEG-lipid incorporation into a supported lipid membrane by SPR. (r) A suspension of small unilamellar vesicles (SUV) of egg yolk lecithin (70 pg/mL) was applied to a CH3-SAM surface. A PEG-lipid solution (100 pg/mL) was then applied, (ii) Three types of PEG-lipids were compared PEG-DMPE (C14), PEG-DPPE (C16), and PEG-DSPE (C18) with acyl chains of 14, 16, and 18 carbons, respectively, (b) Confocal laser scanning microscopic image of an CCRF-CEM cell displays immobilized FITC-oligo(dA)2o hybridized to membrane-incorporated oligo(dT)20-PEG-lipid. (c) SPR sensorigrams of interaction between oligo(dA)2o-urokinase and the oligo (dT)2o-PEG-lipid incorporated into the cell surface, (i) BSA solution was applied to block nonspecific sites on the oligo(dT)20-incorporated substrate, (ii) Oligo(dA)20-urokinase (solid line) or oligo(dT)20-urokinase (dotted line) was applied...

Measurements of pressure-area (jc-A) isotherms and transfers of monolayers on a substrate were carried out by using a computer-controlled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD-20). Maximum surface area on the trough was 475 X 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 0.5 °C). The concentration of lipid solutions was 1 mg/ml and the spreading amount of lipid solutions was 50 - 150 pi. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-i. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically with the usual manner [26,27]. 

As different tissues have different lipid compositions [112], several groups have worked on the composition of the lipid solutions to overcome these initial PAMPA issues. Sugano and coworkers investigated the influence of the chain length of the organic solvent on the permeability of a small set of compounds over a PC membrane [136], They reported an enhanced permeability with... 

Several factors influence a drug s absorption (table 3.1). Aqueous solutions are more easily absorbed than a lipid solution or solid form. Absorption of drugs in solid form is affected by the rate at which it dissolves. Higher concentrations of a drug are more rapidly absorbed than low concentrations. The amount of blood flow to the site also influences absorption heat and vasodilators increase absorption. 

The chloroform solution of lipids (Solution A) is placed in a 50-mL round-bottomed spherical Quick-fit flask. Following evaporation of the solvent in a rotary evaporator at about 37°C, a thin lipid film is formed on the walls of the flask. The film is flushed for about 60 seconds with oxygen-free nitrogen (N2) to ensure complete solvent removal and to replace air. Two milliliters of distilled water and a few glass beads are added into the flask, the stopper is replaced, and the flask shaken vigorously by hand or mechanically until the lipid film has been transformed into a milky suspension. This process is carried out above the liquid-crystalline transition temperature (7/) of the phospholipid component of liposomes (> 7/) by prewarming the water... 

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

Hitherto, property measurements of BLM have been confined mainly to thickness, water permeability, electrical characteristics, and current-voltage. The bifacial tension (y6) of BLM is believed to be very small, and a value of about 1 dyne per cm. has been estimated (10). Since no detailed investigations of the bifacial tension of BLM have been reported, the immediate purpose of this work was to develop suitable techniques for y6 measurements. The results of measurements on BLM formed from various lipid solutions are given. The general applicability of the apparatus and method described here to studying other interfacial and bifacial phenomena is briefly discussed. 

Procedure. To form a BLM, a small amount (.— 0.005 ml.) of lipid solution was applied via a Teflon capillary attached to a micrometer syringe. The formation characteristics leading to the black state were observed under reflected light at 20-40 X magnification. Other precautions that should be exercised are essentially those described previously (10). The bifacial tension of BLM was measured as follows. After the membrane had become completely black (except at the Plateau-Gibbs border), the infusion-withdrawal pump was started. The pressure difference across the BLM was continuously monitored and reached a maximum when the membrane was hemispherical. The interfacial tension was calculated from this point using Equation 3. 

The results of interfacial tension measurements on BLM formed from five different lipid solutions are given in Table I. One of the immediate questions is whether the measured values represent the true bifacial tension of BLM. It is implicitly assumed in order to apply equation 3 that yb is a characteristic property of BLM and should be independent of the extension of the BLM area. It is generally recognized that if the BLM also possessed elastic properties, the measured yb would be different when it is stretched. To answer this question, yb was measured during both expansion and contraction of the membrane. A typical trace of pressure difference vs. time in which the membrane was being expanded and contracted is shown in Figure 3. The symmetric nature of the curve indicates that little hysteresis was present during inflation and deflation of the BLM. Therefore, it seems safe to conclude that for BLM formed from lipid materials alone the membrane does not appear to possess appreciable elastic properties. 

Organic Solvents and Lipid Solutions. Reagent grade solvents were distilled before use. The lipids were dissolved in chloroform-methanol (85 to 15 v./v.) at a concentration of about 0.5 mg. per ml. and stored in glass-stoppered borosilicate glass tubes for not longer than 5 days at 5°C. Lipid mixtures were freshly prepared from the stock solutions of the individual components. 

Prepare the lipid for chromatography by dissolving 50 to 75 mg of crude lipid in a minimum of hexane (5 to 10 mL). Open the stopcock and allow excess solvent to drain from the column until the level of solvent just reaches the top of the silica gel column. Very carefully add the solution of crude lipid to the top of the column. This should be done by using a Pasteur pipet and allowing drops of the solution to run down the inside of the glass column to the top of the silica gel bed. It is important not to disturb the top of the silica gel column. After addition of the lipid solution, begin to collect a fraction from the bottom of the column into a 25-mL Erlen meyer flask. Set the flow rate to about 2 drops per second. When the level of solution in the column reaches the top of the silica gel, turn off the stop-... 

In the following procedure, lipids are dissolved in solvent and hydrogenated in the presence of hydrogen and a catalyst (platinum oxide). After the hydrogenated lipid solution is processed by filtration and evaporation, it can be analyzed by TLC-FID. 

Probably the most popular option is mechanical dispersion, simply because the greatest number of methods that utilize it have been developed. When using mechanical means to form vesicles, the lipid solution first is dried to remove all traces of organic solvent prior to dispersion in an aqueous media. The dispersion process is the key to... 

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