Lipid water solubility

Figure 18.2 presents an overall flow chart of extraction and separation procedures that can be used to isolate, separate, and measure lipid, water-soluble, and residue-associated As species in seafoods. 

Liposomes have shown potential as drug delivery systems. The exact location of a drug molecule in a liposome depends on its physicochemical composition and the composition of the lipids. Water soluble drugs may be included in the aqueous phase, and oil-soluble drugs may be added to the membrane-forming phospholipid. An extensive account of the pharmaceutical use of liposomes is found in the article Liposomes as Pharmaceutical Dosage Forms, by Y. Barenholz, and D.J.A., Crommelin, Volume 9 of the first edition of this encyclopedia. ... 

Molecular manipulation resulting in a lipid/water-solubility suitable for penetration of both lipid membranes and the water compartments such as the interstitial fluids will enhance absorption and facilitate distribution over the various... 

Body fluids or aqueous extracts of tissue contain, in addition to free amino acids, proteins, peptides, lipids, water-soluble organic com-... 

For many years, the rate and extent of absorption in the small intestine were thought to be determined solely by the lipid/water solubility and membrane permeability characteristics of the drug. While this relatively simplistic model worked for many drugs, there are a number of exceptions to this rule, suggesting other forces are at work within the GI system to control the absorption of drugs. It is now known that a complex system of transporter proteins and metabolic enzymes is present within the GI system. Expression of influx transporters in the intestinal epithelial cells can increase absorption of drugs that are substrates for these transporters, whereas efflux transporters can reduce oral absorption of these drugs. In particular, the impact of the... 

The permeation of compounds through excised skin correlates well with the lipid/water solubility ratio (Figure 5.32). The permeability measured... 

To some authors, it seems to be apparent that the microlayer consists of a surface monolayer of adsorbed organic matter of thickness —20 A diluted by a vast excess, 10 —10 times as much, subsurface seawater. It should be noted that a monolayer thickness of this magnitude applies mostly to mono-layers of simple surfactants such as fatty lipids. Water-soluble surfactants of the wet variety (MacIntyre, 1974) can form monolayer films of much greater thickness, with hydrophobic parts of the molecule attached to the interface and hydrophilic parts extending by as much as —1 pm into the aqueous phase. The results of Baler et al. (1974), to be discussed shortly, show that films in a dry state on germanium prisms used for the Blodgett (1934, 1935) type of sampling method have thicknesses determined by ellip-... 

All lipid analyses are preceded by isolation of the total lipid material by solvent extraction with a solvent such as CHCI3. As far as seawater and the microlayer are concerned it cannot be stressed too strongly that this method of isolation specifically excludes water-soluble organic compounds from whatever form of organic analysis that follows. The lipid extract is normally then hydrolysed with aqueous acid or base which breaks the ester linkages, releasing the constituents of the combined lipids. Water-soluble components such as glycerol and the phosphoric acid of phospholipids are removed... 

The most common mixture is that of chloroform and methanol. A typical procedure is that of Bligh and Dyer (1959) where a one-phase methanol system (chloroform-methanol-water, 1 2 0.8, by vol.) rapidly extracts lipids from most tissues and the extract is then diluted with 1 vol. each of chloroform and methanol to yield a two-phase mixture. The upper (aqueous) phase contains water-soluble contaminants while the lower phase contains lipids. Water-soluble contaminants can also be removed from solutions of lipids in organic solvents by passage through Sephadex columns (Rouser et ai, 1967). After rotary evaporation of the solvent, traces of water may remain with the lipid. These can be removed by azeotropic distillation with benzene. 

Regarding phytochemicals, polyphenols and llavonoids in particular, their major route of entry is by oral ingestion, since they are consumed as part of a normal diet. Their bioavailability, similarly to what happens with other xenobiotics ingested orally, can be influenced by several factors. The physicochemical characteristics of the compound, such as the size of the molecule, its lipid/water solubility, or its pKa, can dictate its ability to cross a membrane by simple diffusion or not. For example, due to its size, larger molecules may imply other transport processes rather than diffusion. If the compound is hydrophiUc, it is unlikely to cross a membrane freely, but if it is too lipophilic, it may not be completely soluble in gastric secretions which can also make its absorption difficult. In addition, if the compound has ionic charge, it is not probable to cross biological membranes by diffusion and may involve other process to enter the cell. 

So there it is, perhaps the fundamental dichotomy within toxicology the issue of water-solubility. The behavior of virtually every toxic compound is directed by the fundamental dichotomy of solubility water versus lipid. Compound solubility influences fundamental processes such as absorption from the environment, delivery in the blood, diffusion into the target tissues, excretion from the target tissue, metabolism, sequestration, and whole-body elimination. These functions are not as strongly regulated by molecule size or shape as they are by the relative lipid/water solubility. Contaminant solubility, the road to toxicology, begins here. 

In another investigation of the comparative value of lipid and hydration hypotheses (Eger et aL 1969), the minimal anaesthetic concentrations of the following agents were plotted against the relevant physical properties carbon tetrafluoride, sulphur hexafluoride, nitrous oxide, xenon, cyclopropane, fluorexene (trifluoroethyl vinyl ether), diethyl ether, enflurane (see above), halothane, chloroform, and methoxyflurane (see above). (These anaesthetics have been arranged here in order of increasing lipid/water solubility.) The results of this study showed an excellent correlation between... 

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