Membranes are usually made of phospholipids, which are molecules that have a head and a tail. The head is attracted to water, and the tail, which is made of oil (hydrocarbon), is repelled by water.
In nature, phospholipids are found in stable membranes composed of two layers (a bilayer). In the presence of water, the heads are attracted to water and line up to form a surface facing the water. The tails are repelled by water, and line up to form a surface away from the water. In a cell, one layer of heads faces outside of the cell, attracted to the water in the environment. Another layer of heads faces inside the cell, attraced by the water inside the cell. The hydrocarbon tails of one layer face the hydrocarbon tails of the other layer, and the combined structure forms a bilayer. [ [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html Kimball's Biology Pages,] "Cell Membranes."]
When membrane phospholipids are disrupted, they can reassemble themselves into tiny spheres, smaller than a normal cell, either as bilayers or monolayers. These are liposomes.
The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine and cholesterol. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other.
Liposomes can be composed of naturally-derived phospholipids with mixed lipid chains (like egg phosphatidylethanolamine), or of pure
surfactantcomponents like DOPE ( dioleoylphosphatidylethanolamine). Liposomes, usually but not by definition, contain a core of aqueous solution [Stryer S. (1981) Biochemistry, 213] ; lipid spheres that contain no aqueous material are called micelles, however, reverse micelles [http://en.wikipedia.org/wiki/Micelle#Reverse_Micelles] can be made to encompass an aqueous environment.
The name liposome is derived from two Greek words: 'Lipid' meaning fat and 'Soma' meaning body. A liposome can be formed at a variety of sizes as uni-lamellar or multi-lamellar construction, and its name relates to its structural building blocks, phospholipids, and not to its size. In contrast, the term Nanosome does relate to size and was coined in the early 1990s to denote special liposomes in the low nanometer range; liposome and Nanosome are not synonyms. A liposome does not necessarily have
lipophobiccontents, such as water, although it usually does.
Liposomes were first described by British haematologist
Dr Alec D Bangham FRSin 1961 (published 1964), at the Babraham institute, Cambridge. They were discovered when Bangham and R. W. Horne were testing the institute's new electron microscopeby adding negative stainto dry phospholipids. The resemblance to the plasmalemmawas obvious, and the microscope pictures served as the first real evidence for the cell membrane being a bilayer lipid structure.
Liposomes are used for
drug deliverydue to their unique properties. A liposome encapsulates a region on aqueous solution inside a hydrophobicmembrane; dissolved hydrophilic solutescannot readily pass through the lipids. Hydrophobic chemicals can be dissolved into the membrane, and in this way liposome can carry both hydrophobic molecules and hydrophilic molecules. To deliver the molecules to sites of action, the lipid bilayer can fuse with other bilayers such as the cell membrane, thus delivering the liposome contents. By making liposomes in a solution of DNAor drugs (which would normally be unable to diffuse through the membrane) they can be (indiscriminately) delivered past the lipid bilayer.
Liposomes can also be designed to deliver drugs in other ways. Liposomes that contain low (or high)
pHcan be constructed such that dissolved aqueous drugs will be chargedin solution (i.e., the pH is outside the drug's pI range). As the pH naturally neutralizes within the liposome ( protonscan pass through some membranes), the drug will also be neutralized, allowing it to freely pass through a membrane. These liposomes work to deliver drug by diffusionrather than by direct cell fusion. Another strategy for liposome drug delivery is to target endocytosisevents. Liposomes can be made in a particular size range that makes them viable targets for natural macrophage phagocytosis. These liposomes may be digestedwhile in the macrophage's phagosome, thus releasing its drug. Liposomes can also be decorated with opsonins and ligands to activate endocytosis in other cell types.
* The use of liposomes for transformation or
transfectionof DNA into a host cell is known as lipofection.
Another interesting property of liposomes are their natural ability to target
cancer. The endothelialwall of all healthy human blood vessels are encapsulated by endothelial cells that are bound together by tight junctions. These tight junctions stop any large particle in the bloodfrom leaking out of the vessel. Tumourvessels do not contain the same level of seal between cells and are diagnostically "leaky". This ability is known as the Enhanced Permeability and Retention effect. Liposomes of certain sizes, typically less than 400nm, can rapidly enter tumour sites from the blood, but are kept in the bloodstream by the endothelial wall in healthy tissue vasculature. Anti-cancer drugs such as Doxorubicin(Doxil), Camptothecinand Daunorubicin(Daunoxome) are currently being marketed in liposome delivery systems.
Liposomes can be created by sonicating phospholipids in water [Stryer S. (1981) Biochemistry, 213] . Low
shear rates create multilamellar liposomes, which have many layers like an onion. Continued high-shear sonication tends to form smaller unilamellar liposomes. In this technique, the liposome contents are the same as the contents of the aqueous phase. Sonication is generally considered a "gross" method of preparation, and newer methods such as extrusion are employed to produce materials for human use.
Further advances in liposome research have been able to allow liposomes to avoid detection by the body's immune system, specifically, the cells of
reticuloendothelial system(RES). These liposomes are known as " stealth liposomes", and are constructed with PEG (Polyethylene Glycol) studding the outside of the membrane. The PEG coating, which is inertin the body, allows for longer circulatory life for the drug delivery mechanism. However, research currently seeks to investigate at what amount of PEG coating the PEG actually hinders binding of the liposome to the delivery site. In addition to a PEG coating, most stealth liposomes also have some sort of biological species attached as a ligand to the liposome in order to enable binding via a specific expression on the targeted drug delivery site. These targeting ligands could be monoclonal antibodies(making an immunoliposome), vitamins, or specific antigens. Targeted liposomes can target nearly any cell type in the body and deliver drugs that would naturally be systemically delivered. Naturally toxic drugs can be much less toxic if delivered only to diseased tissues. Polymersomes, morphologically related to liposomes can also be used this way.
* [http://www.liposome.org International Liposome Society]
* [http://www.controlledreleasesociety.org/ Controlled Release Society]
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Look at other dictionaries:
liposome — [ lipozom ] n. m. • v. 1975; en angl. 1968; mot angl.; cf. lipo et some ♦ Petite vésicule artificielle formée de plusieurs feuillets phospholipidiques renfermant une solution aqueuse. Les liposomes peuvent véhiculer des substances actives vers… … Encyclopédie Universelle
liposome — liposome. См. липосома. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
liposome — Liposome Липосома Микроскопический сферический мембранный пузырек (диаметр около 2 мкм). Получается искусственно в лабораторных условиях путем добавления водного раствора к фосфолипидному гелю. Защитная оболочка этого пузырька напоминает… … Толковый англо-русский словарь по нанотехнологии. - М.
liposome — ☆ liposome [lip′ə sōm΄, lī′pəsōm] n. [ LIPO + SOME ] a synthetic, microscopic globule consisting of layers of lipids encapsulating certain substances (as enzymes or drugs): used to introduce these substances into a cell or specified tissue … English World dictionary
Liposome — Unter einem Liposom versteht man eine bestimmte, kugelförmige Anordnung von oberflächenaktiven Molekülen in einer Flüssigkeit. Üblicherweise finden Phospholipide wie Lecithin als oberflächenaktive Substanzen Verwendung. Liposome bilden sich aus,… … Deutsch Wikipedia
Liposome — Schéma d un liposome formé par des phospholipides en solution aqueuse. Un liposome est une vésicule artificielle formée par des bicouches lipidiques concentriques, emprisonnant entre elles des compartiments aqueux. On en obtient à partir d une… … Wikipédia en Français
liposome — 1. A spherical particle of lipid substance suspended in an aqueous medium within a tissue. 2. Any small, roughly spherical artificial vesicle consisting of a lipid bilayer enclosing some of the suspending medium. [lipo + G. som … Medical dictionary
liposome — n. a microscopic spherical membrane enclosed vesicle or sac (20–30 nm in diameter) made artificially in the laboratory by the addition of an aqueous solution to a phospholipid gel. The membrane resembles a cell membrane and the whole vesicle is… … The new mediacal dictionary
liposome — A synthetic microscopic spherical structure consisting of a phospholipid bilayer membrane containing a user defined aqueous solution. Liposomes can be used to transport relatively toxic drugs into diseased cells, where they can exert their… … Glossary of Biotechnology
liposome — noun Date: 1968 an artificial vesicle composed of one or more concentric phospholipid bilayers and used especially to deliver microscopic substances (as drugs or DNA) to body cells • liposomal adjective … New Collegiate Dictionary