Amphipathic Lipids Selforient At Oilwater Interfaces

They Form Membranes, Micelles, Liposomes, & Emulsions

In general, lipids are insoluble in water since they contain a predominance of nonpolar (hydrocarbon) groups. However, fatty acids, phospholipids, sphin-golipids, bile salts, and, to a lesser extent, cholesterol contain polar groups. Therefore, part of the molecule is hyd rophobic, or water-insoluble; and part is hydro-philic, or water-soluble. Such molecules are described as amphipathic (Figure 14-22). They become oriented at oil:water interfaces with the polar group in the water phase and the nonpolar group in the oil phase. A bi-layer of such amphipathic lipids has been regarded as a basic structure in biologic membranes (Chapter 41). When a critical concentration of these lipids is present in an aqueous medium, they form micelles. Aggregations of bile salts into micelles and liposomes and the formation of mixed micelles with the products of fat digestion are important in facilitating absorption of lipids from the intestine. Liposomes may be formed by sonicating an amphipathic lipid in an aqueous medium. They consist of spheres of lipid bilayers that enclose part of the aqueous medium. They are of potential clinical use—particularly when combined with tissue-specific antibodies—as carriers of drugs in the circulation, targeted to specific organs, eg, in cancer therapy. In addition, they are being used for gene transfer into vascular cells and as carriers for topical and transdermal

Figure 14-20. Dolichol—a C95 alcohol.

Figure 14-20. Dolichol—a C95 alcohol.

HH Malondialdehyde

Endoperoxide

HH Malondialdehyde

Endoperoxide

Hydroperoxide ROOH

Figure 14-21. Lipid peroxidation. The reaction is initiated by an existing free radical (XI, by light, or by metal ions. Malondialdehyde is only formed by fatty acids with three or more double bonds and is used as a measure of lipid peroxidation together with ethane from the terminal two carbons of <<3 fatty acids and pen-tane from the terminal five carbons of o6 fatty acids.

Hydroperoxide ROOH

Figure 14-21. Lipid peroxidation. The reaction is initiated by an existing free radical (XI, by light, or by metal ions. Malondialdehyde is only formed by fatty acids with three or more double bonds and is used as a measure of lipid peroxidation together with ethane from the terminal two carbons of <<3 fatty acids and pen-tane from the terminal five carbons of o6 fatty acids.

AMPHIPATHIC LIPID A

Aqueous phase

OOOOOO

Nonpolar or hydrophobic groups

Aqueous phase

Polar or hydrophiIic groups

Aqueous phase

"Oil" or nonpolar phase

OOOOOO

Aqueous phase LIPID BILAYER B

Nonpolar phase

Aqueous phase

LIPOSOME (UNILAMELLAR) E

Nonpolar or hydrophobic groups

Aqueous phase

MICELLE C

Polar or hydrophiIic groups

Aqueous phase

OIL IN WATER EMULSION D

MICELLE C

OIL IN WATER EMULSION D

OOOOOO

Aqueous phase LIPID BILAYER B

Nonpolar phase

Aqueous phase

LIPOSOME (UNILAMELLAR) E

Aqueous compartments

Lipid bilayers

Aqueous compartments

Lipid bilayers

LIPOSOME (MULTILAMELLAR) F

Figure 14-22. Formation of lipid membranes, micelles, emulsions, and liposomes from am-phipathic lipids, eg, phospholipids.

delivery of drugs and cosmetics. Emulsions are much larger particles, formed usually by nonpolar lipids in an aqueous medium. These are stabilized by emulsifying agents such as amphipathic lipids (eg, lecithin), which form a surface layer separating the main bulk of the nonpolar material from the aqueous phase (Figure

14-22). SUMMARY

• Lipids have the common property of being relatively insoluble in water (hydrophobic) but soluble in nonpolar solvents. Amphipathic lipids also contain one or more polar groups, making them suitable as constituents of membranes at lipid:water interfaces.

• The lipids of major physiologic significance are fatty acids and their esters, together with cholesterol and other steroids.

• Long-chain fatty acids may be saturated, monounsat-urated, or polyunsaturated, according to the number of double bonds present. Their fluidity decreases with chain length and increases according to degree of unsaturation.

• Eicosanoids are formed from 20-carbon polyunsatu-rated fatty acids and make up an important group of physiologically and pharmacologically active compounds known as prostaglandins, thromboxanes, leukotrienes, and lipoxins.

• The esters of glycerol are quantitatively the most significant lipids, represented by triacylglycerol ("fat"), a major constituent of lipoproteins and the storage form of lipid in adipose tissue. Phosphoacylglycerols are amphipathic lipids and have important roles—as major constituents of membranes and the outer layer of lipoproteins, as surfactant in the lung, as precursors of second messengers, and as constituents of nervous tissue.

• Glycolipids are also important constituents of nervous tissue such as brain and the outer leaflet of the cell membrane, where they contribute to the carbohydrates on the cell surface.

• Cholesterol, an amphipathic lipid, is an important component of membranes. It is the parent molecule from which all other steroids in the body, including major hormones such as the adrenocortical and sex hormones, D vitamins, and bile acids, are synthesized.

• Peroxidation of lipids containing polyunsaturated fatty acids leads to generation of free radicals that may damage tissues and cause disease.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment