Harpers

LIPIDS OF PHYSIOLOGIC SIGNIFICANCE / 121delivery of drugs and cosmetics. Emulsions are muchlarger particles, formed usually by nonpolar lipids in anaqueous medium. These are stabilized by emulsifyingagents such as amphipathic lipids (eg, lecithin), whichform a surface layer separating the main bulk of thenonpolar material from the aqueous phase (Figure14–22).SUMMARY• Lipids have the common property of being relativelyinsoluble in water (hydrophobic) but soluble in nonpolarsolvents. Amphipathic lipids also contain oneor more polar groups, making them suitable as constituentsof membranes at lipid:water interfaces.• The lipids of major physiologic significance are fattyacids and their esters, together with cholesterol andother steroids.• Long-chain fatty acids may be saturated, monounsaturated,or polyunsaturated, according to the numberof double bonds present. Their fluidity decreaseswith chain length and increases according to degreeof unsaturation.• Eicosanoids are formed from 20-carbon polyunsaturatedfatty acids and make up an important group ofphysiologically and pharmacologically active compoundsknown as prostaglandins, thromboxanes,leukotrienes, and lipoxins.• The esters of glycerol are quantitatively the most significantlipids, represented by triacylglycerol (“fat”),a major constituent of lipoproteins and the storageform of lipid in adipose tissue. Phosphoacylglycerolsare amphipathic lipids and have important roles—asmajor constituents of membranes and the outer layerof lipoproteins, as surfactant in the lung, as precursorsof second messengers, and as constituents of nervoustissue.• Glycolipids are also important constituents of nervoustissue such as brain and the outer leaflet of thecell membrane, where they contribute to the carbohydrateson the cell surface.• Cholesterol, an amphipathic lipid, is an importantcomponent of membranes. It is the parent moleculefrom which all other steroids in the body, includingmajor hormones such as the adrenocortical and sexhormones, D vitamins, and bile acids, are synthesized.• Peroxidation of lipids containing polyunsaturatedfatty acids leads to generation of free radicals thatmay damage tissues and cause disease.REFERENCESBenzie IFF: Lipid peroxidation: a review of causes, consequences,measurement and dietary influences. Int J Food Sci Nutr1996;47:233.Christie WW: Lipid Analysis, 2nd ed. Pergamon Press, 1982.Cullis PR, Fenske DB, Hope MJ: Physical properties and functionalroles of lipids in membranes. In: Biochemistry of Lipids,Lipoproteins and Membranes. Vance DE, Vance JE (editors).Elsevier, 1996.Gunstone FD, Harwood JL, Padley FB: The Lipid Handbook.Chapman & Hall, 1986.Gurr MI, Harwood JL: Lipid Biochemistry: An Introduction, 4th ed.Chapman & Hall, 1991.