Thin layer absorption chromatography can achieve very good separation of small lipid samples

Transcription

1 Abbreviations Preface Acknowledgements Lipids: definition, isolation, separation and detection p. 1 Introduction p. 1 Definitions p. 1 Structural chemistry and nomenclature p. 1 Extraction of lipids from natural samples p. 3 Likely components of the crude lipid extract p. 4 General features of lipids important for their analysis p. 4 Chromatographic techniques for separating lipids p. 5 The two phases can be arranged in a variety of ways p. 5 Gas-liquid chromatography is a particularly useful method for volatile derivatives of lipids Absorption column chromatography is used for the separation of large amounts of lipids p. 8 Thin layer absorption chromatography can achieve very good separation of small lipid samples p. xii p. xiv p. xvi p. 6 p. 10 Other useful methods p. 10 Summary p. 10 Further Reading p. 12 Fatty acid structure and metabolism p. 13 Structure and properties p. 13 Saturated fatty acids p. 13 Branched-chain fatty acids p. 15 Unsaturated fatty acids p. 15 Monoenoic (monounsaturated) fatty acids p. 15 Polyenoic (polyunsaturated) fatty acids p. 16 Cyclic fatty acids p. 16 Oxy acids p. 16 Conjugated unsaturated fatty acids p. 18 Fatty aldehydes and alcohols p. 18 Some properties of fatty acids p. 19 Quantitative and qualitative fatty acid analysis p. 19 General principles p. 19 Determination of the structure of an unknown acid p. 21 The biosynthesis of fatty acids p. 21 Conversion of fatty acids into metabolically active thiolesters is often a prerequisite for their metabolism p. 21 Acyl-CoA thiolesters were the first types of activated fatty acids to be discovered p. 22 Acyl-acyl carrier proteins can be formed as distinct metabolic intermediates in some organisms p. 24

2 The biosynthesis of fatty acids can be divided into de novo synthesis and modification reactions p. 25 De novo biosynthesis p. 26 Acetyl-CoA carboxylase p. 26 Fatty acid synthase p. 27 Termination p. 37 Elongation p. 39 Branched-chain fatty acids p. 40 The biosynthesis of hydroxy fatty acids results in hydroxyl groups in different positions along the fatty chain p. 41 The biosynthesis of unsaturated fatty acids is mainly by oxidative desaturation p. 42 Monounsaturated fatty acids p. 42 Polyunsaturated fatty acids p. 46 Formation of polyunsaturated fatty acids in animals p. 49 Biohydrogenation of unsaturated fatty acids takes place in rumen microorganisms p. 51 The biosynthesis of cyclic acids provided one of the first examples of a complex lipid substrate for fatty acid modifications p. 52 The control of fatty acid synthesis can take place at a number of enzyme steps p. 53 Acetyl-CoA carboxylase (ACC) regulation in animals p. 53 Acetyl-CoA carboxylase regulation in other organisms p. 56 Regulation of fatty acid synthase p. 56 Control of animal desaturases p. 58 Degradation of fatty acids p. 59 [beta]-oxidation is the most common type of biological oxidation of fatty acids p. 59 Cellular site of [beta]-oxidation p. 59 Transport of acyl groups to the site of oxidation: the role of carnitine p. 60 Control of acylcarnitine formation is very important p. 61 Enzymes of mitochondrial [beta]-oxidation p. 61 Other fatty acids containing branched chains, double bonds and an odd number of carbons p. 62 atoms can also be oxidized Regulation of mitochondrial [beta]-oxidation p. 64 Fatty acid oxidation in E. coli p. 66 [beta]-oxidation in microbodies p. 67 [alpha]-oxidation of fatty acids is important when structural features prevent [beta]-oxidation p. 68 [omega]-oxidation uses mixed-function oxidases p. 69 Chemical peroxidation is an important reaction of unsaturated fatty acids p. 70 Peroxidation catalysed by lipoxygenase enzymes p. 71 Lipoxygenases are important for stress responses and development in plants p. 72 Essential fatty acids and the biosynthesis of eicosanoids p. 75 The pathways for prostaglandin synthesis are discovered p. 77 Cyclic endoperoxides can be converted into different types of eicosanoids p. 80 New eicosanoids are discovered p. 81

3 The cyclooxygenase products exert a range of activities p. 82 Prostaglandins and other eicosanoids are rapidly catabolized p. 83 Instead of cyclooxygenation, arachidonate can be lipoxygenated or epoxygenated p. 84 Control of leukotriene formation p. 84 Physiological action of leukotrienes p. 86 For eicosanoid synthesis an unesterified fatty acid is needed p. 88 Essential fatty acid activity is related to double bond structure and to the ability of such acids to be converted into a physiologically active eicosanoid p. 89 Summary p. 90 Further Reading p. 91 Lipids as energy stores p. 93 Introduction p. 93 The naming and structure of the acylglycerols (glycerides) p. 93 Triacylglycerols are the major components of natural fats and oils; partial acylglycerols are usually intermediates in the breakdown or synthesis of triacylglycerols p. 93 All natural oils are complex mixtures of molecular species p. 95 The storage of triacylglycerols in animals and plants p. 97 Adipose tissue depots are the sites of TAG storage in animals p. 97 Milk triacylglycerols provide a supply of energy for the needs of the new-born p. 99 Some plants use lipids as a fuel, stored as minute globules in the seed p. 100 The biosynthesis of triacylglycerols p. 102 Pathways for complete (de novo) synthesis build-up TAG from small basic components p. 104 The glycerol 3-phosphate pathway in mammalian tissues provides a link between TAG and p. 104 phospholipid metabolism The dihydroxyacetone phosphate pathway in mammalian tissues is a slight variant to the main glycerol 3-phosphate pathway and provides an important route to ether lipids Formation of triacylglycerols in plants involves the co-operation of different subcellular compartments The monoacylglycerol pathway is important mainly in rebuilding TAG from absorbed dietary fat p. 107 p. 108 p. 111 The catabolism of acylglycerols p. 113 The nature and distribution of lipases p. 113 Animal triacylglycerol lipases play a key role in the digestion of food and in the uptake and release of fatty acids by tissues Plant lipases break down the lipids stored in seeds in a specialized organelle, the glyoxysome p. 114 p. 115 The integration and control of animal acylglycerol metabolism p. 116 Fuel economy: the interconversion of different types of fuels is hormonally regulated to maintain blood glucose concentration within the normal range and ensure storage of excess dietary energy in triacylglycerols The control of acylglycerol biosynthesis is important, not only for fuel economy but for membrane formation, requiring close integration of storage and structural lipid metabolism p. 116 p. 117

4 Mobilization of fatty acids from fat stores is regulated by hormonal balance, which in turn is responsive to nutritional and physiological states p. 121 Wax esters p. 122 Occurrence and characteristics p. 123 Biosynthesis of wax esters involves the condensation of a long-chain fatty alcohol with fatty acyl-coa p. 123 Digestion and utilization of wax esters is poorly understood p. 124 Surface lipids include not only wax esters but a wide variety of lipid molecules p. 125 Summary p. 125 Further Reading p. 126 Dietary lipids p. 127 Lipids in food p. 127 The fats in foods are derived from the structural and storage fats of animals and plants p. 127 The fatty acid composition of dietary lipids depends on the relative contributions of animal and plant structural or storage lipids Industrial processing may influence the chemical and physical properties of dietary fats either beneficially or adversely p. 128 p. 129 Catalytic hydrogenation p. 129 Heating p. 131 Irradiation p. 131 Interesterification p. 131 Fractionation p. 132 Structured fats p. 132 A few dietary lipids may be toxic p. 132 Cyclopropenes p. 133 Long-chain monoenes p. 133 Trans-unsaturated fatty acids p. 133 Lipid peroxides p. 134 Roles of dietary lipids p. 134 Triacylglycerols provide a major source of metabolic energy especially in affluent countries Lipids supply components of organs and tissues for membrane synthesis and other functions p. 134 p. 135 Foetal growth p. 135 Post-natal growth p. 138 Dietary lipids supply essential fatty acids that are essential to life but cannot be made in the animal body p. 140 Historical background: discovery of essential fatty acid deficiency p. 140 Biochemical basis for EFA deficiency p. 141 Functions of essential fatty acids p. 143 Which fatty acids are essential? p. 146 What are the quantitative requirements for essential fatty acids in the diet? p. 147 Dietary lipids supply fat-soluble vitamins p. 150

5 Vitamin A p. 151 Vitamin D p. 153 Vitamin E p. 156 Vitamin K p. 158 Lipids play an important role in enhancing the flavour and texture and therefore the palatability of foods p. 159 Odour p. 159 Taste p. 160 Texture p. 161 Dietary lipids in relation to immune function p. 161 Components of the immune system and their functional assessment p. 161 Summary of lipid effects on different components of immunity p. 162 Influence on target cell composition p. 162 Influence on lymphocyte functions ex vivo p. 163 Influence on antibody production p. 163 Influence on delayed-type hypersensitivity p. 163 Graft versus host and host versus graft reactions and organ transplants p. 163 Survival after infection p. 163 Influence on autoimmune and inflammatory disease processes p. 163 Mechanisms p. 164 Membrane properties p. 164 Availability of eicosanoid precursors p. 164 Availability of vitamin E p. 164 Gene expression p. 164 Implications for dietary advice p. 165 Lipids and cancer p. 165 Dietary lipids and cancer p. 165 Cellular lipid changes in cancer p. 166 Lipids and the treatment of cancer p. 167 Summary p. 167 Further Reading p. 168 Lipid transport p. 170 Digestion and absorption p. 170 Intestinal digestion of dietary fats involves breakdown into their component parts by a variety of digestive enzymes The intraluminal phase of fat absorption involves passage of digestion products into the absorptive cells of the small intestine The intracellular phase of fat absorption involves recombination of absorbed products in the enterocytes and packaging for export into the circulation p. 170 p. 173 p. 174 Malassimilation of lipids, through failure to digest or absorb lipids properly, can arise from defects in the gut or other tissues but may also be induced deliberately p. 175 Transport of lipids in the blood: plasma lipoproteins p. 177

6 Lipoproteins can be conveniently divided into groups according to density p. 177 The apolipoproteins are the protein moieties that help to stabilize the lipid; they also provide specificity and direct the metabolism of the lipoproteins The different classes of lipoprotein particles transport mainly triacylglycerols or cholesterol through the plasma Specific lipoprotein receptors mediate the cellular removal of lipoproteins and of lipids from the circulation p. 179 p. 179 p. 182 Membrane receptors p. 183 The LDL-receptor p. 183 The LDL-receptor-related protein and other members of the LDL-receptor family p. 184 Scavenger receptors p. 186 The lipoprotein particles transport lipids between tissues but they interact and are extensively remodelled in the plasma compartment p. 186 Species differ quantitatively in their lipoprotein profiles p. 190 The co-ordination of lipid metabolism in the body p. 190 The sterol regulatory element binding protein (SREBP) system controls pathways of cholesterol accumulation in cells and may also control fatty acid synthesis The peroxisome proliferator activated receptor (PPAR) system regulates fatty acid metabolism in liver and adipose tissue p. 191 p. 194 Other nuclear receptors are activated by fatty acids and affect gene expression p. 196 Adipose tissue secretes hormones and other factors that may themselves play a role in regulation of fat storage p. 197 Diseases involving changes or defects in lipid metabolism p. 199 Atherosclerosis p. 200 Risk factors for CHD and the effects of diet p. 205 Hyperlipoproteinaemias (elevated circulating lipoprotein concentrations) are often associated with increased incidence of cardiovascular disease p. 208 Obesity and diabetes are associated with increased risk of cardiovascular diseases p. 208 Hypolipoproteinaemias are rare conditions of abnormally low plasma lipoprotein concentrations p. 212 Summary p. 212 Further Reading p. 213 Lipids in cellular structures p. 215 Cell organelles p. 215 Glycerolipids p. 217 Phosphoglycerides are the major lipid components of most biological membranes p. 217 Phosphonolipids constitute a rare class of lipids found in few organisms p. 218 Glycosylglycerides are particularly important components of photosynthetic membranes p. 219 Betaine lipids are important in some organisms p. 221 Ether-linked lipids and their bioactive species p. 221 Sphingolipids p. 222 Sterols p. 228

7 Major sterols p. 228 Other steroids p. 229 Membrane structure p. 229 Early models already envisaged a bilayer of lipids but were uncertain about the location of the proteins The lipid-globular protein mosaic model now represents the best overall picture of membrane structure Membrane structure is not static but shows rapid movement of both lipid and protein components p. 229 p. 230 p. 232 Further remarks on the lipid composition of membranes p. 234 Transbilayer asymmetry is an essential feature of all known biological membranes p. 234 Lateral heterogeneity is probably important in some membranes at least p. 237 Physical examination of membranes reveals their fluid properties p. 238 General functions of membrane lipids p. 238 Membrane lipids are modified to maintain fluidity at low temperatures p. 242 Lipids and membrane fusion p. 245 Lipids and proteins interact in order to determine membrane structure and shape p. 247 Why are there so many membrane lipids? p. 249 Liposomes and drug delivery systems p. 250 Lipid anchors for proteins p. 251 Acylation p. 252 Prenylation p. 252 GPI-anchors p. 252 Lipids as components of the surface layers of different organisms p. 254 Cutin, suberin and waxes - the surface coverings of plants p. 254 Mycobacteria contain specialized cell-wall lipids p. 255 Lipopolysaccharide forms a major part of the cell envelope of Gram-negative bacteria p. 256 Gram-positive bacteria have a completely different surface structure p. 258 Insect waxes p. 259 Lipids of the skin--the mammalian surface layer and skin diseases p. 259 Summary p. 261 Further Reading p. 263 Metabolism of structural lipids p. 267 Phosphoglyceride biosynthesis p. 267 Tracer studies revolutionized concepts about phospholipids p. 267 Formation of the parent compound, phosphatidate, is demonstrated p. 267 A novel cofactor for phospholipid synthesis was found by accident p. 268 The core reactions of glycerolipid biosynthesis are those of the Kennedy pathway p. 268 The zwitterionic phosphoglycerides can be made using CDP-bases p. 270 CDP-diacylglycerol is an important intermediate for phosphoglyceride formation in all organisms p. 271 Phospholipid formation in E. coli is entirely via CDP-diacylglycerol p. 271

8 Differences between phosphoglyceride synthesis in different organisms p. 272 Plasmalogen biosynthesis p. 273 Platelet activating factor (PAF): a biologically active phosphoglyceride p. 276 Degradation of phospholipids p. 277 General features of phospholipase reactions p. 278 Phospholipase A activity is used to remove a single fatty acid from intact phospholipids p. 280 Phospholipase B and lysophospholipases p. 281 Phospholipases C and D remove water-soluble moieties p. 282 Phospholipids may also be catabolized by non-specific enzymes p. 283 Metabolism of glycosylglycerides p. 283 Biosynthesis of galactosylglycerides takes place in chloroplast envelopes p. 283 Catabolism of glycosylglycerides p. 284 Relatively little is known of the metabolism of the plant sulpholipid p. 284 Metabolism of sphingolipids p. 285 Biosynthesis of the sphingosine base and ceramide p. 285 Cerebroside biosynthesis p. 286 Formation of neutral glycosphingolipids p. 286 Ganglioside biosynthesis p. 286 Sulphated sphingolipids p. 288 Sphingomyelin is both a sphingolipid and a phospholipid p. 289 Catabolism of the sphingolipids p. 289 Cholesterol biosynthesis p. 291 Acetyl-CoA is the starting material for terpenoid as well as fatty acid synthesis p. 291 Further metabolism generates the isoprene unit p. 293 Higher terpenoids are formed by a series of condensations p. 293 A separate way of forming the isoprene unit occurs in plants p. 293 Sterol synthesis requires cyclization p. 294 Cholesterol is an important metabolic intermediate p. 294 It is important that cholesterol concentrations in plasma and tissues are regulated within certain limits and complex regulatory mechanisms have evolved p. 296 Specific roles p. 297 Pulmonary surfactant p. 298 Lipid storage diseases (lipidoses) p. 300 The 'phosphatidylinositol cycle' in cell signalling p. 302 A wider range of lipid signalling molecules p. 304 Phospholipase D in cell signalling p. 305 Role of sphingolipids in cellular signalling p. 307 Summary p. 310 Further Reading p. 312 Index p. 315

9 Table of Contents provided by Blackwell's Book Services and R.R. Bowker. Used with permission.