Compound LipidsClinical importance of LipidsSDKMarch 2013
Compound LipidsDefinition:• They are lipids that contain additionalsubstances, e.g., sulfur, phosphorus, aminogroup, carbohydrate, or proteins beside fattyacid and alcohol.• Compound or conjugated lipids are classifiedinto the following types according to the natureof the additional group:1. Phospholipids2. Glycolipids.3. Lipoproteins4. Sulfolipids and amino lipids.
A-PhospholipidsDefinition: Phospholipids or phosphatides are compound lipids,which contain phosphoric acid group in their structure.Importance:1. They are present in large amounts in the liver and brain as wellas blood.2. The membranes bounding cells and subcellular organelles arecomposed mainly of phospholipids.3. They are important components of the lipoprotein coatessential for secretion and transport of plasma lipoproteincomplexes. Thus, they are lipotropic agents that prevent fattyliver.4. Myelin sheath of nerves is rich with phospholipids.
5-Important in digestion and absorption of neutral lipidsand excretion of cholesterol in the bile.6-Important function in blood clotting and plateletaggregation.7-They provide lung alveoli with surfactants thatprevent its irreversible collapse.8-Important role in signal transduction across the cellmembrane.9-Phospholipase A2 in snake venom hydrolysesmembrane phospholipids into hemolyticlysolecithin or lysocephalin.10-They are source of polyunsaturated fatty acids forsynthesis of eicosanoids.Phospholipids
Sources: They are found in all cells (plant and animal),milk and egg-yolk in the form of lecithins.Structure: phospholipids are composed of:1. Fatty acids (a saturated and an unsaturated fattyacid).2. Nitrogenous base (choline, serine, threonine, orethanolamine).3. Phosphoric acid.4. Fatty alcohols (glycerol, inositol or sphingosine).Phospholipids
Classification of PhospholipidsClassified into 2 groups according to the type of thealcohol present into two types:A-Glycerophospholipids: Phospholipids that contain glycerolare called glycerophospholipids (or phosphoglycerides).1.Phosphatidic acids[Phosphatidic acid is the simplest phosphoglyceride, and is theprecursor of the other members of this group.2. Lecithins3.Cephalins.4.Plasmalogens.5.Inositides.6.Cardiolipin.B-Sphingophospholipids: They contain sphingosine as analcohol and are named Sphingomyelins.
A-Glycerophospholipids1-Phosphatidic acids:They are metabolic intermediates insynthesis of triglycerides and glycerophospholipids in thebody and may have function as a second messenger. Theyexist in two forms according to the position of the phosphateCH2 OC HOCH2CCO PR1R2OOα-Phosphatidic acidOHOHOSaturatedfatty acidPolyunsaturatedfatty acidPhosphateCH2 OC HCH2COR1Oβ-Phosphatidic acidSaturatedfatty acidPolyunsaturatedfatty acidPhosphate PHOOHOC R2OββααααO
2-Lecithins:• Definition: Lecithins are glycerophospholipids thatcontain choline as a base beside phosphatidic acid.Lecithins are important in the metabolism of fat by theliver.• Structure:• Glycerol is connected at C2 or C3 with a polyunsaturated fatty acid,• At C1 with a saturated fatty acid,• At C3 or C2 by phosphate to which the choline base is connected.• The common fatty acids in lecithins are stearic,palmitic, oleic, linoleic, linolenic and arachidonicacids.
Lysolecithin causes hemolysis of RBCs. This partially explainstoxic the effect of snake venom,.The venom contains lecithinase, which hydrolyzes thepolyunsaturated fatty converting lecithin into lysolecithin.CH2 OC HOCH2CCO PR1R2OOα-LecithinOOHOCH2 OC HCH2COR1Oβ-LecithinPOHOC R2OCH2 CH2 NCH3CH3CH3+OCH2CH2NCH3CH3CH3+CholineCholineO
Lung surfactant• Is a complex of dipalmitoyl-lecithin, sphingomyelin and agroup of apoproteins called apoprotein A, B, C, and D.• It is produced by type II alveolar cells and is anchored to thealveolar surface of type II and I cells.• It lowers alveolar surface tension and improves gasexchange besides activating macrophages to kill pathogens.• In premature babies, this surfactant is deficient and theysuffer from respiratory distress syndrome.• Glucocorticoids increase the synthesis of the surfactantcomplex and promote differentiation of lung cells.
3-Cephalins (or Kephalins):• Definition: They are phosphatidyl-ethanolamine orserine.• Cephalins occur in association with lecithins in tissuesand are isolated from the brain (Kephale = head).• Structure: Cephalins resemble lecithins in structureexcept that choline is replaced by ethanolamine, serineor threonine amino acids.
• Certain cephalins are constituents of the complex mixture ofphospholipids, cholesterol and fat that constitute the lipidcomponent of the lipoprotein “thromboplastin” whichaccelerates the clotting of blood by activation of prothrombinto thrombin in presence of calcium ions.CH2 OC HOCH2CCO PR1R2OOα-CephalinOOHOCH2 CH2 NH2 EthanolamineHO CH2 CH COOH SerineNH2HO CH CH COOH ThreonineNH2CH3
4-Plasmalogens:• Definition: Plasmalogens are found in the cellmembrane phospholipids fraction of brain and muscle(10% of it is plasmalogens), liver, semen and eggs.• Structure: Plasmalogens resemble lecithins andcephalins in structure but differ in the presence ofα,β-unsaturated fatty alcohol rather than a fatty acidat C1 of the glycerol connected by ether bond.• At C2 there is an unsaturated long-chain fatty acid,however, it may be a very short-chain fatty acidCH2C HOCH2CO PR2Oα-PlasmalogenOOHOCH2 CH2 NCH3CH3CH3+α,β-Unsaturatedfatty alcoholCH CH R1OCH2C HOCH2CO PR2Oα-PlasmalogenOOHOCH2 CH2 NCH3CH3CH3+α,β-Unsaturatedfatty alcoholCH CH R1O
5-Inositides:• Definition:• - They are phosphatidyl inositol.• Structure: They are similar to lecithins or cephalins but theyhave the cyclic sugar alcohol, inositol as the base.• They are formed of glycerol, one saturated fatty acid, oneunsaturated fatty acid, phosphoric acid and inositolCH2C HOCH2CO PR2Oα-PhosphatidylinositolOOHOC R1OOHHOHOHHOHHOHOHH H12 3456CH2C HOCH2CO PR2Oα-PhosphatidylinositolOOHOC R1OOHHOHOHHOHHOHOHH H12 3456
Function:• Phosphatidyl inositol is a major component of cell membranephospholipids particularly at the inner side.• They play a major role as second messengers during signaltransduction for certain hormone..• On hydrolysis by phospholipase C, phosphatidyl-inositol-4,5-diphosphate produces diacyl-glycerol and inositol-triphosphate bothact to liberate calcium from its intracellular stores to mediate thehormone effects.5-Inositides:
6-Cardiolipins:• Definition: They are diphosphatidyl-glycerol. They are foundin the inner membrane of mitochondria.• Initially isolated from heart muscle (cardio).• It is formed of 3 molecules of glycerol, 4 fatty acids and 2phosphate groups.• Function: Used in serological diagnosis of autoimmunitydiseases.CH2C HOCH2CO PR2OCardiolipinOOHOC R1OOCH2CH OHCH2 CH2CH OCH2COPR3OOOHOCR4 OOCH2C HOCH2CO PR2OCardiolipinOOHOC R1OOCH2CH OHCH2 CH2CH OCH2COPR3OOOHOCR4 OO
B-Sphingophospholipids1-Sphingomyelins• Definition: Sphingomyelins are found in large amounts inbrain and nerves and in smaller amounts in lung, spleen,kidney, liver and blood.• Structure: Sphingomyelins differ from lecithins and cephalinsin that they contain sphingosine as the alcohol instead ofglycerol,• They contain two nitrogenous bases: sphingosine itself andcholine.• Thus, sphingomyelins contain sphingosine base, one long-chain fatty acid, choline and phosphoric acid.
Ceramide• This part of sphingomyelin in which the amino group ofsphingosine is attached to the fatty acid by an amide linkage.• Ceramides have been found in the free state in the spleen, liverand red cells.CH CH NHCH2CHCH(CH2)12CH3OHSphingosineC R1OOP OOHO CH2 CH2 NCH3CH3CH3+CholineFatty acidPhosphateCeramideSphingomyelinCH CH NHCH2CHCH(CH2)12CH3OHSphingosineC R1OOP OOHO CH2 CH2 NCH3CH3CH3+CholineFatty acidPhosphateCeramideSphingomyelin
B-Glycolipids• Definition: They are lipids that contain carbohydrate residues withsphingosine as the alcohol and a very long-chain fatty acid (24 carbonseries).• They are present in cerebral tissue, therefore are called cerebrosidesClassification: According to the number and nature of the carbohydrateresidue(s) present in the glycolipids the following are1. Cerebrosides. They have one galactose molecule (galactosides).2. Sulfatides. They are cerebrosides with sulfate on the sugar (sulfatedcerebrosides).3. Gangliosides. They have several sugar and sugaramine residues.
1-Cerebrosides:• Occurrence: They occur in myelin sheath of nerves and white matter of thebrain tissues and cellular membranes. They are important for nerveconductance.• Structure: They contain sugar, usually β-galactose and may be glucose orlactose, sphingosine and fatty acid, but no phosphoric acid.CH CH NHCH2CHCH(CH2)12CH3OHSphingosineC R1OOPsychosinFatty acidCeramideCerebrosideOOHH HHOHHOHCH2OHHGalactoseCH CH NHCH2CHCH(CH2)12CH3OHSphingosineC R1OOPsychosinFatty acidCeramideCerebrosideOOHH HHOHHOHCH2OHHGalactose
2-Sulfatides:• They are sulfateed cerebroside, in which the sulfate group isusually attached to the –OH group of C3 or C6 of galactose.• Sulfatides are usually present in the brain, liver, muscles andtestes.CH CH NHCH2CHCH2(CH2)12CH3OHC R1OOSulfatides (sulfated cerebroside)OOHH HHOHHOSO3HCH2OHHCH CH NHCH2CHCH2(CH2)12CH3OHC R1OOSulfatides (sulfated cerebroside)OOHH HHOHHOSO3HCH2OHH
3-Gangliosides:• They are more complex glycolipids that occur in the gray matter of thebrain, ganglion cells, and RBCs.• They transfer biogenic amines across the cell membrane and act as a cellmembrane receptor.• The most simple type of it the monosialoganglioside,. It works as areceptor for cholera toxin in the human intestine.Ceramide-Glucose-Galactose-N-acetylgalactosamine-GalactoseMonosialogangliosideSialic acidCeramide-Glucose-Galactose-N-acetylgalactosamine-GalactoseMonosialogangliosideSialic acid
C-Lipoproteins• Definition: Lipoproteins are lipids combined with proteins in the tissues.The lipid component is phospholipid, cholesterol or triglycerides. Theholding bonds are secondary bonds.• They include:1. Structural lipoproteins: These are widely distributed in tissues beingpresent in cellular and subcellular membranes. In lung tissues acting as asurfactant in a complex of a protein and lecithin. In the eye, rhodopsin ofrods is a lipoprotein complex.• Transport lipoproteins:• These are the forms present in blood plasma. They are composed of aprotein called apolipoprotein and different types of lipids. (Cholesterol,cholesterol esters, phospholipids and triglycerides). As the lipid contentincreases, the density of plasma lipoproteins decreases
• Plasma lipoproteins can be separated by two methods:1. Ultra-centrifugation: Using the rate of floatation in sodiumchloride solution leading to their sequential separation intochylomicrons, very low density lipoproteins (VLDL or pre-β-lipoproteins), low density lipoproteins (LDL or β-lipoproteins), high density lipoproteins (HDL or α-lipoproteins) and albumin-free fatty acids complex.2. Electrophoresis: is the migration of charged particles in anelectric field either to the anode or to the cathode. Itsequentially separates the lipoproteins into chylomicrons, pre-β-, β-, and α-lipoprotein and albumin-free fatty acids complex.Polar lipids(phospholipids)Nonpolar lipids(cholesterol and its estersand triacylglycerols)Structure of a plasma lipoprotein complexPolar apolipoproteins
a) Chylomicrons: They have the largest diameter and the leastdensity. They contain 1-2% protein only and 98-99% fat. Themain lipid fraction is triglycerides absorbed from the intestineand they contain small amounts of the absorbed cholesterol andphospholipids.b) Very low-density lipoproteins (VLDL) or pre-β-lipoproteins: Their diameter is smaller than chylomicrons.They contain about 7-10% protein and 90-93% lipid. The lipidcontent is mainly triglycerides formed in the liver. They containphospholipid and cholesterol more than chylomicrons.c) Low-density lipoproteins (LDL) or β-lipoproteins: Theycontain 10-20% proteins in the form of apolipoprotein B. Theirlipid content varies from 80-90%. They contain about 60% oftotal blood cholesterol and 40% of total blood phospholipids. Astheir percentage increases, the liability to atherosclerosisincreases.
d) High-density lipoproteins (HDL) or α-Lipoproteins:They contain 35-55% proteins in the form ofapolipoprotein A. They contain 45-65% lipids formedof cholesterol (40% of total blood content) andphospholipids (60% of total blood content). They act ascholesterol scavengers, as their percentage increases,the liability to atherosclerosis decreases. They arehigher in females than in males. Due to their highprotein content they possess the highest density.e) Albumin-free fatty acids complex: It is a proteolipidcomplex with 99% protein content associated withlong-chain free fatty acids for transporting them.
Cholesterol:• Importance: -• It is the most important sterol in animal tissues as free alcoholor in an esterified form (with linoleic, oleic, palmitic acids orother fatty acids).• Steroid hormones, bile salts and vitamin D are derivativesfrom it.• Tissues contain different amounts of it that serve a structuraland metabolic role, e.g., adrenal cortex content is 10%,whereas, brain is 2%, others 0.2-0.3%.• Source: - It is synthesized in the body from acetyl-CoA(1gm/day, cholesterol does not exist in plants) and is also takenin the diet (0.3 gm/day as in, butter, milk, egg yolk, brain, meatand animal fat).
Physical propeties:It has a hydroxyl group on C3, a double bondbetween C5 and C6, 8 asymmetric carbon atoms and a side chainof 8 carbon atoms.• It is found in all animal cells, corpus luteum and adrenal cortex,human brain (17% of the solids).• In the blood (the total cholesterol amounts about 200 mg/dL ofwhich 2/3 is esterified, chiefly to unsaturated fatty acids whilethe remainder occurs as the free cholesterol.CH3CH3HOCH3CH3CH3CholesterolCH3CH3HOCH3CH3CH3Cholesterol
• Chemical properties Intestinal bacteria reduce cholesterolinto coprosterol and dihydrocholesterol.• - It is also oxidized into 7-Dehydrocholesterol and furtherunsaturated cholesterol with a second double bond between C7and C8. When the skin is irradiated with ultraviolet light 7-dehydrocholesterol is converted to vitamin D3. This explainsthe value of sun light in preventing rickets.CH3CH3HOCH3CH3CH3Coprosterol,in fecesHCH3CH3HOCH3CH3CH3Dihydrocholesterol,in blood and other tissuesHCH3CH3HOCH3CH3CH3Coprosterol,in fecesHCH3CH3HOCH3CH3CH3Dihydrocholesterol,in blood and other tissuesH
• Ergosterol differs from 7-dehydrocholesterol in the sidechain. Ergosterol is converted to vitamin D2 by irradiation withUV Ergosterol and 7- dehydrocholesterol are called Pro-vitamins D or precursors of vitamin D.• - It was first isolated from ergot, a fungus then from yeast.Ergosterol is less stable than cholesterol (because of having 3double bonds).CH3CH3HOCH3CH3CH37-dehydrocholesterolCH3CH3HOCH3CH3CH3ErgosterolCH3CH3CH3HOCH3CH3CH37-dehydrocholesterolCH3CH3HOCH3CH3CH3ErgosterolCH3
Steroids• Steroids constitute an important class of biologicalcompounds.• Steroids are usually found in association with fat.They can be separated from fats after saponificationsince they occur in the unsaponifiable residue.• They are derivatives of cholesterol that is formed ofsteroid ring or nucleus.• Biologically important groups of substances, which containthis ring, are:1. Sterols.2. Adrenal cortical hormones.3. Male and female sex hormones.4. Vitamin D group.5. Bile acids.6. Cardiac glycosides.
• General consideration about naturally occurring steroids:A typical member of this group is cholesterol. Certain facts have to beconsidered when drawing steroid formula:1) There is always oxygen in the form of hydroxyl or ketone on C3.2) Rings C and D are saturated (stable).3) Methyl groups at C18 C19. In case of vitamin D, the CH3 group at C19becomes a methylene group (=CH2) and the ring B is opened, whereas, thismethyl group is absent in female sex hormones (estrogens).4) In estrogens (female sex hormones) ring A is aromatic and there is no methylgroup on C10.CH3CH3HOSteroid ring123 456 7891011121314 1516171819A BC DCH3CH3HOSteroid ring123 456 7891011121314 1516171819A BC D
• Bile acids:• They are produced from oxidation of cholesterol in the liverproducing cholic and chenodeoxycholic acids that areconjugated with glycine or taurine to produce glycocholic,glycochenodeoxycholic, taurocholic andtaurochenodeoxycholic acids. They react with sodium orpotassium to produce sodium or potassium bile salts.• Their function is as follows:1. Emulsification of lipids during digestion.2. Help in digestion of the other foodstuffs.3. Activation of pancreatic lipase.4. Help digestion and absorption of fat-soluble vitamins.5. Solubilizing cholesterol in bile and prevent gall stoneformation.6. Choleretic action (stimulate their own secretion).7. Intestinal antiseptic that prevent putrefaction
CH3CH3HOCH3CSodium-tauro orglyco-cholateCH3CH3HOCH3CSodium-tauro orglyco-chenodeoxycholateOHOH OHO OR1 or R2 R1 or R2(CH2)2 SO3-Na+H2NCH2 COO-Na+H2NSodium taurateSodium glycateR1 R2