PHOSPHATIDYLCHOLINE AND RELATED LIPIDS

Transcription

1 PHSPHATIDYLLINE AND RELATED LIPIDS STRUCTURE, CCURRENCE, BIEMISTRY AND ANALYSIS 1. Phosphtidylcholine Structure nd ccurrence Phosphtidylcholine (once given the trivil nme lecithin ) is usully the most bundnt phospholipid in niml nd plnts, often mounting to lmost 50% of the totl, nd s such it is obviously the key building block of membrne bilyers. In prticulr, it mkes up very high proportion of the outer leflet of the plsm membrne. Phosphtidylcholine is lso the principl phospholipid circulting in plsm, where it is n integrl component of the lipoproteins, especilly the HDL. n the other hnd, it is less often found in bcteril membrnes, perhps 10% of species. It is neutrl or zwitterionic phospholipid over ph rnge from strongly cid to strongly lkline. In niml tissues, some of its membrne functions pper to be shred with the structurlly relted sphingolipid sphingomyelin lthough the ltter hs mny unique properties of its own. CR' R''C phosphtidylcholine P N( 3 ) 3 H P 2 N( 3 ) 3 1,2-dihexdecnoyl-sn-glycero-3-phosphocholine In niml tissues, phosphtidylcholine tends to exist in minly in the dicyl form, but smll proportions (in comprison to phosphtidylethnolmine nd phosphtidylserine) of lkylcyl nd lkenylcyl forms my lso be present. Dt for the compositions of these vrious forms from bovine hert muscle re listed in our web pges on Ether lipids. As generliztion, niml phosphtidylcholine tends to contin lower proportions of rchidonic nd docoshexenoic cids nd more of the C 18 unsturted ftty cids thn the other zwitterionic phospholipid, phosphtidylethnolmine. The sturted ftty cids re most bundnt in position sn-1, while the polyunsturted components re concentrted in position sn-2. Indeed, C 20 nd C 22 polyenoic cids re exclusively in position sn-2. Dietry fctors obviously influence ftty cid compositions, but in compring niml species, it would be expected tht the structure of the phosphtidylcholine in the sme metboliclly ctive tissue would be somewht similr in terms of the reltive distributions of ftty cids between the two positions. Tble 1 lists some representtive dt. There re some exceptions to the rule. The phosphtidylcholine in some orgns contins reltively high proportions of disturted moleculr species. For exmple, it is well known tht lung phosphtidylcholine in most if not ll niml species studied to dte contins high proportion (50% or more) of diplmitoyl-phosphtidylcholine. It ppers tht this is the min surfce-ctive component, providing lveolr stbility by decresing the surfce tension t the lveolr surfce to W.W. Christie 1

2 very low level. Also, the internl lipids of the niml cell nucleus (fter the externl membrne hs been removed) contin high proportion of disturted phosphtidylcholine, mounting to 10% of the volume indeed. This is synthesised entirely within the nucleus, unlike phosphtidylinositol for exmple, nd in contrst to other cellulr lipids its composition cnnot be chnged by extreme dietry mnipultion. It hs been suggested tht it my hve role in stbilizing or regulting the structure of the chromtin, s well s being source of dicylglycerols with signlling function. Tble 1. Positionl distribution of ftty cids in the phosphtidylcholines of some niml tissues. Position Ftty cid 16:0 16:1 18:0 18:1 18:2 20:4 22:6 Rt liver [1] sn trce sn Rt hert [2] sn sn Rt lung [3] sn sn Humn plsm [4] sn trce - sn Humn erythrocytes [4] sn sn Bovine brin (gry mtter) [5] sn sn trce Chicken egg [6] sn sn trce , Wood, R. nd Hrlow, R.D. Arch. Biochem. Biophys., 131, (1969); 2, Kuksis, A. et l. J. Lipid Res., 10, (1969); 3, Kuksis, A. et l. Cn. J. Physiol. Phrm., 46, (1968); 4, Mri, L. nd Kuksis, A. J. Lipid Res., 10, (1969); 5, Ybuuchi, H. nd 'Brien, J.S. J. Lipid Res., 9, (1968); 6, Kuksis, A. nd Mri, L. Lipids, 2, (1967). The positionl distributions of ftty cids in phosphtidylcholine in representtive plnts nd yest re listed in Tble 2. In the leves of the model plnt Arbidopsis thlin, sturted ftty cids re concentrted in position sn-1, but monoenoic ftty cids re distributed pproximtely eqully between the two positions, nd there is prepondernce of di- nd triunsturted ftty cids in position sn-2. The sme is true for soyben lecithin. The pttern is somewht similr for the yest Lipomyces lipoferus, except tht much of the 16:1 is in position sn-1 in this instnce. W.W. Christie 2

3 Tble 2. Composition of ftty cids (mol %) in positions sn-1 nd sn-2 in the phosphtidylcholine from plnts nd yest. Position Ftty cid 16:0 16:1 18:0 18:1 18:2 18:3 A. thlin (leves) [1] sn-1 42 trce sn-2 1 trce trce Soyben lecithin [2] sn sn L. lipoferus [3] sn trce sn trce , Browse, J., Wrwick, N., Somerville, C.R. nd Slck, C.R. Biochem. J., 235, (1986). 2, Blnk, M.L., Nutter, L.J. nd Privett,.S. Lipids, 1, (1966). 3, Hley, J.E. nd Jck, R.C. Lipids, 9, (1974). 2. Phosphtidylcholine Biosynthesis nd Biologicl Function There re three mechnisms for the biosynthesis of phosphtidylcholine. Choline itself is not synthesised s such by niml cells nd is n essentil nutrient. It must be obtined from dietry sources or by degrdtion of existing choline-contining lipids, for exmple those produced by the second pthwy described below. nce tken up into cells, choline is immeditely phosphorylted by choline kinse in the cytoplsm of the cell to phosphocholine, which is rected with cytidine triphosphte (CTP) to form cytidine diphosphocholine. The membrne-bound enzyme CDPcholine:1,2-dicylglycerol cholinephosphotrnsferse in the endoplsmic reticulum ctlyses the rection of the lst compound with sn-1,2-dicylglycerol to form phosphtidylcholine. This is the min pthwy for the synthesis of phosphtidylcholine in nimls nd plnts, nd it is nlogous to the biosynthesis of phosphtidylethnolmine (see our web pges on this lipid). ATP H N( 3 ) 3 choline cytidine ADP CTP PPi P N( 3 ) 3 P P N( 3 ) 3 phosphocholine cytidine diphosphocholine CDP-choline CR' R''C H sn-1,2-dicylglycerol CR' R''C P N( 3 ) 3 phosphtidylcholine The discovery of the importnce of this pthwy depended little on serendipity in tht in experiments in the lb of Professor Eugene Kennedy, smples of denosine triphosphte (ATP) contined some cytidine triphosphte (CTP) s n impurity. However, luck is of little vlue without W.W. Christie 3

4 receptive minds, nd Kennedy nd co-workers demonstrted tht the impurity ws n importnt metbolite tht ws essentil for the formtion of phosphtidylcholine. The bove rection, together with the biosynthetic mechnism for phosphtidylethnolmine, is significntly different from tht for phosphtidylglycerol, phosphtidylinositol nd crdiolipin. Both mke use of nucleotides, but with the ltter, the nucleotide is covlently linked directly to the lipid intermedite, i.e. cytidine diphosphte dicylglycerol. The source of the sn-1,2-dicylglycerol precursor, which is lso key intermedite in the formtion of phosphtidylethnolmine nd phosphtidylserine, nd of tricylglycerols, is phosphtidic cid. In this instnce, the importnt enzyme is phosphtidic cid phosphtse, which is present minly in the endoplsmic reticulum of the cell. CR CR' P 3 H phosphtidic cid CR CR' H sn-1,2-dicylglycerol The second pthwy for biosynthesis of phosphtidylcholine involves sequentil methyltion of phosphtidylethnolmine, with S-denosylmethionine s the source of methyl groups, with monond dimethyl-phosphtidylethnolmine (see the web pges) s intermedites nd ctlysed by the enzyme phosphtidylethnolmine N-methyltrnsferse. A single enzyme (~20 Kd) ctlyses ll three rections nd is locted minly in the endoplsmic reticulum where it spns the membrne. This is mjor pthwy in the liver, but not in other niml tissues or in generl in higher orgnisms. It my be the min route to phosphtidylcholine in those bcteril species tht produce this lipid nd in yests. R''C CR' P NH 3 H phosphtidylethnolmine R''C CR' P NH 3 H phosphtidylmonomethylethnolmine R''C CR' P N( 3 ) 3 phosphtidylcholine R''C CR' P NH( 3 ) 2 H phosphtidyldimethylethnolmine This liver enzyme is especilly importnt when choline is deficient in the diet. A by-product of the biosynthesis of phosphtidylcholine from phosphtidylethnolmine is the conversion of S- denosylmethionine to S-denosylhomocysteine, which is hydrolysed in the liver to denosine nd homocysteine. It is noteworthy tht elevted plsm homocysteine is risk fctor for crdiovsculr disese nd myocrdil infrction. Phosphtidylcholine biosynthesis by both pthwys in the liver is necessry for norml secretion of the plsm lipoproteins (VLDL nd HDL), nd it is relevnt to number of humn physiologicl conditions. W.W. Christie 4

5 In one bcteril species, third pthwy for phosphtidylcholine biosynthesis tht is cholinedependent hs been found in which the lipid is formed in one step vi condenstion of choline directly with CDP-dicylglycerol. While phosphtidylcholine is mjor lipid in yests, recent work suggests tht it is not essentil if suitble lterntive growth substrtes re vilble, unlike higher orgnisms where perturbtion of phosphtidylcholine synthesis cn led to inhibition of growth or even cell deth. Enhnced synthesis of phosphtidylcholine ppers to occur in cncer cells nd solid tumours, nd this my prove to be trget for therpeutic gents. Whtever the mechnism of biosynthesis in tissues, it is pprent tht the ftty cid compositions nd positionl distributions on the glycerol moiety re determined post synthesis by extensive remodelling involving hydrolysis (phospholipses A 1, A 2 or B) nd re-cyltion, process tht is sometimes termed the Lnds cycle. The re-cyltion step is ctlysed by specific lysophosphtidylcholine cyltrnsferse, which hs been locted within the endoplsmic reticulum in orgns such s the liver, dipose tissue nd pncres. For exmple, the highly sturted moleculr species of phosphtidylcholine found in the nucleus re formed from species with more conventionl composition by remodelling. In plnts, ftty cids esterified to phosphtidylcholine cn serve s substrtes for desturses, nd this mens tht the ftty cid composition chnges lso fter the initil synthetic process. The process is further complicted in plnts in tht biosynthesis or prtil synthesis (vi lysophosphtidylcholine) occurs in different orgnelles, such s the endoplsmic reticulum, plstids nd mitochondri, from different ftty cid pools or with differing specificities. Becuse of the generlly cylindricl shpe of the molecule, phosphtidylcholine spontneously orgnizes into bilyers, so it is idelly suited to serve s the bulk structurl element of biologicl membrnes. Such properties re essentil to ct s blnce to those lipids tht do not form bilyers or tht form specific microdomins such s rfts. In ddition to its function s membrne constituent, phosphtidylcholine my hve role in signlling vi the genertion of dicylglycerols, especilly in the nucleus. Although the pool of the precursor is so gret in mny tissues tht turnover is not esily mesured, the presence of phospholipses C nd D specific for phosphtidylcholine, which re ctivted by number of gonists, suggests such function especilly in the cell nucleus. Dicylglycerols formed in this wy would be much more sturted thn those derived from phosphtidylinositol, nd would not be expected to be s ctive. The plsmlogen form of phosphtidylcholine my lso hve signlling function, s thrombin tretment of endothelil cells ctivtes selective hydrolysis (phospholipse A 2 ) of moleculr species contining rchidonic cid in the sn-2 position, relesing this ftty cid for eicosnoid production. The dicyl form of phosphtidylcholine my hve relted function in signl trnsduction in other tissues. In ddition, it is known tht the enzyme 3-hydroxybutyrte dehydrogense requires to be bound to phosphtidylcholine before it cn function optimlly. Phosphtidylcholine is the biosynthetic precursor of sphingomyelin nd s such must hve some influence on the mny metbolic pthwys tht constitute the sphingomyelin cycle. It is lso precursor for phosphtidic cid, lysophosphtidylcholine nd pltelet-ctivting fctor, ech with importnt signlling functions, nd of phosphtidylserine. n ctbolism of choline-contining lipids, much of the choline is re-used for phosphtidylcholine biosynthesis, often fter being returned to the liver. Some is oxidized in the kidney nd liver to betine, which serves s donor of methyl groups for S-denosylmethionine production. A proportion is used in nervous tissues for production of cetylcholine, which is neurotrnsmitter of importnce to lerning, memory nd sleep. Some choline is lost through excretion of phosphtidylcholine in the bile. W.W. Christie 5

6 3. Lysophosphtidylcholine Lysophosphtidylcholine, with one mole of ftty cid per mole of lipid in position sn-1, is found in smll CR' mounts in most tissues. It is formed by hydrolysis of H phosphtidylcholine by the enzyme phospholipse A 2, s prt of the de-cyltion/re-cyltion cycle tht P N( 3 ) 3 controls its overll moleculr species composition. It cn lso be formed indvertently during extrction of lipids lysophosphtidylcholine from tissues if the phospholipse is ctivted by creless hndling. There is lso phospholipse A 1, which is ble to cleve the sn-1 ester bond. In plsm of niml species, pprecible mounts of lysophosphtidylcholine re formed by specific enzyme system, lecithin:cholesterol cyltrnsferse (LCAT), which is secreted from the liver. The enzyme ctlyses the trnsfer of the ftty cids of position sn-2 of phosphtidylcholine to the free cholesterol in plsm, with formtion of cholesterol esters nd of course of lysophosphtidylcholine. Identifiction of highly specific phospholipse A 2 in peroxisomes tht genertes 2-rchidonoyl lysophosphtidylcholine suggests tht this my be of relevnce to eicosnoid genertion nd signlling. Lysophosphtidylcholine hs lso been shown to be pthologicl component of oxidized lipoproteins (LDL) in plsm nd of therosclerotic lesions. Recently, it hs been found to hve some functions in cell signlling, nd specific receptors (coupled to G proteins) hve been identified. It ctivtes the specific phospholipse C tht releses dicylglycerols nd inositol triphosphte with resultnt increses in intrcellulr C 2 nd ctivtion of protein kinse C. It lso ctivtes the mitogen-ctivted protein kinse in certin cell types. Steroyl lysophosphtidylcholine hs been shown to be protective ginst lethl sepsis in experimentl nimls by vrious mechnisms, including stimultion of neutrophils to eliminte invding pthogens through peroxide-dependent rection. 4. Pltelet-Activting Fctor Pltelet-ctivting fctor (PAF) or 1-lkyl-2-cetyl-sn-glycero-3-phosphocholine is n ether nlogue of phosphtidylcholine tht is biologiclly ctive. For convenience, it is discussed in our web pges on Ether lipids. 5. Anlysis Anlysis of phosphtidylcholine presents no prticulr problems. It is redily isolted by thin-lyer or high-performnce liquid chromtogrphy methods. Determintion of the diplmitoyl species in lung surfctnt is more demnding tsk, but specific methods hve been published. Phospholipse A 2 from snke venom is used in methods to determine the position of ftty cids on the glycerol moiety. Modern mss spectrometry methodology hs gretly simplified the tsk of moleculr species nlysis. Recommended Reding o Becker, K.P. nd Hnnun, Y.A. Dicylglycerols. In: Bioctive Lipids, pp (edited by A. Nicolou nd G. Kokotos, The ily Press, Bridgwter) (2004). W.W. Christie 6

7 o Christie, W.W. Lipid Anlysis - 3rd Edition (ily Press, Bridgwter) (2003). o de Kroon, A.I.P.M. Metbolism of phosphtidylcholine nd its implictions for lipid cyl chin composition in Scchromyces cerivisie. Biochim. Biophys. Act, 1771, (2007). o Hunt, A.N. Dynmic lipidomics of the nucleus. J. Cell. Biochem., 97, (2006). o Kent, C. Regultory enzymes of phosphtidylcholine biosynthesis: personl perspective. Biochim. Biophys. Act, 1733, (2005). o Li, Z. nd Vnce, D.E. Phosphtidylcholine nd choline homeostsis. J. Lipid Res., 49, (2008). o Sohlenkmp, C., Lopez-Lr, I.M. nd Geiger,. Biosynthesis of phosphtidylcholine in bcteri. Prog. Lipid Res., 42, (2003). o Vnce, D.E., Li, Z. nd Jcobs, R.L. Heptic phosphtidylethnolmine N-methyltrnsferse, unexpected roles in niml biochemistry nd physiology. J. Biol. Chem., 282, (2007). o Vnce, D.E. nd Vnce, J. (editors) Biochemistry of Lipids, Lipoproteins nd Membrnes. 4 th Edition. (Elsevier, Amsterdm) (2002) severl chpters. o Veldhuizen, R., Ng, K., rgeig, S. nd Possmyer, F. The role of lipids in pulmonry surfctnt. Biochim. Biophys. Act, 1408, (1998). o Xu, Y. Sphingosylphosphorylcholine nd lysophosphtidylcholine: G protein-coupled receptors nd receptor-medited signl trnsduction. Biochim. Biophys. Act, 1582, (2002). o Yn, J.-J., nd eleven others. Therpeutic effects of lysophosphtidylcholine in experimentl sepsis. Nture Medicine, 10, (2004). W.W. Christie Scottish Crop Reserch Institute (nd Mylnefield Lipid Anlysis), Invergowrie, Dundee (DD2 5DA), Scotlnd 2/6/2008 W.W. Christie 7