Incorporation of isomeric octadecenoic acids into alk-1-enyl moieties of cardiac glycerophospholipids of the rat

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

Characterization of trans-monounsaturated alkenyl chains in total plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) from sheep heart

In the present study, we investigated the alkenyl chains from sheep heart plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) after their conversion into trimethylene dioxyalkanyl (TMDOA) derivatives. Particular attention was given to monounsaturated alkenyl chains (C18 mainly). For this purpose, a combination of silver ion TLC and GLC on highly polar, very long capillary columns was applied to TMDOA derivatives. Approximately 30 different alkenyl chains could be separated, and the main observation was that the component previously reported as a cis-9 18:1 alkenyl chain in plasmalogens embraces in fact a wide range of trans and cis isomers, in amounts equal to 7.9 and 5.6%, respectively, of total alkenyl chains. Concerning the trans-monoenoate fraction, isomers with their ethylenic bond spanning from delta6-delta8 to delta16 were tentatively identified on the basis of their distribution profile, which was similar to that of trans-18:1 acids prepared and isolated from sheep adipose tissue. The main trans-monoenoic C18 alkenyl chain in sheep heart plasmalogens would thus have its double bond in position 11, which seems logical, as alkenyl chains are derived from the corresponding alcohols, themselves issued from the corresponding FA, and in this particular case, vaccenic (trans-11 18:1) acid. cis-Monoenoic C18 alkenyl chains also appear more complex than realized earlier, showing in particular isomers with their ethylenic bond farther than the delta9 position, in addition to the main isomer derived from oleic acid. Several trans-16:1 alkenyl chains could be observed (totaling ca. 1%), but cis-16:1 isomers were present in trace amounts only.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

Modification of alkenyl chain profile in plasmalogens of rat heart mitochondria by dietary trielaidin

Effects of dietary trielaidin upon the alkenyl chain profile of plasmalogens were studied using heart mitochondria of rats fed a semi-purified diet containing 10% of fat supplement in which elaidic acid accounted for 69% of total fatty acids. Alkyl substituted dioxane (ASD) derivatives of the alkenyl groups of plasmalogens were prepared and analyzed by silver nitrate TLC and by GLC on different phases (BDS and OV-275). After two months of feeding the experimental diet, 40% of the ASD contained a trans-octadecenyl chain, suggesting that dietary elaidic acid was reduced in vivo to the corresponding alcohol and incorporated into plasmalogens. There was a simultaneous decrease in the percentage of ASD containing saturated chains, but the percentage of ASD substituted with cis-octadecenyl chains was not significantly affected. These observations suggested that elaidic acid may compete with saturated fatty acids, but not with cis-octadecenoic acids during the plasmalogen biosynthesis. Feeding trielaidin did not seem to have any significant influence on the relative proportions of plasmalogens, which accounted for 11-12%, on a phosphorus basis, of total heart mitochondria phospholipids.

Formation of ether lipids and wax esters in mammalian cells. Specificity of enzymes with regard to carbon chains of substrates

The incorporation of radioactivity from individual constituents of an equimolar mixture of saturated straight-chain alcohols (14:0, 16:0, 18:0, 20:0) and of nearly uniform mixtures of isomeric cis- or trans-octadecenols (delta 8-delta 16) into alkyl, alk-1-enyl and acyl moieties of diradylglycerophosphocholines and diradylglycerophosphoethanolamines and into alkyl and acyl moieties of wax esters was studied in rat brain as well as in L 1210 and S 180 ascites cells. The pattern of incorporation of radioactivity from the substrates into alkyl and alk-1-enyl moieties of ether phospholipids and into alkyl moieties of wax esters reveals the following: (1) The enzymes catalyzing the biosynthesis of alkylacylglycerols, the common intermediates of cholinephospholipids and ethanolaminephospholipids, have no substrate specificity with regard to position of the double bond of either cis- or trans-octadecenols or of intermediate ether lipids derived therefrom. (2) CDPcholine:diradylglycerol cholinephosphotransferases exhibit a strong preference for alkylacylglycerols with cis-8, cis-9 and cis-10-octadecenyl moieties, but no preference for the double bond position in the trans-octadecenylacylglycerols. (3) CDPethanolamine:diredylglycerol ethanolaminephosphotransferases have no substrate specificity with regard to position of the double bond in cis- or trans-octadecenyl moieties of alkylacylglycerols. (4) THe enzyme systems catalyzing the biosynthesis of alkylacylglycerophosphocholines and alkylacylglycerophosphoethanolamines exhibit substrate specificity with regard to chain-length of saturated alcohols and intermediate ether lipids derived therefrom. (5) Alkylacylglycerophosphoethanolamine desaturase and (6) wax ester synthase are highly specific for alkylacylglycerophosphoethanolamines and long-chain alcohols, respectively, with regard to chain-length of saturated alkyl moieties, but not with regard to position of double bonds of cis- or trans-octadecenyl moieties.