Substrate specificity of enzymes catalyzing the biosynthesis of ionic alkoxylipids from alcohols

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.

Metabolism of long-chain fatty acids, alcohols and alkylglycerols in the fish parasite Paratenuisentis ambiguus (Acanthocephala)

Specific differences between the acyl composition of lipids of the helminth Paratenuisentis ambiguus and its host eel, as shown previously, prompted us to study the lipid metabolism in this intestinal fish parasite. Adults and larvae of P. ambiguus were fed various lipid precursors, e.g., fatty acids, long-chain alcohols and 1-O-alkylglycerols, which may occur as common nutrients of intestinal parasites. Incorporation of [1-14C]palmitic acid into neutral and polar lipids was found to be similar under aerobic and near-anaerobic conditions. In adult parasites maintained in culture medium supplemented with glucose, [1-14C]palmitic acid was incorporated mainly into triacylglycerols and phosphatidylcholines, whereas [1-14C]oleic acid was incorporated preferentially into triacylglycerols. In fasted adults, as well as in larvae, [1-14C]oleic acid was mainly transferred to phosphatidylcholines. Lipolytic activity was detected in adult parasites that had been incubated with radioactive trioleoylglycerol. [1-14C]Hexadecan-1-ol was oxidized in P. ambiguus at a high rate to labeled palmitic acid, which was incorporated into various lipid classes of P. ambiguus. Small but significant proportions of radioactivity from hexadecan-1-ol were incorporated into ether glycerolipids of the parasite. A more direct precursor in ether glycerolipid metabolism, i.e., rac-1-O-[1'-14C]hexadecylglycerol, was incorporated into alkyl and 1'-alkenyl moieties of choline and etha-nolamine ether-glycerophospholipids of P. ambiguus in high yield. High proportions of labeled diacylglycerols, triacylglycerols and steryl esters were detected in surface lipids as well as lipid extracts of the culture media after incubation of P. ambiguus with [1-14C]palmitic or [1-14C]oleic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

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.