The regulation of CoA-independent transacylation reactions in neuronal nuclei by lysophospholipid, free fatty acid, and lysophospholipase: the control of nuclear lyso platelet-activating factor metabolism.
CoA-independent transacylase activities generating alkylacylglycerophosphocholine (AAGPC) from alkylglycerophosphocholine (1-alkyl GPC) were considerably enriched in neuronal nuclei isolated from rabbit cerebral cortex. Specific nuclear transacylation activities were 13 times the corresponding microsomal values. Several lysophospholipids, notably 1-acyl glycerophosphocholine (1-acyl GPC), 1-alkenyl GPC and 1-alkenyl GPE (1-alkenyl glycerophosphoethanolamine) inhibited the transacylation of 1-alkyl GPC. The inhibitory effects of 1-acyl GPC were seen in the presence of MAFP (methyl arachidonoylfluorophosphonate) or free oleate, compounds that inhibit neuronal nuclear lysophospholipase. When neuronal nuclei were preincubated with 1-alkyl GPC, the radioactive AAGPC product served as donor in transacylation reactions, to generate 1-alkyl GPC. In these nuclear reactions, 1-palmitoyl GPE and 1-palmitoyl GPC appeared to be poor acceptor substrates, when compared with corresponding 1-alkyl and 1-alkenyl analogues. The presence of free oleate or MAFP in the reactions containing 1-acyl GPC boosted the release of 1-alkyl GPC from AAGPC. These observations are of particular relevance to brain ischemia in which lysophospholipid, free fatty acid, and platelet-activating factor (PAF) levels rise dramatically. PAF can be made by the nuclear acetylation of 1-alkyl GPC, which is formed by nuclear transacylation mechanisms. Yet transacylase also removes 1-alkyl GPC, and thus this enzyme activity can regulate 1-alkyl GPC availability. Our observations indicate that lysophospholipids promote the formation of 1-alkyl GPC from nuclear AAGPC via transacylation, while free fatty acid likely prolongs the lifetime of 1-acyl lysophospholipids substrates by lysophospholipase inhibition. Similarly, once 1-alkyl GPC is formed, other lysophospholipids effectively compete with this 1-alkyl analogue and reduce its conversion back to AAGPC by transacylation. Free oleate, in this case, sustains 1-acyl lysophospholipid inhibitors of 1-alkyl GPC transacylation. Thus the cycle of transacylation may favour 1-alkyl GPC formation during ischemia, increasing levels of 1-alkyl GPC for nuclear acetylation reactions and PAF formation. The nuclear generation of PAF is of considerable importance as PAF can play regulatory roles in transcription events associated with inflammation.
Baker
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RR
Department of Biochemistry, University of Toronto, Ontario, Canada.
Chang
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HY
D000215:Acylation; D000818:Animals; D001095:Arachidonic Acids / Q000378:metabolism; D002417:Cattle; D002467:Cell Nucleus / Q000201:enzymology / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism; D002845:Chromatography; D003065:Coenzyme A / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005227:Fatty Acids / Q000378:metabolism*; D007511:Ischemia; D008245:Lysophospholipase / Q000378:metabolism*; D008246:Lysophospholipids / Q000378:metabolism*; D008861:Microsomes / Q000378:metabolism; D008956:Models, Chemical; D009474:Neurons / Q000378:metabolism*; D019301:Oleic Acid / Q000378:metabolism; D063065:Organophosphonates / Q000378:metabolism; D010972:Platelet Activating Factor / Q000031:analogs & derivatives* / Q000378:metabolism; D011817:Rabbits; D013552:Swine
C036761:1-alkyl-2-acyl-sn-glycero-3-phosphocholine; D001095:Arachidonic Acids; D005227:Fatty Acids; D008246:Lysophospholipids; C029271:O-deacetyl platelet activating factor; D063065:Organophosphonates; D010972:Platelet Activating Factor; C104810:methyl arachidonylfluorophosphonate; D019301:Oleic Acid; D008245:Lysophospholipase; D003065:Coenzyme A