Regulation of the synthesis of platelet-activating factor and its inactive storage precursor (1-alkyl-2-acyl-sn-glycero-3-phosphocholine) from 1-alkyl-2-acetyl-sn-glycerol by rabbit platelets.
We have established previously that 1-alkyl-2-acetyl-sn-glycerol (alkylacetyl-G) can be converted into at least six metabolites by rabbit platelets, including alkylacetyl-sn-(glycero-3-phosphocholine) (-GPC), i.e. platelet-activating factor (PAF) and 1-alkyl-2-acyl-sn- (alkylacyl)-GPC. Since part of the biological functions of alkylacetyl-G can be explained by its metabolic conversion to PAF and also to alkylacyl-GPC as an inactive storage precursor of PAF, the present study focused on the regulation of the synthesis of PAF and alkylacyl-GPC from alkylacetyl-G. Our results document the presence of a specific dithiothreitol (DTT)-insensitive cholinephosphotransferase in saponin-permeabilized rabbit platelets and show that DTT potentiates the production of PAF from alkylacetyl-G but inhibits the formation of phosphatidylcholine from diolein. We also demonstrated that the availability of CDP-choline controls the generation of PAF from alkylacetyl-G. Furthermore, when CTP: phosphocholine cytidylyltransferase is activated to produce more CDP-choline through the translocation of this enzyme from the cytosol to membranes by incubating the rabbit platelets with 0.2 mM sodium oleate, the production of PAF from alkylacetyl-G is increased 5-fold. More importantly, our experiments reveal the presence of two metabolic pathways that are responsible for the synthesis of alkylacyl-GPC from alkylacetyl-G, with each producing a unique molecular species composition of the stored PAF precursor, alkylacyl-GPC. The latter is enriched in polyunsaturates (70.7-78.5% 20:4) when formed through the remodeling pathway of PAF cycle via alkylacetyl-G (DTT-insensitive cholinephosphotransferase)----alkylacetyl-GPC----alkyllyso-GPC---- alkylacyl-GPC . Alkylacyl-GPC containing saturated species (71.8% 16:0) is generated by the retroconversion/de novo pathway according to the reaction scheme of alkylacetyl-G----alkyl-G----alkyllyso-glycero-3-phosphate (-GP)----alkylacyl-GP----alkylacyl-G (DTT-sensitive cholinephosphotransferase)----alkylacyl-GPC. Inactivation of PAF through the remodeling/PAF cycle can generate alkylacyl-GPC at both low (1.75 x 10(-7) M) and high (10(-6) M) concentrations of PAF whereas the conversion of alkylacetyl-G to alkylacyl-GPC via PAF through the remodeling pathway only occurs at a low concentration (1.75 x 10(-7) M). At a high concentration (10(-6) M), alkylacetyl-G is converted to alkylacyl-GPC via the retroconversion/de novo route. These data suggest that the formation of PAF by the DTT-insensitive cholinephosphotransferase activity limits the amounts of alkylacyl-GPC produced from alkylacetyl-G through this remodeling pathway (PAF cycle).(ABSTRACT TRUNCATED AT 400 WORDS)
Lee
T C
TC
Medical Sciences Division, Oak Ridge Associated Universities, Tennessee 37831-0117.
Malone
B
B
Blank
M L
ML
Fitzgerald
V
V
Snyder
F
F
D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D019916:Choline-Phosphate Cytidylyltransferase; D003566:Cytidine Diphosphate Choline / Q000378:metabolism; D002798:Diacylglycerol Cholinephosphotransferase / Q000378:metabolism; D004075:Diglycerides / Q000378:metabolism; D004229:Dithiothreitol / Q000494:pharmacology; D005995:Glyceryl Ethers / Q000378:metabolism*; D007700:Kinetics; D009713:Nucleotidyltransferases / Q000378:metabolism; D019301:Oleic Acid; D009829:Oleic Acids / Q000494:pharmacology; D010713:Phosphatidylcholines / Q000096:biosynthesis; D010972:Platelet Activating Factor / Q000031:analogs & derivatives* / Q000096:biosynthesis*; D011817:Rabbits; D010738:Type C Phospholipases / Q000378:metabolism
C036761:1-alkyl-2-acyl-sn-glycero-3-phosphocholine; D004075:Diglycerides; D005995:Glyceryl Ethers; D009829:Oleic Acids; D010713:Phosphatidylcholines; D010972:Platelet Activating Factor; D019301:Oleic Acid; C013173:osteum; D003566:Cytidine Diphosphate Choline; C055394:1-palmityl-2-acetylglycerol; D009713:Nucleotidyltransferases; D019916:Choline-Phosphate Cytidylyltransferase; D002798:Diacylglycerol Cholinephosphotransferase; D010738:Type C Phospholipases; D004229:Dithiothreitol; C013965:diolein