In vivo metabolism of a new class of biologically active phospholipids: 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine,a platelet activating-hypotensive phospholipid

This report describes the in vivo metabolism of a new class of naturally occurring biologically active phospholipids (1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholines) that can cause hypotension and platelet aggregation. After intravenous injection in male rats, the acetylated ether phospholipid (1-[1′,2′-³H]alkyl) is rapidly cleared ($\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?30}\text{s}$) from blood and its metabolites are found in a variety of tissues. The tissues containing the highest levels of radioactivity are lung, liver, spleen, and kidney. Chromatographic results showed that a considerable portion of the active lipid is not readily catabolized in some of the major tissues examined; however, inactive metabolites were also found, mainly 1-alkyl-2-lyso-$\text{sn}$-glycero-3-phosphocholine and 1-alkyl-2-acyl-$\text{sn}$-glycero-3-phosphocholine; the latter has a long chain fatty acid at the $\text{sn}$-2 position instead of the acetate. The findings are consistent with our earlier data that show these same tissues have the most active enzyme systems for metabolizing 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine.     

Comparison of 1-O-alkyl-, 1-O-alk-1'-enyl-, and 1-O-acyl-2-acetyl-sn-glycero-3-phosphoethanolamines and -3-phosphocholines as agonists of the platelet-activating factor family

Four naturally occurring platelet-activating factor (PAF) analogs, 1-alk-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine, 1-hexade-canoyl-2-acetyl-sn-glycero-3-phosphocholine, 1-octadecanoyl-2-acetyl-sn-glycero-3-phosphocholine, and 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamine, stimulated human neutrophils (PMN) to mobilize Ca²⁺, degranulate, and produce Superoxide anion. They were, respectively, 5-, 300-, 500-, and 4000-fold weaker than PAF in each assay; inhibited PMN-binding of [³H]PAF at concentrations paralleling their biological potencies; and showed sensitivity to the inhibitory effects of PAF antagonists. PAF and the analogs, moreover, desensitized PMN responses to each other but not to leukotriene B₄ and actually increased (or primed) PMN responses to N-formyl-MET-LEU-PHE. Finally, 5-hydroxyicosatetraenoate-enhanced PMN responses to PAF and the analogs without enhancing the actions of other stimuli. It stereospecifically raised each analog's potency by as much as 100-fold and converted a fifth natural analog, 1-alk-1'-enyl-2-acetyl-sn-glycero-3-phosphoethanolamine from inactive to a weak stimulator of PMN. PAF and its analogs thus represent a structurally diverse family of cell-derived phospholipids which can activate, prime, and desensitize neutrophils by using a common, apparently PAF receptor-dependent mechanism.     

Characterization of the transacylase activity of rat liver 60-kDa lysophospholipase-transacylase. Acyl transfer from the sn-2 to the sn-1 position

Rat liver 60-kDa lysophospholipase-transacylase catalyzes not only the hydrolysis of 1-acyl-sn-glycero-3-phosphocholine, but also the transfer of its acyl chain to a second molecule of 1-acyl-sn-glycero-3-phosphocholine to form phosphatidylcholine (H. Sugimoto, S. Yamashita, J. Biol. Chem. 269 (1994) 6252–6258). Here we report the detailed characterization of the transacylase activity of the enzyme. The enzyme mediated three types of acyl transfer between donor and acceptor lipids, transferring acyl residues from: (1) the sn-1 to -1(3); (2) sn-1 to -2; and (3) sn-2 to -1 positions. In the sn-1 to -1(3) transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1(3) positions of glycerol and 2-acyl-sn-glycerol, producing 1(3)-acyl-sn-glycerol and 1,2-diacyl-sn-glycerol, respectively. In the sn-1 to -2 transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to not only the sn-2 positions of 1-acyl-sn-glycero-3-phosphocholine, but also 1-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. 1-Acyl-sn-glycero-3-phospho-myo-inositol and 1-acyl-sn-glycero-3-phosphoserine were much less effectively transacylated by the enzyme. In the sn-2 to -1 transfer, the sn-2 acyl residue of 2-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1 position of 2-acyl-sn-glycero-3-phosphocholine and 2-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. Consistently, the enzyme hydrolyzed the sn-2 acyl residue from 2-acyl-sn-glycero-3-phosphocholine. By the sn-2 to -1 transfer activity, arachidonic acid was transferred from the sn-2 position of donor lipids to the sn-1 position of acceptor lipids, thus producing 1-arachidonoyl phosphatidylcholine. When 2-arachidonoyl-sn-glycero-3-phosphocholine was used as the sole substrate, diarachidonoyl phosphatidylcholine was synthesized at a rate of 0.23 μmol/min/mg protein. Thus, 60-kDa lysophospholipase-transacylase may play a role in the synthesis of 1-arachidonoyl phosphatidylcholine needed for important cell functions, such as anandamide synthesis.     

Stereospecific activity of 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine and comparison of analogs in the degranulation of platelets and neutrophils

1-O-Hexadecyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine (platelet activating factor) stimulated the degranulation of rabbit platelets and human neutrophils, whereas the enantiomer, 3-O-hexadecyl-2-O-acetyl-$\text{sn}$-glycero-1-phosphocholine, was inactive. The analogs compared had the following relative potencies in degranulating platelets and neutrophils: 1-O-hexadecyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine > 1-O-hexadecyl-2-O-ethyl-$\text{sn}$-glycero-3-phosphocholine >$\text{rac}$-1-O-octadecyl-2-O-ethylglycero-3-phosphocholine = 1-O-hexadecyl-2-O-methyl-$\text{sn}$-glycero-3-phosphocholine >$\text{rac}$-1-O-dodecyl-2-O-ethyl-glycero-3-phosphocholine. The deacetylated compound, 1-O-hexadecyl-2-lyso-$\text{sn}$-glycero-3-phosphocholine, and 1-O-hexadecyl-2,2-dimethylpropanediol-3-phosphocholine were inactive. The active analogs selectively desensitized the response to each other in the neutrophils. It is suggested that these compounds may activate cells through interaction with a stereospecific receptor.     

Metabolism of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine by cell cultures

The metabolism of 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine (platelet-activating factor) was studied using various cultured cell lines. All incubations were done in the presence of bovine serum albumin and serum-free media, since albumin eliminates the adsorption of 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine to cultureware and serum enzymes interfere. Human leukemia (HL-60) cells, MDCK canine kidney cells, and transformed and nontransformed clones of mouse C3H1OT1/2 cells display varying rates of uptake, degradation, and capacities for reacylation of 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine. HL-60 cells displayed the highest uptake rate (24.6 pmol/mg cell protein/15 min). Whereas C3H10T1/2 cells in culture showed uptake rates comparable to other cells tested, they displayed a relative metabolic inertness towards 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine.     

Development of a novel scintillation proximity radioimmunoassay for platelet-activating factor measurement: comparison with bioassay and GC/MS techniques

A novel, facile and sensitive scintillation proximity radioimmunoassay (SPRIA) for quantitation of PAF has been developed. No separation of antibody bound [3H]PAF from free [3H]PAF is required as the assay employs protein A - coated fluomicrospheres (beads containing scintillant). The assay system was suitable for the quantitation of 0.03 to 2 pmol of 1-hexadecyl-2-acetyl-sn-glycero-3- phosphocholine. The cross-reactivity was high with 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine but was very low with PAF analogs such as 1-alkyl- and 1-acyl-2-lyso-sn-glycero-3-phosphocholine, 1-acyl-2-acetyl-sn-glycero-3-phosphocholine, and 1-alk-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine. The specificity of SPRIA was higher than that of bioassay (platelet degranulation assay). PAF receptor antagonists (L-652,731, WEB2086, and FR900452) at up to 10 nmol per tube had no affect on the SPRIA. These observations indicate that the specificity of the PAF antibody is quite different from that of the platelet receptor. The values obtained using SPRIA for the measurement of PAF produced in polymorphonuclear leukocytes with stimuli are comparable to those obtained by SIM/GC/MS analysis.     

Inactivation of platelet activating factor by rabbit platelets. Lyso-platelet activating factor as a key intermediate with phosphatidylcholine as the source of arachidonic acid in its conversion to a tetraenoic acylated product

[3H]PAF (platelet activating factor or 1-alkyl-2-acetyl-GPC) is converted to 1-alkyl-2-lyso-GPC and 1-alkyl-2-acyl-GPC by rabbit platelets (GPC is sn-glycero-3-phosphocholine). The deacetylation reaction does not involve the transfer of the acetate of PAF to any other lipid class and added exogenous lyso-PAF readily mixes with the cellular pool of the [3H]lyso-PAF intermediate formed from [3H]PAF. [3H]1-Alkyl-2-acyl-GPC produced during the inactivation of [3H]PAF contained primarily the tetraenoic acyl species (approximately 80% of the 3H in this fraction). The source of the arachidonic acid used for the reacylation of the lyso-PAF intermediate is the diacyl species, phosphatidylcholine.     

The animal ether phospholipid platelet-activating factor stimulates acidification of the incubation medium by cultured soybean cells

Addition of the animal ether phospholipid platelet-activating factor, 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, (PAF) stimulates medium acidification in cultured soybean (Glycine max L.) cells. The pH of the medium after 8–10 hours is on the average one pH unit lower than in controls. With fusicoccin an average pH difference of 1.7 units is reached. Phospholipids, glycerol, 1-oleyl-2-acetyl-sn-glycerol, 1-0-hexadecyl-sn-glycerol, and triolein at the same concentrations as PAF had no stimulatory effect on medium acidification. The detergents CHAPS and deoxycholate lead to alkalinization of the medium whereas lysophosphatidylcholine (LPC), a detergent with structural similarity to PAF, shows no effect.Abbreviations CHAPS (3-((3-cholamylopropyl) dimethylamino)-1-propanesulfonate) - DOC deoxycholic acid - FC fusicoccin - LPC lysophosphatidylcholine - OAG 1-oleyl-2-acetyl-sn-glycerol - PAF platelet-activating factor = 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine - IAA indole-3-acetic acid     

Lysophospholipase and lysophosphatidylcholine:Lysophosphatidylcholine transacylase from rat lung: Evidence for a single enzyme and some aspects of its specificity

An enzyme preparation was isolated from rat lung cytosol with the capability to transfer the fatty acyl chain from 1-acyl-sn-glycero-3-phosphocholine to water and to another molecule of 1-acyl-sn-glycero-3-phosphocholine. The evidence presented to indicate that a single protein confers both activities includes: (a) both normal and sodium dodecyl sulfate polyacrylamide disc gel electrophoresis showed a single protein band, and (b) heat treatment and preincubation with increasing amounts of diisopropylfluorophosphate resulted in concomitant loss of fatty acid and phosphatidylcholine formation. The enzyme converted 1-[9,10-³H₂]stearoyl-sn-glycero-3-phospho[¹⁴C-methyl]choline into phosphatidylcholine with an isotopic ³H/¹⁴C ratio twice that of the substrate, even when an excess of unlabeled fatty acid was present. The acyl group from palmitoyl-propanediol (1,3)-phosphocholine and palmitoyl-propanediol (1,3)-phosphoethanolamine could be transferred to lysophosphatidylcholine acceptor to yield phosphatidylcholine. Neither acylglycerols and cholesterol nor glycero-3-phosphate and glycero-3-phosphocholine served as acyl acceptors. Lysophosphatidylethanolamine and lysophosphatidyglycerol were converted also into the corresponding diacylphospholipids. Palmitoyllysophosphatidylcholine is preferentially converted into phosphatidylcholine when compared with stearoyllysophosphatidylcholine. The possible involvement of the enzyme in the synthesis of dipalmitoylphosphatidylcholine for the production of lung surfactant is discussed.     

Maintenance of respiratory control by beef heart mitochondria incubated at 25 degrees C: response to protective agents and to protective agents and to prior stress.

Lysophospholipase D (EC 3.1.4.-) activity was demonstrated in rat kidneys, intestines, lungs, testes, and liver. The liver enzyme was studied in greatest detail and its labeled products were identified by chemical and Chromatographic techniques. This enzyme hydrolyzes 1-[1-¹⁴C]hexadecyl-sn-glycero-3-phosphoethanolamine and 1-[1-¹⁴C]hexadecyl-sn-glycero-3-phosphocholine to yield 1-[1-¹⁴C]hexadecyl-sn-glycero-3-phosphate; the initial product is subsequently dephosphorylated by a phosphohydrolase in microsomes to form 1-[1-¹⁴C]hexadecyl-sn-glycerol. The possibility that phospholipase C and a phosphotransferase were responsible for the formation of 1-[1-¹⁴C]hexadecyl-sn-glycero-3-phosphate was ruled out. Neither 1-[1-¹⁴C]hexadecyl-2-acyl-sn-glycero-3-phosphoethanolamine nor 1-[1-¹⁴C]hexadecyl-2-acyl-sn-glycero-3-phosphocholine was hydrolyzed. The enzyme requires Mg²⁺, is inhibited by Ca²⁺, and is stimulated by high salt concentrations; it is localized in the microsomal fraction and has a pH optimum between 7.0 and 7.6. Inhibition by sulfhydryl reagents and protection by glutathione and dithiothreitol suggest that a sulfhydryl group is required for activity. The enzyme is inhibited by detergents and by organic solvent extraction. It appears to be tightly bound to the microsomes, since repeated freeze-thawing or sonication did not release the activity, and trypsin digestion (either in the presence or in the absence of 0.04% deoxycholate) did not destroy the activity. Lysophospholipase D was previously known to occur only in brain (R. L. Wykle and J. M. Schremmer, 1974, J. Biol. Chem., 249, 1742–1746).     

Biosynthesis of pulmonary surfactant: Comparison of 1-palmitoyl-sn-glycero-3-phosphocholine and palmitate as precursors of dipalmitoyl-sn-glycero-3-phosphocholine in adenoma alveolar type II cells

Urethan-induced pulmonary adenomas of mice are composed of cells that appear to be morphologically identical to alveolar type II cells and synthesize disaturated diacyl-sn-glycero-3-phosphocholine, the major component of pulmonary surfactant. 1-[1-¹⁴C]Palmitoyl-sn-glycero-3-phosphocholine and [1-¹⁴C]palmitic acid were compared as precursors of disaturated diacyl-sn-glycero-3-phosphocholine in the adenoma type II cells by incubating both substrates with whole adenomas. When the precursors were compared at equal concentrations (100 μm) in the presence of albumin (1 mg/ml), the rates of incorporation of 1-[1-¹⁴C]palmitoyl-sn-glycero-3-phosphocholine and [1-¹⁴C]palmitic acid into diacyl-sn-glycero-3-phosphocholine were 5.2 and 2.9 nmol/min · g tissue, respectively. The concentration of monoacyl-sn-glycero-3-phosphocholine (lysolecithin) in the blood plasma of BALB/c mice was 150 μm. In short-term labeling experiments, the label in disaturated diacyl-sn-glycero-3-phosphocholine was equally distributed between the sn-1 and sn-2 positions when 1-[1-¹⁴C]palmitoyl-sn-glycero-3-phosphocholine was the precursor, whereas 75 to 80% was in the sn-2 position when [1-¹⁴C]palmitic acid was the precursor. The ratios are consistent with incorporation of 1-palmitoyl-sn-glycero-3-phosphocholine via the lysolecithin:lysolecithin transacylase reaction and incorporation of palmitate via acylation of 1-palmitoyl-sn-glycero-3-phosphocholine by acyl-CoA:lysolecithin acyltransferase. 1-[1-¹⁴C]Palmitoyl-sn-glycero-3-phospho-[³H-methyl]choline was incorporated into total cellular diacyl-sn-glycero-3-phosphocholine with an isotope ratio similar to that of the precursor; the disaturated species was more enriched in ¹⁴C. These findings indicate the cells take up intact monoacyl-sn-glycero-3-phosphocholine and incorporate it into diacyl-sn-glycero-3-phosphocholine. The ability of the cells to utilize intact lysophosphoglycerides for synthesis of cellular lipids was further demonstrated by showing that ether analogs, 1-alkyl-sn-glycero-3-phosphocholine and 1-alkyl-sn-glycero-3-phosphoethanolamine, are taken up and acylated by the cells. Activities of lysolecithin:lysolecithin transacylase and acyl-CoA:lysolecithin acyltransferase were measured in subcellular fractions of the adenoma type II cells; the specific activities of the enzymes were 2.1 nmol/min · mg soluble protein and 21 nmol/min · mg microsomal protein, respectively. The total activity of the acyltransferase in the cell fractions was about four-fold higher than the activity of the transacylase. Characteristics of the two enzymes were studied and are discussed. The findings indicate that exogenous 1-palmitoyl-sn-glycero-3-phosphocholine and palmitic acid both serve as efficient precursors of disaturated diacyl-sn-glycero-3-phosphocholine in the adenoma alveolar type II cells.     

Activities of enzymes that metabolize platelet-activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) in neutrophils and eosinophils from humans and the effect of a calcium ionophore

Enzymatic systems in human blood cells are described for the activation and inactivation of a biologically active phospholipid (1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine) with hypotensive, platelet-aggregating, and inflammatory properties. The results document the presence of alkyldihydroxyacetone-phosphate synthase (forms the $\text{O}$-alkyl linkage in lipids), 1-alkyl-2-lyso-$\text{sn}$-glycero-3-phosphocholine:acetyl-CoA acetyltransferase (produces the biologically active molecule), and 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine: acetylhydrolase (destroys the biological activity) in human neutrophils and eosinophils. Both the acetyltransferase and acetylhydrolase activities are increased severalfold after treatment of normal neutrophils with ionophore A23187; however, alkyldihydroxyacetone-phosphate synthase activity is not influenced by the ionophore. Eosinophils isolated from patients with eosinophilia have significantly greater activities of all the enzymes studied than the eosinophils isolated from normal individuals. Our results indicate the acetyltransferase responsible for 1-alkyl-2-acetyl-$\text{sn}$-glycero-3-phosphocholine synthesis may serve an important role in human blood cells that release this biologically active phospholipid. Moreover, the acetyltransferase activity was found to be dramatically influenced by calcium flux.     

Characterization of the enzymatic hydrolysis of acetate from alkylacetylglycerols in the de novo pathway of PAF biosynthesis

This report describes the partial characterization of the enzymatic activity responsible for the hydrolysis of acetate from 1-alkyl-2-acetyl-sn-glycerol, the immediate precursor in the de novo synthesis of PAF (platelet-activating factor or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by Ehrlich ascites cells. The highest acetylhydrolase activity for this neutral lipid was associated with the membrane fractions from Ehrlich ascites cells (> 90% of total activity); only a minimal level of activity (< 10%) was observed in the cytosol which contrasts with the cytosolic site of PAF acetylhydrolase in normal cells. Hydrolysis of 1-[³H]hexadecyl-2-acetyl-sn-glycerol by the membrane fraction at pH 7.5 and 37°C gave apparent values for K_m and V_max of 45 μM and 179 nmol/min per mg protein, respectively. Hydrolysis of acetate from 1-[³H]hexadecyl-2-acetyl-sn-glycerol by the membrane fraction was not affected by 5 mM concentrations of Ca⁺², Mg⁺² or EDTA, but was significantly inhibited (80% reduction) by 10 mM NaF. Based on differences in both the subcellular distribution and response to inhibition by NaF, the neutral lipid acetylhydrolase does not appear to be the same enzyme that hydrolyzes acetate from platelet-activating factor. In contrast to inhibition of diacylglycerol lipase by p-chloromercuribenzoate and N-ethylmaleimide, we found no significant inhibition of acetate hydrolysis from 1-[³H]hexadecyl-2-acetyl-sn-glycerol by either of these compounds. Also, p-nitrophenyl acetate (a nonspecific esterase substrate) failed to inhibit acetate hydrolysis of 1-[³H])hexadecyl-2-acetyl-sn-glycerol. Our studies of this enzyme would indicate that it may play an important role in regulating the levels of platelet-activating factor synthesized by the de novo pathway via hydrolysis of the immediate precursor of PAF.     

Biochemical studies of the excitable membrane of Paramecium tetraurelia. II. Phospholipids of ciliary and other membranes

The phospholipids of cilia and deciliated bodies of Paramecium tetraurelia were isolated and characterized. 1-alkyl-2-acyl-sn-glycero-3-(2′-aminoethyl) phosphonate (GAEPL), phosphatidylethanolamine, and 1-alkyl-2-acyl-sn-glycero-3-phosphorylcholine (GPC) were the major lipids of Paramecium, and the minor lipids included phosphatidylinositol, cardiolipin, ceramide-(2-aminoethyl) phosphonate (CAEP), ceramide phosphorylethanolamine (COPE) and four sphingolipids whose identity was not established. The deciliated bodies contained 4% cardiolipin, 15% GAEPL, 41% phosphatidylethanolamine, 30% GPC and 3% each of CAEP and phosphatidylinositol; the cilia contained no cardiolipin, 24% GAEPL, 37% phosphatidylethanolamine, 15% GPC, 15% CAEP, 3% phosphatidylinositol, 2% COPE and small amounts (approx. 1%) of the four uncharacterized sphingolipids. No alteration in phospholipid composition was found among cells harvested in the various stages of growth. The phospholipids of six Paramecium mutants of three distinct phenotypes (pawn, paranoiac and fast) were also examined. Only one significant difference was found on comparison of the whole cell, deciliated body and cilia fraction of the mutants with the analogous fractions from wild type cells: the fast mutant, fA 97, had two extra, minor phospholipids (approx. 2%) in the deciliated body fraction that were tentatively identified as 1,2-diacyl-sn-glycero-3-(2′-aminoethyl) phosphonate (AEPL) and 1-alkyl-2-acyl-sn-glycero-3-phosphorylethanolamine (GPE).     

1-O-Alkyl-2-acyl-sn-glycero-3-phosphocholine: A novel source of arachidonic acid in neutrophils stimulated by the calcium ionophore A23187

Rabbit peritoneal neutrophils incorporated [¹⁴C]arachidonic acid into seven molecular species of choline-containing phosphoglycerides. These 2-[¹⁴C]arachidonoyl species differed with respect to the alkyl ether or acyl residue bound at the $\text{sn}$-1 position; four of the seven were ether-linked. Stimulation with calcium ionophore A23187 induced a proportionate release of arachidonate from all seven molecular species: 40% of the released arachidonate came from alkyl ether species. Thus, 1-$\text{O}$-alkyl-2-arachidonoyl-$\text{sn}$-glycero-3-phosphocholine (GPC) is a significant source of metabolizable arachidonic acid. Since 1-$\text{O}$-alkyl-2-lyso-GPC is the metabolic precussor of platelet activating factor, these results further interrelate pathways forming arachidonate metabolites and platelet activating factor; they also supply a rationale for the observation that both classes of stimuli form concomitantly during cell activation.     

Metabolic behavior of acetyl glyceryl ether phosphorylcholine on interaction with rabbit platelets

The metabolic fate of 1-O-[³H]alkyl-2-acetyl-sn-glycero-3-phosphorylcholine ([³H]-AGEPC) upon interaction with rabbit platelets was investigated. [³H]AGEPC was converted to a product identified as the long-chain fatty acyl analog. The reaction was unaffected by extracellular calcium. After a lag time of 30 to 60 s the kinetics of the conversion was linear. The rate of the reaction was found to be a function of platelet and AGEPC concentrations. Of the [³H]AGEPC (10^?9m) 85 ± 5% was processed into the-long chain fatty acyl analog within 1 h when incubated at 37 2C with a 1.25 × 10⁹ platelets per milliliter suspension. A maximal number of 1200 to 3600 [³H]AGEPC molecules were converted to the long-chain fatty acyl derivative per minute per platelet in the presence of 2 mm EDTA. Under similar conditions the 1-O-[³H]alkyl-2-(lyso)-sn-glycero-3-phosphorylcholine ([³H]lysoGEPC) also was transformed to a comparable long-chain fatty acyl derivative at a much slower rate and to a lower extent. No significant increase in lysoGEPC was noted in incubation mixtures containing [³H]AGEPC. The possible direct transacylation of AGEPC upon interaction with platelets is discussed as well as the possible involvement of this reaction in directly triggering the platelet response to AGEPC stimuli.     

Monoclonal anti-idiotypic antibodies to platelet activating factor (PAF) and their interaction with PAF receptors.

Monoclonal anti-idiotypic antibodies (3C3F3E4 and 10D3F8H7) that interact with platelet activating factor (PAF) receptors were generated using an auto-anti-idiotypic approach by immunizing mice with an aldehydic analog of PAF coupled to bovine thyroglobulin. The resulting hybridomas were screened for anti-idiotypic antibody (anti-anti-PAF) with F(ab')2 fragments of affinity-purified polyclonal rabbit anti-PAF antibody. These antibodies displayed internal image properties of PAF and were considered as Ab2 beta according to the following criteria: (a) they bound to F(ab')2 fragments of the affinity-purified rabbit polyclonal anti-PAF antibody that had high affinity for PAF; (b) they inhibited [3H]PAF binding to rabbit polyclonal anti-PAF antibody and its F(ab')2 fragment in a concentration-dependent manner; (c) they displaced [3H]PAF from the anti-PAF antibody/[3H]PAF complex specifically; (d) they inhibited [3H]PAF binding to PAF receptors on rabbit platelet membranes dose dependently; (e) they displaced [3H]PAF from the [3H]PAF/PAF receptor complex specifically; and (f) they stimulated rabbit platelets to aggregate, and this aggregation could be inhibited or totally blocked by specific PAF receptor antagonists WEB 2086 and SRI 63-441. All of the above are consistent with the first successful production of monoclonal antibodies that mimic PAF and interact specifically with the PAF binding domain of PAF receptors on rabbit platelet membranes.     

Metabolism of platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and 1-alkyl-2-acetyl-sn-glycerol by human endothelial cells

The metabolism of platelet activating factor (1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine) and 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol was studied in cultures of human umbilical vein endothelial cells. Human endothelial cells deacetylated 1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine to the corresponding lyso compound (1-[1,2-3H]alkyl-2-lyso-sn-glycerol-3-phosphocholine) and a portion was converted to 1-[1,2-3H]alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholine. Lyso platelet activating factor (lyso-PAF) (1-[1,2-3H]alkyl-2-lyso-sn-glycero-3-phosphocholine) was detected in the media very early during the incubation and the amount remained higher than the level of the lyso product observed in the cells. Cellular levels of 1-[1,2-3H]alkyl-2-lyso-sn-glycero-3-phosphocholine were significantly higher than the acylated product (1-[1,2-3H]alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholine) at all times during the 60-min incubation period, which suggests that the ratio of acetylhydrolase to acyltransferase activities is greater in endothelial cells than in most other cells. When endothelial cells were incubated with 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol, a known precursor of PAF, 1-[1,2-3H]alkyl-sn-glycerol was the major metabolite formed (greater than 95% of the 3H-labeled metabolites during 20- and 40-min incubations). At least a portion of the acetate was removed from 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol by a hydrolytic factor released from the endothelial cells into the medium during the incubations. Only negligible amounts of the total cellular radioactivity (0.2%) was incorporated into platelet activating factor (1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine); therefore, it is unlikely that the previously observed hypotensive activity of 1-alkyl-2-acetyl-sn-glycerols can be explained on the basis of the conversion to platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by endothelial cells. Results of this investigation indicate that endothelial cells play an important role in PAF catabolism. Undoubtedly, the endothelium is important in the regulation of PAF levels in the vascular system.     

On the phospholipid metabolism of glial cell primary cultures

Primary cultures prepared from newborn rat brain, consisted after 16 or 17 days mainly of astrocytes and of oligodendrocytes. 1-Alkenyl-sn-glycero-3-phosphoethanolamine (lysoplasmalogen) was used as substrate for studies on the metabolism of ethanolamine-glycerophospholipids. After 3 hr incubation two main products were observed: a) 1-alkenyl-2-acyl-sn-glycero-3-phosphoethanolamine (=ethanolamine plasmalogen) and b) 1-alkenyl-2-acyl-sn-glycero-3-phosphocholine (=choline plasmalogen). The acylation rate reached saturation at about 10 nmol substrate/mg cell protein with aV _max of 30 nmol×mg cell protein^–1×3 hr^–1. This acylated compound amounted to almost 60% of all radioactivity internalized, whereas the second product, choline plasmalogen, came to 20%. Unchanged substrate was found within the cells only in small amounts, even at maximum substrate internalization. These results were discussed in comparison with those obtained with 1-alkyl-sn-glycero-3-phosphoethanolamine under the same conditions (25).     

Lipid Acetylation Reactions and the Metabolism of Platelet-Activating Factor

In this review properties of lipid acetyltransferase enzymes are outlined. The three activities of interest are lyso PAF acetyltransferase (acetyl CoA: 1-alkyl-sn-glycero-3-phosphocholine acetyltransferase), AGP acetyltransferase (acetyl CoA: 1-alkyl sn-glycero-3-phosphate acetyltransferase) and a transacetylase activity that can transfer acetyl groups from PAF to lipid acceptors in the formation of 1-alkenyl-2-acetyl-sn-glycero-3-phosphoethanolamine and N-acetyl sphingosine (C₂ ceramide). This review focuses on the role of acetyltransferases and transacetylases within the metabolism of platelet-activating factor and specifically addresses characteristics of the enzymes, including subcellular localization, substrate selectivity, and enzymatic regulation