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Biomedical subjects

F Snyder

Publications and source records attributed to F Snyder.

At least 91 records · Page 5Linked to original sources
PubMed1985

Alkyldihydroxyacetonephosphate synthase mechanism: 18O studies of fatty acid release from acyldihydroxyacetone phosphate.

Alkyldihydroxyacetonephosphate synthase (alkyl-DHAP synthase) catalyzes the exchange of the ester-linked fatty acid of 1-O-acyldihydroxyacetone phosphate (1-O-acyl-DHAP) for a fatty alcohol that is attached in an ether linkage to form 1-O-alkyldihydroxyacetone phosphate (1-O-alkyl-DHAP). In our continuing investigation of the mechanism of this enzyme, we have examined the fatty acid released during the reaction. In contrast to the reports of others using whole microsomes, we found that the cleavage of fatty acid by purified preparations of alkyl-DHAP synthase was dependent on the presence of the cosubstrate, fatty alcohol. Furthermore, the amount of fatty acid produced was equivalent to the alkyl-DHAP formed. Our previously proposed detailed mechanism for alkyl-DHAP synthase predicted that the fatty acid should retain both of the carboxyl ester oxygens upon cleavage. Reactions carried out with palmitoyl-[18O]DHAP as substrate yielded [18O]palmitic acid as the product in agreement with this scheme.

Acylation↗
PubMed1985

Metabolism of platelet-activating factor by rat alveolar macrophages: lyso-PAF as an obligatory intermediate in the formation of alkylarachidonoyl glycerophosphocholine species.

1-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (alkylacetyl-GPC; platelet-activating factor; PAF) is actively taken up and metabolized by rat alveolar macrophages maintained in culture. The major metabolic products are lyso-PAF (alkyllyso-GPC) and alkylacyl-GPC. Lyso-PAF accumulates primarily in the media, whereas alkylacyl-GPC is predominantly associated with cellular lipids. The addition of unlabeled lyso-PAF to incubations initiated with [3H]PAF results in an increase in the amount of lyso-[3H]PAF product formed and a decrease in the final product, [3H]alkylacyl-GPC; however, the total amount of [3H]PAF metabolized remains unchanged. Unlabeled lyso-PAF thus enters the metabolic pool of the cell and competes with the deacetylated product of [3H]PAF, i.e., lyso-PAF, for acylation. High-performance liquid chromatography demonstrated that the reacylated product derived from lyso-PAF consisted primarily of the arachidonoyl-containing species that exists as the 16:0-20:4 molecular species. These results document that PAF is inactivated in rat alveolar macrophages via a deacetylation-reacylation reaction with lyso-PAF as an obligatory intermediate. The sequestering of arachidonic acid into the PAF precursor pool and the substantial amount of lyso-PAF secreted by macrophages into the extracellular fluid appear to be significant events in the inactivation process.

Animals↗
PubMed1985

Acylation of lysophospholipids by rabbit alveolar macrophages. Specificities of CoA-dependent and CoA-independent reactions.

Intact alveolar macrophages were found to acylate alkyl- and acyllysophospholipids with a high selectivity for arachidonate. A specific mechanism appears responsible for the incorporation of arachidonate into lysophospholipids in intact cells since the kinetic pattern for the formation of the 20:4 species was different from all other species. This specificity was investigated in more detail by examining the enzymatic acylation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine by macrophage membranes; in the absence of CoA, ATP, and Mg2+, this lysophospholipid was acylated with a high preference for arachidonate that was independent of added free fatty acids. The addition of CoA alone increased the rate of acylation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine, mainly due to an increase in the formation of species other than those containing arachidonate. When CoA, ATP, and Mg2+ were present, the macrophage membranes catalyzed the acylation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine without preference for arachidonate. A different apparent Km and Vmax was observed for reactions involving each cofactor condition. We conclude that the acylation of alkyl- and acyllysophospholipids by rabbit alveolar macrophages occurs by three separate mechanisms: a CoA-independent transacylation, a CoA-dependent transacylation (reverse reaction catalyzed by acyl-CoA acyltransferase), and an acyl-CoA-dependent acylation. The CoA-independent transacylation reaction is unique in that it is specific for arachidonate and accounts for the selective acylation of alkyl- and acyllysophospholipids by arachidonate in membrane preparations of alveolar macrophages. This reaction appears to be extremely important in the remodeling of phospholipid molecular species and the mobilization of arachidonate into ether-linked lipids. The transfer of arachidonate to 1-alkyl-2-lyso-sn-glycero-3-phosphocholine also is of importance in the final inactivation step for platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine), whereby 1-alkyl-2-arachidonoyl-sn-glycerol-3-phosphocholine (a stored precursor of both platelet activating factor and arachidonic acid metabolites) is formed.

Acylation↗
PubMed1985

Metabolism of unique diarachidonoyl and linoleoylarachidonoyl species of ethanolamine and choline phosphoglycerides in rat testes.

Selected molecular species of rat testicular 1,2-diradyl-sn-glycero-3-phosphocholines and 1,2-diradyl-sn-glycero-3-phosphoethanolamines were quantitated as their diradylglycerobenzoate derivatives, using a recently developed high-performance liquid chromatographic method. Increased amounts of docosapentaenoic acid were found in glycerophospholipids containing ether moieties compared with the diacyl phospholipids (e.g., docosapentaenoate-containing species comprised more than 80% of the alkylacyl subclass of the ethanolamine phosholipids as opposed to 29.3% of the diacyl subclass). Within 2 h after intratesticular injections of [5,6,8,9,11,12,14,15-3H]arachidonic acid, the 20:4-20:4 and 18:2-20:4 molecular species of the diacyl subclass of both the choline and ethanolamine glycerophosphatides had the highest specific radioactivities. These unique molecular species (20:4-20:4 and 18:2-20:4) also exhibited the largest percentage decrease in specific radioactivity 24 h after the intratesticular injections of [3H]arachidonic acid, which indicates these two species possess a high metabolic turnover. Two of the arachidonate-containing molecular species (18:1-20:4 and 18:0-20:4) in the ethanolamine plasmalogens showed only a small decrease in specific radioactivity, whereas a third species (16:0-20:4) actually had a 44% increase in specific radioactivity 24 h after the intratesticular injections of [3H]arachidonate. These data indicate that the 20:4-20:4, 18:2-20:4 and 18:1-20:4 species of phosphatidylcholine and/or phosphatidylethanolamine are most rapidly labeled after administration of [3H]arachidonic acid and that they appear to serve as the source of the [3H]arachidonate that is ultimately transferred to ethanolamine plasmalogens.

Animals↗
PubMed1985

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.

Acetylation↗
PubMed1985

Facile synthesis of platelet-activating factor and racemic analogues containing unsaturation in the sn-1-alkyl chain.

Platelet-activating factor, 1 (PAF, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine), and octadecyl-PAF were synthesized chemically as the racemates. The sn-1-O-alkyl isomers were isolated after treatment of the racemates with phospholipase A2 and subsequent reacetylation of the 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholines released. Analogues of PAF containing unsaturated alkyl moieties at the sn-1 position (2, 4, 5) were synthesized by utilizing the methoxyethoxymethyl protecting group as a novel method for preparing unsaturated alkyl lipids. This procedure provides a facile means for preparing unsaturated either phospholipids of defined structure that may be tritiated to high radiospecific activity for metabolic studies. Unsaturation in the alkyl chain had minimal effect on the bioactivities examined in this study.

Animals↗
PubMed1985

The adrenergic system and the cardiovascular effects of platelet activating factor (1-0-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) in SHR and WKY rats.

1-0-Hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (1-hexadecyl-2-acetyl-GPC, platelet activating factor, PAF) was previously shown to produce profound hypotension and sympathetic activation in conscious rats. To determine the role of the sympatho-adrenomedullary system in the cardiovascular responses elicited by 1-hexadecyl-2-acetyl-GPC, the vasoactive phospholipid was administered (1 nmol per 300 g) to a) intact, b) bilaterally demedullated, and c) propranolol- (a beta-adrenoceptor blocker) treated SHR and WKY rats. The hypotensive response to 1-hexadecyl-2-acetyl-GPC was prolonged in demedullated or propranolol-pretreated WKY rats and in propranolol-treated SHR rats. The extreme tachycardia produced by 1-hexadecyl-2-acetyl-GPC in both the WKY and SHR rats was abolished by propranolol pretreatment. Pressor responses to norepinephrine during the 1-hexadecyl-2-acetyl-GPC-induced hypotension in propranolol-pretreated rats were suppressed in both the normotensive and SHR rats. Plasma acetylhydrolase activity, which inactivates PAF, was higher in hypertensive (SHR) rats or demedullated WKY rats than in the normotensive (WKY) rats. These results show that the tachycardia evoked by 1-hexadecyl-2-acetyl-GPC is mediated solely by sympathetic activation and the beta-adrenergic receptors and further indicate the major role of the sympathetic system and beta-adrenoceptors in recuperation from 1-hexadecyl-2-acetyl-GPC-induced shock. The data also suggest that acetylhydrolase in serum is an important regulatory enzyme for controlling PAF levels in the vascular compartment.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1985

Cardiac output and peripheral vascular resistance during intravenous infusion of 1-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine in conscious normotensive and spontaneously hypertensive rats.

Cardiovascular responses to intravenous 1-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (56 and 112 pmoles/kg/min) were examined in conscious Wistar-Kyoto (WKY) normotensive rats and spontaneously hypertensive rats (SHR) instrumented with arterial and venous catheters, and an electromagnetic flow probe around the ascending aorta. The results suggest that this compound reduces mean blood pressure via a dose related fall in peripheral vascular resistance and cardiac index in conscious SHR and WKY rats.

Animals↗
PubMed1984

Release of arachidonic acid from 1-alkyl-2-acyl-sn-glycero-3-phosphocholine, a precursor of platelet-activating factor, in rat alveolar macrophages.

Platelet activating factor and the bioactive metabolites of arachidonic acid are secreted by alveolar macrophages in response to stimulation by phagocytic agents or calcium ionophore. We have previously shown a deacylation-acetylation sequence in the formation of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) from alkylacyl-(long chain)-GPC (Albert, D.H. and Snyder, F. (1983) J. Biol. Chem. 258, 97-102). This sequence may be an important source of 20:4 during inflammatory reactions since, in alveolar macrophages, the ether lipid precursor of PAF represents 35% of the choline glycerophospholipids and has a much higher content (35%) of 20:4 in the sn-2 position than does diacyl-GPC (17%). Alveolar macrophages prelabeled with 14C-labeled fatty acids (16:0, 18:1, 18:2 and 20:4) and [1-3H]alkyllyso-GPC were used to study the release of fatty acids from ether-linked and diacyl phospholipids. Each of these fatty acids was incorporated primarily into the choline glycerophospholipids of alveolar macrophages. The release of 20:4 from macrophage phospholipids was increased by treatment of the labeled cells with the calcium ionophore A23187 (2 microM) or zymosan (1 mg/ml), whereas the release of 16:0, 18:1 and 18:2 was not increased above control levels by either stimuli. Although more of the labeled 20:4 is released from the diacyl-GPC (50% of the total released), substantial amounts (44%) of 20:4 are derived from alkylacyl-GPC after incubating the stimulated cells for 60 min. The loss of 20:4 continued from the diacyl species throughout the incubation period studied, whereas a slower net release of 20:4 lost from the alkylacyl-GPC fraction was evident after 2 h. We conclude that the deacylation-reacylation cycle is an important aspect of the metabolism of 20:4 and alkylacyl-GPC during inflammatory stimulation of alveolar macrophages and that the deacylation of this ether-linked phospholipid (which is the first step in the formation of PAF) is responsible for a significant amount of the 20:4 released.

Animals↗
PubMed1984

Conversion of 1-alkyl-2-acetyl-sn-glycerols to platelet activating factor and related phospholipids by rabbit platelets.

The metabolic pathway for 1-alkyl-2-acetyl-sn-glycerols, a recently discovered biologically active neutral lipid class, was elucidated in experiments conducted with rabbit platelets. The total lipid extract obtained from platelets incubated with 1-[1,2-(3)H]alkyl-2-acetyl-sn-glycerols or 1-alkyl-2-[3H]acetyl-sn-glycerols contained at least six metabolic products. The six metabolites, identified on the basis of chemical and enzymatic reactions combined with thin-layer or high-performance liquid chromatographic analyses, corresponded to 1-alkyl-sn-glycerols, 1-alkyl-2-acetyl-sn-glycero-3-phosphates, 1-alkyl-2-acyl (long-chain)-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholines, and 1-alkyl-2-acetyl-sn-glycero-3-phosphocholines (platelet activating factor). These results indicate that the metabolic pathway for enzymatic activities: choline- and ethanolamine- phosphotransferases, acetyl-hydrolase, an acyltransferase, and a phosphotransferase. The step responsible for the biosynthesis of platelet activating factor would appear to be the most important reaction in this pathway and this product could explain the hypotensive activities previously described for alkylacetyl-(or propionyl)-glycerols. Of particular interest was the preference exhibited for the utilization of the 1-hexadecyl-2-acetyl-sn-glycerol species in the formation of platelet activating factor.

Animals↗
PubMed1984

Novel quantitative method for determination of molecular species of phospholipids and diglycerides.

A novel method is described for the quantitative analysis of subclasses (alk-1-enylacyl, alkylacyl, and diacyl types) and molecular species within each subclass of glycerophosphatides. Diradylglycerols from phospholipase C hydrolysis of the phospholipids are converted to benzoate derivatives, the benzoates are separated into their respective subclasses by thin-layer chromatography, and quantitated by measuring absorbance at 230 nm. Molecular species within individual subclasses are separated using a combination of argentation thin-layer chromatography and reversed-phase high-performance liquid chromatography with direct, on-line quantitation at 230 nm. We applied the method to the analysis of ethanolamine phosphatides from beef brain and were able to quantitate the three diradylglycerol subclasses (alk-1-enylacyl, alkylacyl, and diacyl types) as well as ca. 29 molecular species within each of these subclasses. This new quantitative approach for the analysis of specific molecular species of glycerolipids should be applicable to studies involving a variety of biologically important lipids, such as phosphatidylcholine, phosphatidylinositol, platelet activating factor, plasmalogens, and neutral type glycerolipids including diacylglycerols.

Animals↗
PubMed1984

Liposomal preparations of calcium- or zinc-DTPA have a high efficacy for removing colloidal Ytterbium-169 from rat tissues.

Results from this study demonstrate that liposomes increase the effectiveness of chelating agents in removing heavy metals from contaminated tissues in vivo. We compared the ability of free and liposomal preparations of Ca- and Zn-diethylenetriaminepentaacetic acid (DTPA) to remove 169Yb from tissues of rats previously injected intravenously with either soluble or colloidal forms of 169Yb . Although single injections of liposomal Zn-DTPA were better than the free chelator in reducing the body burden of 169Yb administered in a soluble form (citrate), the liposomal preparations of Zn-DTPA or Ca-DTPA were even more effective in removing the 169Yb that had been injected as the colloidal form. However, a second injection of liposomal Zn-DTPA given 8 days after the initial treatment was not as effective in removing 169Yb as a second injection of the free chelator. Whether injected in the free or liposomal forms, Ca-DTPA was more effective than Zn-DTPA in removing the colloidal 169Yb . Significantly lower amounts of colloidal 169Yb remained in the liver, kidney, muscle, bone, and blood of rats after injection of the liposomal preparations of Ca- or Zn-DTPA than in the corresponding organs of the controls (P less than 0.01). The liposomal preparations were also more efficient in reducing the retention of colloidal 169Yb in bone and blood than the free chelators (P less than 0.01).

Animals↗
PubMed1984

A new class of antihypertensive neutral lipid: 1-alkyl-2-acetyl-sn-glycerols, a precursor of platelet activating factor.

A new type of neutral lipid is described that possesses hypotensive activity in genetic hypertensive (SHR) and normotensive (WKY) rats. 1-Alkyl-2-acetyl-sn-glycerols and 1-alkyl-2-propionyl-sn-glycerols are both equally effective in eliciting the hypotensive response. Requirement for the 1-alkyl and 2-acetyl or 2-propionyl structure of the active isomer was documented by the negative responses obtained with closely related neutral lipid analogs (1-alkyl-2-acyl-, 1-alkyl-3-acetyl-, 1-acyl-2-acetyl-, 1-alkyl-2,3-diacetyl-, and 1-alkyl-glycerols). Although less potent than PAF (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine), the 1-alkyl-2-acetyl-sn-glycerols produce a response of significantly longer duration and may have fewer immediate side effects than PAF. The mechanism for the biological activity is unknown; however, we have demonstrated previously that the enzymatic synthesis of 1-alkyl-2-acetyl-sn-glycerols to PAF occurs via a specific cholinephosphotransferase and therefore the observed blood pressure response might be due to the conversion of the neutral lipid precursor to PAF in vivo.

Animals↗
PubMed1984

Vertebrate class distribution of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetylhydrolase in serum.

Serum from numerous mammals and lower vertebrates contains an enzyme activity that is specific for the hydrolysis of the acetate moiety of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF, platelet activating factor). Acetylhydrolase (EC 3.1.1.47, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetylhydrolase) was found in all mammalian sera with activity ranging from 11 (fetal calf) to 178 (rabbit) pmol acetate liberated/microliter serum/min. The enzyme is not present in avian serum but is a constituent of reptiles and bony fishes.

Animals↗
PubMed1984

Hypotension produced by platelet-activating factor is reversed by thyrotropin-releasing hormone.

Platelet-activating factor (PAF), a vasoactive phospholipid implicated in anaphylactic reactions, causes severe hypotension in experimental animals that is highly resistant to pharmacological therapy. In the present studies, we showed that PAF (1 nmol/600 g body weight, IV) produced profound hypotension in unanesthetized guinea pigs that was promptly and completely reversed by thyrotropin-releasing hormone (TRH) (2 mg/kg, IV) or by the synthetic TRH analog MK771 (2 mg/kg, IV). TRH also reversed this hypotension when administered intracerebroventricularly (ICV) at a dose (0.02 mg/kg) that was systemically ineffective. The opiate receptor antagonist naloxone (5 mg/kg) was less effective than TRH in reversing the cardiovascular consequences of PAF administration. These data suggest that TRH reverses PAF-induced shock through central receptor-mediated mechanisms. This therapeutic action of TRH may partially account for the beneficial cardiovascular effects of this peptide in anaphylactic shock.

Animals↗
PubMed1984

Cytotoxicity of platelet activating factor and related alkyl-phospholipid analogs in human leukemia cells, polymorphonuclear neutrophils, and skin fibroblasts.

A series of 11 alkyl-phospholipid analogs, structurally related to platelet activating factor (L-PAF), were analyzed for cytotoxic activity in human leukemic (HL-60) cells, human polymorphonuclear neutrophils, and Detroit 551 human skin fibroblasts. The order of selectiveness of the analogs in their cytotoxic response toward HL-60 cells in comparison to neutrophils is 1-alkyl-2-acetamide-GPC greater than 1-alkyl-2-methoxy-GPC greater than D-PAF greater than 1-acyl-2-lyso-GPC greater than 1-alkyl-2-lyso-GPC greater than L-PAF. A time-sequenced progression of events caused by the most potent cytotoxic alkyl-phospholipid analogs was characterized by (a) a rapid decrease in the cellular uptake and incorporation of 3H-thymidine into DNA that was detectable 4 hr after exposure to the analog, (b) a release of lactate dehydrogenase activity into the media at 8 hr after exposure, and (c) a decrease in cell number due to cell death that begins at 12 hr after exposure. Treatment of HL-60 cells with 1-alkyl-2-methoxy-GPC for 1 hr destroyed 40% of the cells after a subsequent 24-hr incubation period. The varied biologic activities of L-PAF, including how it affects serotonin release from platelets, blood pressure in rats, and cytotoxic responses in normal and leukemic cells, are discussed in relation to its D-enantiomer, 3-alkyl-2-acetyl-GPC, and the 2-acetamide analog. This report characterizes the kinetic events of the cellular responses in both normal and HL-60 cells in relation to the antineoplastic activities of unnatural ether-linked phospholipid analogs that are structurally related to L-PAF.

Antineoplastic Agents↗