Identification of substrate orienting and phosphorylation sites within tryptophan hydroxylase using homology-based molecular modeling.

Tryptophan hydroxylase (TPH) is the initial and rate-limiting enzyme in the biosynthesis of serotonin. The inherent instability of TPH has prevented a crystallographic structure from being resolved. For this reason, multiple sequence alignment-based molecular modeling was utilized to generate a full-length model of human TPH. Previously determined crystal coordinates of two highly homologous proteins, phenylalanine hydroxylase and tyrosine hydroxylase, were used as templates. Analysis of the model aided rational mutagenesis studies to further dissect the regulation and catalysis of TPH. Using rational site-directed mutagenesis, it was determined that Tyr235 (Y235), within the active site of TPH, appears to be involved as a tryptophan substrate orienting residue. The mutants Y235A and Y235L displayed reduced specific activity compared to wild-type TPH ( approximately 5 % residual activity). The K(m) of tryptophan for the Y235A (564 microM) and Y235L (96 microM) mutant was significantly increased compared to wild-type TPH (42 microM). In addition, kinetic analyses were performed on wild-type TPH and a deletion construct that lacks the amino terminal autoregulatory sequence (TPH NDelta15). This sequence in phenylalanine hydroxylase (residues 19 to 33) has previously been proposed to act as a steric regulator of substrate accessibility to the active site. Changes in the steady-state kinetics for tetrahydrobiopterin (BH(4)) and tryptophan for TPH NDelta15 were not observed. Finally, it was demonstrated that both Ser58 and Ser260 are substrates for Ca(2+)/calmodulin-dependent protein kinase II. Additional analysis of this model will aid in deciphering the regulation and substrate specificity of TPH, as well as providing a basis to understand as yet to be identified polymorphisms.

Exploring the nature of molecular recognition in nicotinic acetylcholine receptors.

Nicotinic acetylcholine receptors (nAChRs) are the subject of ever increasing interest because of their presumed involvement in the etiology of numerous clinical disorders. Unfortunately, the absence of atomic-level structural data, as well as the pharmacological complexity of these receptors leaves many fundamental questions unanswered. An understanding of how ligands interact with the receptor and, in-turn, how these interactions lead to pharmacological effect is vital in the advancement of nAChR-based therapeutics. We will first explore physico-chemical themes that are evidenced to be of particular importance in nAChR molecular recognition; these are- pi-cation interaction, conformational entropy and stereochemistry. The second objective of this review is an interpretive encapsulation of the extensive and disparate body of structure-activity data that now exists for nAChRs. Finally, this review will advocate a re-investigation of distance geometry based methods as well as the need for additional approaches in nicotinic receptor pharmacophore generation.

AhpF can be dissected into two functional units: tandem repeats of two thioredoxin-like folds in the N-terminus mediate electron transfer from the thioredoxin reductase-like C-terminus to AhpC.

AhpF, the flavin-containing component of the Salmonella typhimurium alkyl hydroperoxide reductase system, catalyzes the NADH-dependent reduction of an active-site disulfide bond in the other component, AhpC, which in turn reduces hydroperoxide substrates. The amino acid sequence of the C-terminus of AhpF is 35% identical to that of thioredoxin reductase (TrR) from Escherichia coli. AhpF contains an additional 200-residue N-terminal domain possessing a second redox-active disulfide center also required for AhpC reduction. Our studies indicate that this N-terminus contains a tandem repeat of two thioredoxin (Tr)-like folds, the second of which contains the disulfide redox center. Structural and catalytic properties of independently expressed fragments of AhpF corresponding to the TrR-like C-terminus (F[208-521]) and the 2Tr-like N-terminal domain (F[1-202]) have been addressed. Enzymatic assays, reductive titrations, and circular dichroism studies of the fragments indicate that each folds properly and retains many functional properties. Electron transfer between F[208-521] and F[1-202] is, however, relatively slow (4 x 10(4) M(-)(1) s(-)(1) at 25 degrees C) and nonsaturable up to 100 microM F[1-202]. TrR is nearly as efficient at F[1-202] reduction as is F[208-521], although neither the latter fragment, nor intact AhpF, can reduce Tr. An engineered mutant AhpC substrate with a fluorophore attached via a disulfide bond has been used to demonstrate that only F[1-202], and not F[208-521], is capable of electron transfer to AhpC, thereby establishing the direct role this N-terminal domain plays in mediating electron transfer between the TrR-like part of AhpF and AhpC.

Molecular recognition in nicotinic acetylcholine receptors: the importance of pi-cation interactions.

We explore the significance of pi-cation interactions in the binding of ligands to nicotinic acetylcholine receptors. Specifically, the Austin method of semiempirical molecular orbital theory is utilized to estimate the interaction of aromatic amino acid side chains with the cation-containing heterocyclic ring fragments of nicotinic ligands. Variational interaction energies (E(i)) of side chain-ligand fragment pairs are shown to be distance-dependent and follow a Morse-like potential function. The tryptophan side chain shows the most pronounced interaction with the cation fragments, followed by tyrosine and phenylalanine. For a given side chain, cationic fragments exhibit characteristically different E(i) profiles, with the azabicyclo[2.2.1]heptane fragment of the potent nicotinic ligand epibatidine eliciting the greatest interaction energy of the study set. Most significantly, the minimum energy values calculated for numerous fragments correlate with the binding affinity of the parent ligands- we show this to be the case for heteropentameric (alpha4beta2) and homopentameric (alpha7) nicotinic acetylcholine receptor subtypes. Furthermore, intermolecular distances corresponding to the Morse-like potential minimum also correlate with high-affinity binding. A number of parallel calculations were conducted at the Hartree-Fock 6-31G ab initio level of theory in an effort to substantiate these findings.

The heterocyclic substituted pyridine derivative (+/-)-2-(-3-pyridinyl)-1-azabicyclo[2.2.2]octane (RJR-2429): a selective ligand at nicotinic acetylcholine receptors.

The present report describes in vitro studies demonstrating that the heterocyclic substituted pyridine compound (+/-)-2-(3-pyridinyl)-1-azabicyclo[2.2.2]octane (RJR-2429) is extremely potent in activating human muscle nicotine ACh receptor (nAChR) (EC50 = 59 +/- 17 nM; Emax = 110 +/- 09% vs. nicotine). RJR-2429 is markedly less potent in activating nAChRs in the clonal cell line PC12, with EC50 = 1100 +/- 230 nM and Emax = 85 +/- 20% when compared with nicotine. The activation of a putative alpha 3 beta 4-containing nAChR in PC12 by RJR-2429 reveals a potency intermediate between nicotine and epibatidine (EC50 of 20,000 nM for nicotine and 30 nM for epibatidine). Dose-response curves for agonist-induced ileum contraction indicate that RJR-2429 is equipotent with nicotine, having an EC30 of approximately 2 microM. RJR-2429 binds with high affinity to alpha 4 beta 2 receptor subtype (Ki = 1.0 +/- 0.3 nM), and chronic exposure results in significant up-regulation of the high-affinity [3H]nicotine binding sites. In addition, RJR-2429 does not activate nAChRs present in rat thalamic preparations but is a potent inhibitor of this receptor subtype. It antagonizes nicotine-stimulated ion flux in thalamic synaptosomes with an IC50 of 154 +/- 37 nM. It also is a potent partial agonist at nAChRs mediating dopamine release from rat synaptosomal preparations (EC50 = 2 +/- 1 nM; Emax = 40%; epibatidine and nicotine have EC50 values of 0.4 and 100 nM, respectively). A model for the structure-activity profile of RJR-2429, nicotine and epibatidine was derived by molecular forcefield and quantum mechanics calculations and may provide important clues for the development of ligands selective for nAChR subtypes as probes in the life sciences or as potential therapeutic tools.

Ferrocenoyl derivatives of alamethicin: redox-sensitive ion channels.

The synthesis and single-channel characterization of two redox-active C-terminal derivatives of alamethicin are herein described. The reduced [Fe(II)] forms of ferrocenoyl-alamethicin (Fc-ALM) and 1'-carboxyferrocenoyl-alamethicin (cFc-ALM) are shown to form voltage-dependent ion channels at cis positive potentials in planar lipid bilayers (PLB) with conductance properties similar to those of alamethicin. In situ oxidation of Fc-ALM [to Fe(III)] in the PLB apparatus causes a time-dependent elimination of channel openings, which can be restored by an increase in the transbilayer potential. In contrast, oxidation of cFc-ALM leads to the formation of shorter-lived channels. Pretreatment of the ferrocenoyl peptides with oxidizing agent alters their single-channel properties in a qualitatively similar manner, establishing that the changes in channel properties in the presence of oxidizing agents are due specifically to ferrocenoyl oxidation. We suggest that the redox sensitivity of these ferrocene-containing ion channels may be governed by a combination of the following factors: (1) changes in hydrophobicity; (2) alteration of peptide molecular dipole; and (3) alterations in tendencies toward self-association. However, oxidation induced changes in peptide conformation cannot be ruled out. Our results provide evidence that it is possible to engineer channel-forming peptides that respond to specific changes in the chemical environment.

Stereospecific labeling of the glycerol moiety: synthesis of 1,2-dioleoyl-sn-[3-3H]glycero-3-phospho(1-rac-glycerol).

1,2-Dioleoyl-sn-[3-3H]glycero-3-phospho(1-rac-glycerol) was synthesized from 1,2-dioleoyl-sn-glycerol using a new radiosynthetic procedure. 1,2-Dioleoyl-sn-glycerol was oxidized to the corresponding aldehyde using pyridinium dichromate and pyridine. The aldehyde was reduced to the radiolabeled alcohol using tritiated sodium borohydride and crown ether. This material was then converted to the phosphocholine derivative using 2-chloro-2-oxo-1,3,2-dioxaphospholane, followed by displacement with trimethylamine. In the last step, the 1,2-dioleoyl-sn-[3-3H]glycero-3-phosphocholine was converted to 1,2-dioleoyl-sn-[3-3H]glycero-3-phospho-(1-rac-glycerol) via a classic transphosphatidylation reaction using glycerol and cabbage phospholipase D. A theoretical explanation of unusual chemical behavior of the primary alcohol of diglycerides is also given, based on semi-empirical calculations.

Biosynthetic conversion of phosphatidylglycerol to sn-1:sn-1' bis(monoacylglycerol) phosphate in a macrophage-like cell line.

Bis(monoacylglycerol) phosphate has a unique stereoconfiguration of sn-glycero-1-phospho-1'-sn-glycerol and is synthesized from exogenous phosphatidylglycerol by macrophages. Previous work by our laboratory showed that the macrophage-like cell line RAW 264.7 synthesizes sn-glycero-1-phospho-1'-sn-glycerol bis(monoacylglycerol) phosphate. Here we describe studies using RAW 264.7 cells that examine the biosynthetic pathway by which bis(monoacylglycerol) phosphate is formed. Experiments were conducted using precursors that were specifically radiolabeled on the glycerol backbone in order to examine the stereoconfiguration of the intermediates and products formed in intact RAW 264.7 cells. The results of our studies indicate that a complex series of reactions are involved in the synthesis of bis(monoacylglycerol) phosphate. In this proposed pathway phosphatidylglycerol is hydrolyzed to form 1-acyllysophosphatidylglycerol which is then acylated on the headgroup glycerol to form the sn-glycero-1-phospho-1'-sn-glycerol enantiomer of bis(monoacylglycerol) phosphate. The sn-glycero-1-phospho-1'-sn-glycerol enantiomer of bis(monoacylglycerol) phosphate is then thought to undergo a stereoconversion that proceeds via the required removal of the acyl group at the sn-1 position. The resulting sn-glycero-1-phospho-1'-sn-glycerol enantiomer of lysophosphatidylglycerol with the acyl moiety on the original headgroup glycerol is then acylated to form sn-glycero-1-phospho-1'-sn-glycerol bis(monoacylglycerol) phosphate.

Regulation of rat hepatic lipase by the composition of monomolecular films of lipid.

The regulation of hepatic lipase (HL) by the lipid composition of monomolecular substrate films was examined using a monolayer technique at constant surface pressure. HL-catalyzed hydrolysis of triacylglycerol, a poor substrate for HL in pure monomolecular films, was activated by diradylglycerol and its phosphorylated derivatives in mixed films containing 10 mol % triacylglycerol. When triacylglycerol was progressively diluted with dialkylglycerol, triacylglycerol hydrolysis by HL was maximal between 90 and 98 mol % dialkylglycerol. The best activators, dialkylphosphatidic acid and dialkylphosphatidylethanolamine, increased triacylglycerol hydrolysis 13-14-fold, and the enhancement of HL-catalyzed triacylglycerol hydrolysis by the activator lipids was inversely related to the average mean molecular area of the mixed films. The hydrolysis of 5 mol % triacylglycerol in mixed films that also contained phosphatidylcholine and 0-20 mol % cholesterol was inhibited approximately 80% when the concentration of cholesterol was 10-13 mol %. Interestingly, between 15 and 17 mol % cholesterol the hydrolysis rate was restored to about 50% of the uninhibited rate, but at 20 mol % cholesterol this value decreased back to 80% inhibition of hydrolysis. The hydrolysis of phosphatidylethanolamine in mixed films with 0-20 mol % cholesterol decreased approximately 30% in films containing 10-12 mol % cholesterol. However, at 15 mol % cholesterol the hydrolysis rate was restored to the same level observed for a pure phosphatidylethanolamine film. This enhancement of HL activity occurred at about the same cholesterol concentration as the restoration of triacylglycerol hydrolysis observed for the triacylglycerol/phosphatidylcholine/cholesterol films.(ABSTRACT TRUNCATED AT 250 WORDS)

A facile synthesis of 1-O-alkyl-2-(R)-hydroxypropane-3-phosphonocholine (lyso-phosphono-platelet activating factor).

The synthesis of 1-O-alkyl-2-(R)-hydroxypropane-3-phosphonocholine is described. An efficient alkylation procedure using (NaH/DMSO) catalysis is also described and applied to the synthetic scheme. The key intermediate 1-O-alkyl-2-(R)-O-benzyl-3-bromopropane was phosphonylated using tris(methylsilyl)phosphite; the resulting phosphonic acid was coupled to choline using trichloroacetonitrile/pyridine or triisopropylbenzenesulfonyl chloride/pyridine followed by catalytic hydrogenation to yield 1-O-alkyl-2(R)-hydroxypropane-3-phosphonocholine.

Propranolol, a phosphatidate phosphohydrolase inhibitor, also inhibits protein kinase C.

Propranolol, a beta-adrenergic receptor antagonist, also inhibits phosphatidate phosphohydrolase, the enzyme that converts phosphatidic acid into diacylglycerol. This latter effect has prompted recent use of propranolol in studies examining the importance of diacylglycerol and phosphatidic acid in cellular signalling events. Here, we show that propranolol is also an inhibitor of protein kinase C. At concentrations greater than or equal to 20 microM, propranolol reduced [3H]phorbol dibutyrate binding (IC50 = 200 microM) and phorbol myristate acetate-stimulated superoxide anion release (IC50 = 130 microM) in human neutrophils. Scatchard analysis showed that propranolol lowers the number of phorbol diester binding sites without significantly affecting their affinity. In vitro kinetic analysis, performed in a mixed micellar assay with protein kinase C purified from human neutrophils, suggested a competitive inhibition of propranolol with the cofactor phosphatidylserine. Complex kinetic patterns were observed with respect to diacylglycerol and ATP, approximating competitive and noncompetitive inhibition, respectively. Taken together, these results suggest that the drug interacts at the level of the regulatory domain of the enzyme. Fifty % inhibition occurred at approximately 150 microM propranolol. Similar levels of inhibition were obtained using exogenous (histone) and endogenous (p47-phox, a NADPH oxidase component) substrates. Protein kinase C-alpha and protein kinase C-beta, two protein kinase C isozymes present in human neutrophils, were inhibited by propranolol in a comparable manner. In the range of concentrations tested (30-1000 microM), neither cAMP-dependent protein kinase nor neutrophil protein tyrosine kinases were affected. The racemic form of propranolol and the (+) and the (-) stereoisomers were equally active, and other beta-adrenergic receptor antagonists (pindolol) and agonists (isoproterenol) were inactive. This suggests that the inhibitory action of propranolol on protein kinase C is related to the amphipathic nature of the drug rather than to its beta-adrenergic receptor blocking ability. Analogs of propranolol were synthesized and found to be more potent protein kinase C inhibitors, with IC50 values in the 10-20 microM range. We conclude that the ability of propranolol to inhibit both protein kinase C and PA phosphohydrolase complicates interpretation of results when this drug is used in signal transduction studies. In addition, propranolol may be a useful prototype for the synthesis of new protein kinase C inhibitors.

Inhibition of lecithin:cholesterol acyltransferase activity by synthetic phosphatidylcholine species containing eicosapentaenoic acid or docosahexaenoic acid in the sn-2 position.

Phospholipids isolated from the plasma of monkeys fed a diet enriched in fish oil were poor substrates for cholesteryl ester (CE) synthesis by the lecithin:cholesterol acyltransferase (LCAT) reaction relative to those from animals fed a lard containing diet when the phospholipids were used for the preparation of recombinant particles by cholate dialysis (Parks, J. S., B. C. Bullock, and L. L. Rudel. 1989. J. Biol. Chem. 264: 2545-2551). The purpose of the present study was to directly test the influence of eicosapentaenoic acid (20:5 n-3) and docosahexaenoic acid (22:6 n-3) in the sn-2 position of phosphatidylcholine (PC) on the activity of LCAT. PC species containing 1-palmitoyl-2-oleoyl PC (POPC), 1-palmitoyl-2-linoleoyl PC (PLPC), 1-palmitoyl-2-arachidonoyl PC (PAPC), 1-palmitoyl-2-eicosapentaenoyl PC (PEPC), or 1-palmitoyl-2-docosahexaenoyl PC (PDPC) were purchased or synthesized and made into recombinant particles of uniform size and composition with [14C]cholesterol and apoA-I using the cholate dialysis procedure. The recombinant particles (PC:cholesterol:apoA-I molar ratio = 42:1.9:1) exhibited the following order of reactivity towards purified human LCAT in vitro: POPC greater than PLPC greater than PEPC = PAPC greater than PDPC. The apparent Vmax/Km for recombinant particles containing PEPC and PDPC was 17% and 7% that of particles containing POPC, respectively. There was a linear decrease in CE formation when the percentage of PEPC or PDPC was increased from 0 to 100% relative to POPC in recombinant particles with a constant PC:cholesterol:apoA-I molar ratio, suggesting that the PEPC and PDPC were competitive inhibitors of the LCAT reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Conversion of 1-O-[3H]alkyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine to lyso platelet-activating factor by the CoA-independent transacylase in membrane fractions of human neutrophils.

The first step in the synthesis of platelet-activating factor (PAF) in stimulated neutrophils is generally accepted to be hydrolysis of 1-O-alkyl-2-acyl-sn-glycero-3-phosphorylcholine (1-O-alkyl-2-acyl-GPC), with 1-O-alkyl-2-arachidonoyl-GPC being the preferred precursor. Characterization of the enzymatic activity responsible for the hydrolysis of 1-O-alkyl-2-arachidonoyl-GPC has been hampered by lack of an active and reliable cell-free system for study. In the present studies, membrane preparations containing 1-O-[3H]alkyl-2-arachidonoyl-GPC were prepared from intact human neutrophils that had been labeled using 1-O-[3H]hexadecyl-2-lyso-GPC. When the labeled membrane preparations were incubated in the presence of unlabeled 1-O-alkyl-2-lyso-GPC (5 microM), rapid deacylation (up to 25% of the label in 10 min) of the 1-O-[3H]alkyl-2-arachidonoyl-GPC to 1-O-[3H]alkyl-2-lyso-GPC (lyso-PAF) was observed. The deacylation activity appeared to be the same in preparations from resting or stimulated cells. No requirement for Ca2+, various nucleotides, or protein kinase activation could be demonstrated. A number of observations indicated that [3H]lyso-PAF is formed in the system by the action of the CoA-independent transacylase present in the cells rather than by phospholipase A2. Both 1-O-alkyl-2-lyso-GPC and 1-acyl-2-lyso-GPC elicited deacylation of 1-O-[3H]alkyl-2-arachidonoyl-GPC, whereas neither 3-O-alkyl-2-lyso-GPC nor 1-O-alkyl-2-O-methyl-rac-glycero-3-phosphorylcholine, which should act as detergents but are not transacylase substrates, effected deacylation. The deacylation activity and CoA-independent transacylase activities were blocked in parallel by a number of inhibitors and by heat inactivation. In preparations containing 1-O-alkyl-2-[3H]arachidonoyl-GPC, no release of free [3H]arachidonic acid was observed. However, a shift of the [3H]arachidonate into exogenous 1-O-tetradecyl-2-lyso-GPC was observed in the system. These findings are consistent with the generation of [3H]lyso-PAF by the CoA-independent transacylase activity.

Selective priming of rate and duration of the respiratory burst of neutrophils by 1,2-diacyl and 1-O-alkyl-2-acyl diglycerides. Possible relation to effects on protein kinase C.

Both 1,2-diacyl- and 1-O-alkyl-2-acyl-sn-glycerols are released during stimulation of human polymorphonuclear leukocytes (PMNL). 1,2-Diacylglycerols have received intense interest as intracellular "second messengers" due to their ability to activate protein kinase C (Ca2+ phospholipid-dependent enzyme). However, little is known about bioactivities of the alkylacylglycerols. This study compared the ability of 1,2-diacyl- and 1-O-alkyl-2-acylglycerols to modulate the respiratory burst of stimulated PMNL, a response which depends on the activation of an NADPH oxidase to generate bactericidal species of reduced oxygen. Direct stimulation by N-formyl-Met-Leu-Phe caused an abrupt release of H2O2 which ceased within 2.5 min. Preincubation with diacylglycerols (1-oleoyl-2-acetylglycerol,5-30 microM, and 1,2-dioctanoylglycerol,2-5 microM) caused a decrease in lag time, 3-fold increase in initial rate of H2O2 release, and marked prolongation of the response to N-formyl-Met-Leu-Phe (features characteristic of a priming effect). Preincubation with alkylacylglycerols (1-O-delta 9-octadecenyl-2-acetylglycerol, 5-30 microM, and 1-O-octyl-2-octanoylglycerol, 20-50 microM) primed initiation (shortened lag time and increased velocity) but, in contrast to diacylglycerols, did not alter duration of H2O2 release. While low concentrations of diacylglycerols (5-30 microM) primed PMNL, higher concentrations (greater than or equal to 70 microM) stimulated the cells directly. In contrast, higher (70-100 microM) concentrations of alkylacylglycerols did not prime the responses but, in fact, inhibited priming (especially of duration) induced by diacylglycerol. The high concentrations of alkylacylglycerol also inhibited direct stimulation induced by high concentrations of diacylglycerol. Direct stimulation by high concentrations of diacylglycerol probably involves activation of protein kinase C, whereas alkylacylglycerol was found to inhibit activation of protein kinase C by diacylglycerol in vitro. Thus, diacylglycerols are complete priming agonists, altering both rate and duration of the response. In contrast, alkylacylglycerols may have biphasic, concentration-related effects in modulation of functions of PMNL. At low concentrations, they may facilitate initiation of functional events; however, as their concentration increases, they may serve to terminate responses. The distinct priming effects of these diglycerides also reveal that priming can involve at least two distinct events: 1) initiation and 2) prolongation.(ABSTRACT TRUNCATED AT 400 WORDS)

Alkyl-linked diglycerides inhibit protein kinase C activation by diacylglycerols.

Alkylacylglycerols are synthesized when choline-phospholipids are degraded by a phospholipase C. This class of compounds has been shown to have biological activities; however, the mechanism of action is unknown. A series of alkyl-linked diglycerides were synthesized and tested for activity in an in vitro assay for protein kinase C. When protein kinase C activity was stimulated with the synthetic diacylglyceride analog 1-oleoyl-2-acetyl-sn-glycerol, the addition of alkyl glycerides caused a concentration-dependent inhibition of protein kinase C activity. Comparison of the protein kinase C inhibition by this series of 1-O-alkyl-2-acyl analogs revealed that both saturated and unsaturated long-chain groups in position 1 were effective and that dietherglycerols with short-chain moieties in position 2 were also effective. It is concluded from these studies that the biological activity of alkyl-linked glycerides may be expressed through protein kinase C inhibition.

The degradation of platelet-activating factor and related lipids: susceptibility to phospholipases C and D.

1-O-Octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) is an ether-linked lipid that exhibits selective cytotoxicity toward several types of tumor cells and is relatively inactive toward normal cells under the same conditions of treatment. The mechanis of this selective cytotoxicity is unknown. We conducted studies to determine whether this compound is metabolized by phospholipases C and D and, if so, whether sensitive and resistant cells differ in their ability to degrade ET-18-OCH3 by these enzymes. We have examined the metabolism of the L-isomer of ET-18-OCH3, 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (L-ET-18-OCH3), by lysophospholipase D of rat liver microsomes and by a phospholipase D from the marine bacterium Vibrio damsela. The metabolism of L-ET-18-OCH3 was also examined in cell culture using Madin-Darby canine kidney cells, human promyelocytic leukemia cells and human myelocytic leukemia cells. In these studies, L-ET-18-OCH3 and related 1-O-alkyl-linked phosphocholine analogs radiolabeled with 3H in the 1-O-alkyl chain were used. L-ET-18-OCH3 was not hydrolyzed by lysophospholipase D from rat liver microsomes under conditions where cleavage of 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine was observed. However, phospholipase D from the marine bacterium V. damsela readily hydrolyzed L-ET-18-OCH3 to 1-O-[3H]octadecyl-2-O-methyl-sn-glycero-3-phosphate, demonstrating that L-ET-18-OCH3 can be degraded by a phospholipase D. Platelet-activating factor (PAF) and lyso-PAF were also substrates for the bacterial phospholipase D.(ABSTRACT TRUNCATED AT 250 WORDS)

Reacylation of platelet activating factor with eicosapentaenoic acid in fish-oil-enriched monkey neutrophils.

Platelet activating factor (PAF) is rapidly metabolized via a deacetylation: reacylation pathway which shows striking specificity for arachidonate at the sn-2 position of the 1-O-alkyl-2-acyl-GPC thus formed. We have now examined the effects of a diet enriched in fish oils on the metabolism of PAF and specificity for arachidonate in the reacylation reaction. [3H]PAF was incubated for various lengths of time with neutrophils from monkeys fed a control diet or one enriched in fish oils. The [3H]PAF added to the cell suspension was rapidly converted to 1-O-alkyl-2-acyl-GPC. Reverse-phase HPLC analysis of the acyl chains added at the sn-2 position revealed that arachidonate was the major fatty acid incorporated into the 1-O-alkyl-2-acyl-GPC formed by neutrophils from monkeys on the control diet. In contrast, both 1-O-alkyl-2-arachidonoyl-GPC and 1-O-alkyl-2-eicosapentaenoyl-GPC were formed by the fish-oil-enriched neutrophils. We also report on the fatty acid composition of neutrophil phospholipids during such a diet.

1-O-alkyl-2-N-methylcarbamyl-glycerophosphocholine: a biologically potent, non-metabolizable analog of platelet-activating factor.

We prepared unlabeled and 3H-labeled analogs of platelet-activating factor (PAF) containing a N-methylcarbamyl residue at the sn-2 position. PAF and its methylcarbamyl analog competed for binding to high affinity receptors on human polymorphonuclear neutrophils; their respective dissociation constants for these receptors were 0.2 and 1.1 nM. The binding affinities of the two analogs correlated precisely with their capacities to stimulate neutrophil degranulation responses. Unlike PAF, however, the methylcarbamyl analog completely resisted metabolic inactivation by neutrophils and by human sera. Thus, these compounds' biological potencies are determined predominantly by receptor binding: cellular metabolism of the ligands neither contributes to nor appreciably limits their stimulating actions.

Diacylglycerols and mezerein activate neutrophils by a phorbol myristate acetate-like mechanism.

rac-1-O-Myristoyl-2-O-acetylglycerol, rac-1-O-palmitoyl-2-O-acetylglycerol, and rac-1-O-oleoyl-2-O-acetylglycerol acted like phorbol myristate acetate and mezerein in stimulating human neutrophil aggregation. Responses to these agents were equally influenced by cytochalasin B, extracellular calcium and magnesium, arachidonate antimetabolites, and procedures that rendered the cells desensitized to other agonists. The compounds also inhibited the binding of [3H]-phorbol myristate acetate to its receptor on neutrophils. Thus, these agents are biologically homologous. They act by binding to a common receptor. This receptor may function physiologically as a transducer for endogenous glycerides that form in cells challenged by other stimuli.

Interactions of arachidonate metabolism and protein kinase C in mediating neutrophil function.

Diglyceride activators of protein kinase C (i.e., 1-0-myristoyl-, 1-0-palmitoyl-, and 1-0-oleoyl-2-acetylglycerol) interacted synergistically with an arachidonate metabolite, 5-hydroxyicosatetraenoate, to stimulate neutrophil degranulation and superoxide anion generation. Contrastingly, combinations of 15-hydroxyicosatetraenoate with the glycerides or 5-hydroxyicosatetraenoate with a dialkylglyceride (1-0-hexadecyl-2-ethylglycerol) produced no such synergy. The data support a view of stimulus-response coupling wherein protein kinase C is activated in parallel with the mobilization of arachidonate. Respective products of these events, e.g., phosphorylated proteins and hydroxyicosatetraenates, then interact to mediate function.