Hydrolysis of thioester analogs by rat liver phospholipase A1

Current Knowledge on Mammalian Phospholipase A1, Brief History, Structures, Biochemical and Pathophysiological Roles

Phospholipase A₁ (PLA₁) is an enzyme that cleaves an ester bond at the sn-1 position of glycerophospholipids, producing a free fatty acid and a lysophospholipid. PLA₁ activities have been detected both extracellularly and intracellularly, which are well conserved in higher eukaryotes, including fish and mammals. All extracellular PLA₁s belong to the lipase family. In addition to PLA₁ activity, most mammalian extracellular PLA₁s exhibit lipase activity to hydrolyze triacylglycerol, cleaving the fatty acid and contributing to its absorption into the intestinal tract and tissues. Some extracellular PLA₁s exhibit PLA₁ activities specific to phosphatidic acid (PA) or phosphatidylserine (PS) and serve to produce lysophospholipid mediators such as lysophosphatidic acid (LPA) and lysophosphatidylserine (LysoPS). A high level of PLA₁ activity has been detected in the cytosol fractions, where PA-PLA₁/DDHD1/iPLA₁ was responsible for the activity. Many homologs of PA-PLA₁ and PLA₂ have been shown to exhibit PLA₁ activity. Although much has been learned about the pathophysiological roles of PLA₁ molecules through studies of knockout mice and human genetic diseases, many questions regarding their biochemical properties, including their genuine in vivo substrate, remain elusive.

Yeast phosphatidic acid phosphatase Pah1 hops and scoots along the membrane phospholipid bilayer

PA phosphatase, encoded by PAH1 in the yeast Saccharomyces cerevisiae, catalyzes the Mg²⁺-dependent dephosphorylation of PA, producing DAG at the nuclear/ER membrane. This enzyme plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. As an interfacial enzyme, PA phosphatase interacts with the membrane surface, binds its substrate, and catalyzes its reaction. The Triton X-100/PA-mixed micellar system has been utilized to examine the activity and regulation of yeast PA phosphatase. This system, however, does not resemble the in vivo environment of the membrane phospholipid bilayer. We developed an assay system that mimics the nuclear/ER membrane to assess PA phosphatase activity. PA was incorporated into unilamellar phospholipid vesicles (liposomes) composed of the major nuclear/ER membrane phospholipids, PC, PE, PI, and PS. We optimized this system to support enzyme-liposome interactions and to afford activity that is greater than that obtained with the aforementioned detergent system. Activity was regulated by phospholipid composition, whereas the enzyme's interaction with liposomes was insensitive to composition. Greater activity was attained with large (≥100 nm) versus small (50 nm) vesicles. The fatty-acyl moiety of PA had no effect on this activity. PA phosphatase activity was dependent on the bulk (hopping mode) and surface (scooting mode) concentrations of PA, suggesting a mechanism by which the enzyme operates along the nuclear/ER membrane in vivo.

The role and characterization of phospholipase A1 in mediating lysophosphatidylcholine synthesis in Trypanosoma brucei

Lysophospholipids are ubiquitous intermediates in a variety of metabolic and signalling pathways in eukaryotic cells. We have reported recently that lysoglycerophosphatidylcholine (lyso-GPCho) synthesis in the insect form of the ancient eukaryote Trypanosoma brucei is mediated by a novel phospholipase A1 (TbPLA1). In the present study, we show that despite equal levels of TbPLA1 gene expression in wild-type insect and bloodstream trypomastigotes, both TbPLA1 enzyme levels and lysoGPCho metabolites are approx. 3-fold higher in the bloodstream form. Both of these parasite stages synthesize identical molecular species of lysoGPCho. TbPLA1 null mutants in the bloodstream form of the parasite are viable, but are deficient in lysoGPCho synthesis, a defect that can be overcome by the expression of an ectopic copy of TbPLA1. The biochemical attributes of TbPLA1-mediated lysoGPCho synthesis were examined in vitro using recombinant TbPLA1. Although TbPLA1 possesses an active-site serine residue, it is insensitive to serine-modifying reagents, such as di-isopropyl fluorophosphate and PMSF, a characteristic shared by lipases that possess lid-sheltered catalytic triads. TbPLA1 does not require metal co-factors for activity, but it does require interfacial activation prior to catalysis. Results from size-exclusion chromatography and binding kinetics analysis revealed that TbPLA1 activation by Triton X-100/GPCho mixed micelle surfaces was not specific and did not require the pre-formation of a specific enzyme-substrate complex to achieve surface binding.

PC and PE synthesis: mixed micellar analysis of the cholinephosphotransferase and ethanolaminephosphotransferase activities of human choline/ethanolamine phosphotransferase 1 (CEPT1)

The human choline/ethanolamine phosphotransferase 1 (CEPT1) gene codes for a dual-specificity enzyme that catalyzes the de novo synthesis of the two major phospholipids through the transfer of a phosphobase from CDP-choline or CDP-ethanolamine to DAG to form PC and PE. We used an expression system devoid of endogenous cholinephosphotransferase and ethanolaminephosphotransferase activities to assess the diradylglycerol specificity of CEPT1. A mixed micellar assay was used to ensure that the diradylglycerols delivered were not affecting the membrane environment in which CEPT1 resides. The CEPT1 enzyme displayed an apparent Km of 36 microM for CDP-choline and 4.2 mol% for di-18:1 DAG with a Vmax of 14.3 nmol min(-1) mg(-1). When CDP-ethanolamine was used as substrate, the apparent Km was 98 microM for CDP-ethanolamine and 4.3 mol% for di-18:1 DAG with a Vmax of 8.2 nmol min(-1) mg(-1). The preferred diradylglycerol substrates used by CEPT1 with CDP-choline as the phosphobase donor were di-18:1 DAG, di-16:1 DAG, and 16:0/18:1 DAG. A major difference between previous emulsion-based assay results and the mixed micelle results was a complete inability to use 16:0(O)/2:0 as a substrate for the de novo synthesis of platelet-activating factor when the mixed micelle assay was used. When CDP-ethanolamine was used as the phosphobase donor, 16:0/18:1 DAG, di-18:1 DAG, and di-16:1 DAG were the preferred substrates. The mixed micelle assay also allowed the lipid activation of CEPT to be measured, and both the cholinephosphotransferase and ethanolaminephosphotransferase activities displayed the unusual property of product activation at 5 mol%, implying that specific lipid activation binding sites exist on CEPT1. The protein kinase C inhibitor chelerythrine and the human DAG kinase inhibitor R59949 both inhibited CEPT1 activity with IC50 values of 40 microM.

Structural and mechanistic features of phospholipases C: effectors of inositol phospholipid-mediated signal transduction

The production of the intracellular second messengers inositol (1,4,5)-trisphosphate (InsP3) and sn 1,2-diacylglycerol (DG) in response to a wide variety of extracellular primary messengers is achieved by an extended family of inositol phospholipid phosphodiesterases termed phospholipases C (PLC, E.C. 3.1.4.11). This family has been the subject of extensive research and it is clear that the different isoenzymes exhibit some common characteristics (e.g., interactions with substrates) and other distinctive features (e.g., modes of regulation). The recent description of the X-ray crystal structure of a mammalian PLC has served to clarify much about the behaviour of the PLCs, emphasising the "modular" structure of these enzymes. The main focus of this review will concern the specific adaptations of PLC molecules which make them efficient lipid-metabolising enzymes. We also describe what is known about how these enzymes interact with their lipid substrates, which will serve as a basis for considering how PLCs may be activated.

Peptide thioester substrates for serine peptidases and metalloendopeptidases

Peptide thioesters are sensitive substrates of various serine peptidases and metalloendopeptidases. Thioester substrates generally have high enzymatic hydrolysis rates and low background hydrolysis rates, and the hydrolysis rates can be easily monitored in the presence of thiol reagents such as 4,4'-dithiodipyridine or 5,5'-dithiobis (2-nitrobenzoic acid). Peptide thioester substrates have been invaluable for the study of enzyme specificity and enzyme inhibitors, especially in cases where no other practical synthetic substrates are available. Tripeptide substrates of the type Boc-Ala-Ala-AA-SBzl, where AA is nearly all of the 20 common amino acids, have now been synthesized and should be useful for the subsite mapping of new serine peptidases and the study of crude cell preparations containing serine peptidases.

A simple and efficient synthesis of fatty thioacids

A rapid and high-yielding two-step synthesis of fatty thioacids from readily available starting materials has been devised. In the first step, an acid chloride is reacted with thioacetic acid at room temperature to produce a mixed thioanhydride, which in the second step is nucleophilically deacetylated with propylamine or butylamine at 5 degrees C. Each step is complete in five minutes and proceeds in quantitative yield. The versatility of the procedure is demonstrated by the synthesis of fatty thioacids from six to sixteen carbons in length.

Phospholipids containing nitrogen- and sulfur-linked chains: kinetics of cholesterol exchange between vesicles

We have examined the kinetics of [14C]cholesterol exchange between unilamellar vesicles formed from the following synthetic glycerophosphatidylcholines: (a) those having acyl (OC(O)R), acylamino (NHC(O)R), carbamoyl (NHC(O)OR), and acylthio (SC(O)R) chains at the sn-2 position, and (b) those having alkyl (OR) and thioalkyl (SR) chains at the sn-1 position. Replacement of the glycerol oxygen atom at the sn-2 position of PC with a NH group did not affect the rate of cholesterol exchange to a significant extent, suggesting that the amide group of sphingomyelin is not primarily responsible for the very slow rate of exchange of cholesterol observed from sphingomyelin vesicles. Replacement of the glycerol oxygen at the sn-2 position of phosphatidylcholine with a sulfur atom caused the rate of spontaneous cholesterol exchange to increase by a factor of 1.6. Substitution of an O-alkyl chain for the acyl chain at the sn-1 position of 2-acylthiophosphatidylcholine or substitution of a thioalkyl chain for the O-alkyl sn-1 chain of 1-alkyl-2-acylaminodeoxyphosphatidylcholine also did not result in a marked difference in cholesterol exchange rate. The data suggest that interactions other than intermolecular hydrogen bonding are involved in determining the rates of intermembrane cholesterol exchange. Significantly, these kinetic studies also lend support to the continued use in model membranes of synthetic sulfur- and nitrogen-substituted phosphatidylcholines, which have been employed to study properties of lipolytic enzymes, since synthetic acylamino- and acylthio-phospholipids form vesicles that give cholesterol exchange rates that closely resemble those found in vesicles prepared with diester-phosphatidylcholines.

Hydrolysis of neutral lipid substrates by rat hepatic lipase

Rat hepatic lipase, an enzyme whose involvement in the catabolism of lipoproteins remains poorly defined, has both neutral lipid and phospholipid hydrolyzing activity. We determined the substrate specificity of hepatic lipase for 1-oleoyl-sn-glycerol, 1,2-dioleoyl-sn-glycerol, and 1,3-dioleoyl-sn-glycerol in the Triton X-100 mixed micellar state, and compared these results to those obtained previously in our laboratory for the phospholipid substrates phosphatidic acid (PA), phosphatidylethanolamine (PE), and phosphatidylcholine (PC). Vmax values were determined by diluting the substrate concentration in the surface of the micelle by Triton X-100. The Vmax values obtained were 144 mumol/min/mg for 1-oleoyl-sn-glycerol, 163 mumol/min/mg for 1,2-dioleoyl-sn-glycerol, and 145 mumol/min/mg for 1,3-dioleoyl-sn-glycerol. These values were higher than those obtained earlier for phospholipids which were 67 mumol/min/mg for PA, 50 mumol/min/mg for PE and 4 mumol/min/mg for PC. In addition, the mole fraction of lipid substrate at half maximal velocity (K) in the surface dilution plot was lower for the neutral lipid substrates as compared to those obtained for the phospholipid substrates. When the hydrolysis of 1,3-dioleoyl-sn-glycerol mixed micelles was studied as a function of time, cleavage at the sn-1 and sn-3 positions occurred at the same rate, suggesting that hepatic lipase is not stereoselective with respect to 1,3-diacyl-sn-glycerol substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Assay strategies and methods for phospholipases

Of the general considerations discussed, the two issues which are most important in choosing an assay are (1) what sensitivity is required to assay a particular enzyme and (2) whether the assay must be continuous. One can narrow the options further by considering substrate availability, enzyme specificity, assay convenience, or the presence of incompatible side reactions. In addition, the specific preference of a particular phospholipase for polar head group, micellar versus vesicular substrates, and anionic versus nonionic detergents may further restrict the options. Of the many assays described in this chapter, several have limited applicability or serious drawbacks and are not commonly employed. The most commonly used phospholipase assays are the radioactive TLC assay and the pH-stat assay. The TLC assay is probably the most accurate, sensitive assay available. These aspects often outweigh the disadvantages of being discontinuous, tedious, and expensive. The radioactive E. coli assay has become popular recently as an alternative to the TLC assay for the purification of the mammalian nonpancreatic phospholipases. The assay is less time consuming and less expensive than the TLC assay, but it is not appropriate when careful kinetics are required. Where less sensitivity is needed, or when a continuous assay is necessary, the pH-stat assay is often employed. With purified enzymes, when free thiol groups are not present, a spectrophotometric thiol assay can be used. This assay is approximately as sensitive as the pH-stat assay but is more convenient and more reproducible, although the substrate is not available commercially. Despite the many assay choices available, the search continues for a convenient, generally applicable assay that is both sensitive and continuous. The spectrophotometric SIBLINKS assay and some of the fluorescent assays show promise of filling this need.

Phospholipids showing complex bilayer phase transitions. II. Four sulfur-containing phosphatidylcholines

The main phase transitions of aqueous dispersions of four synthetic phosphatidylcholines (PCs) containing sulfur atoms in thioester and thioether linkages have been studied by high sensitivity differential scanning calorimetry (DSC). The transition enthalpies ranged from 7.4 to 10.3 kcal mol-1, with values for tm, the temperature of maximal excess heat capacity, in the range 38.0 to 40.8 degrees C. The corresponding values for dipalmitoyl PC (DPPC) are 8.5 kcal mol-1 and 41.7 degrees C. Curve fitting required the sum of from one to four two-state components to give an accurate representation of the observed DSC curves. Comparison of the results given here with those reported by B.Z. Chowdhry, G. Lipka, J. Hajdu and J.M. Sturtevant, (1984) Biochemistry 23, 2044-2049, for PCs containing amide, ether or carbamoyl linkages in place of the usual ester bonds shows that small changes in organic structure can result in large changes in thermotropic behavior. The complexity in the cases showing more than a single two-state component is presumably due to a series of sequential cooperative transitions the character of which is at present unknown.

A facile asymmetric synthesis of glycerol phospholipids via tritylglycidol prepared by the asymmetric epoxidation of allyl alcohol. Thiolester and thioether analogs of phosphatidylcholine

Trityl-glycidol was synthesized by in situ derivatization of glycidol, which was prepared by the catalytic asymmetric epoxidation of allyl alcohol. Depending on the enantiomer of diisopropyl tartrate used with the titanium catalyst, either (R)- or (S)-trityl-glycidol was obtained in a "one pot" synthesis in about 50% overall yield. The optical purity, determined by NMR spectroscopy of a Mosher ester, was greater than 98% ee. Nucleophilic opening of the chiral epoxide with dodecyl mercaptan gave optically active 1-S-dodecyl-3-O-trityl-1-thio-glycerol, which was used to synthesize 1-S-dodecyl-2-O-decanoyl-thio-sn-glycero-3-phosphocholine. Opening of the epoxide with methyl xanthate gave a 1,2-trithiocarbonate derivative of trityl glycerol which can be used to synthesize 1,2-bis(S-decanoyl)-1,2-dithio-sn-glycero-3-phosphocholine. Opening of the epoxide with thiodecanoic acid gave 1-S-decanoyl-3-O-trityl-1-thio-glycerol which was used to synthesize 1-S-decanoyl-2-O-decanoyl-1-thio-sn-glycero-3-phosphocholine.