Alkylphosphate esters as inhibitors of phospholipase D

Efficient secretion expression of phospholipase D in Bacillus subtilis and its application in synthesis of phosphatidylserine by enzyme immobilization

Transphosphatidylation catalyzed by phospholipase D has gained increasing attention for producing phosphatidylserine (PS), which can be used in functional food and medicine. In this study, we investigated the effects of six signal peptides on the secretion of PLD (PLD_sa) from Streptomyces antibioticus TCCC 21059 in the food-grade GRAS bacterium Bacillus subtilis. It indicated that the optimal signal peptide DacB with an Ala-X-Ala sequence motif at the C-terminus showed the highest secretory expression ability, resulting in increased production of 2.84 U/mL PLD_sa. Then PLD_sa was immobilized on the epoxy-based carriers, and one of these carriers allowed PLD_sa loading of up to 2.7 mg/g. The immobilized PLD_sa was more stable over a wide range of pH value (4.5-7.5) and temperature (16 °C-60 °C) than free PLD_sa. Subsequently, the synthesis of PS from soybean phosphatidylcholine (PC) was carried out in purely aqueous solution using immobilized PLD_sa, leading to a high yield of 65%. The immobilized PLD_sa catalyst maintained a relative PS production of 60% after 5 recycles. Notably, the use of toxic solvent was completely eliminated in the whole process, which would be more profitable for the application of PS.

Phospholipase D inhibitors screening: Probing and evaluation of ancient and novel molecules

Phospholipase D (PLD) is a ubiquitous enzyme that cleaves the distal phosphoester bond of phospholipids generating phosphatidic acid (PA). In plants, PA is involved in numerous cell responses triggered by stress. Similarly, in mammals, PA is also a second messenger involved in tumorigenesis. PLD is nowadays considered as a therapeutic target and blocking its activity with specific inhibitors constitutes a promising strategy to treat cancers. Starting from already described PLD inhibitors, this study aims to investigate the effect of their structural modifications on the enzyme's activity, as well as identifying new potent inhibitors of eukaryotic PLDs. Being able to purify the plant PLD from Vigna unguiculata (VuPLD), we obtained a SAXS model of its structure. We then used a fluorescence-based test suitable for high-throughput screening to review the effect of eukaryotic PLD inhibitors described in the literature. In this regard, we found that only few molecules were in fact able to inhibit VuPLD and we confirmed that vanadate is the most potent of all with an IC₅₀ around 58 μM. Moreover, the small-scale screening of a chemical library of 3120 compounds allowed us to optimize the different screening's steps and paved the way towards the discovery of new potent inhibitors.

Inhibition of phospholipase D alpha by N-acylethanolamines

N-Acylethanolamines (NAEs) are endogenous lipids in plants produced from the phospholipid precursor, N-acylphosphatidylethanolamine, by phospholipase D (PLD). Here, we show that seven types of plant NAEs differing in acyl chain length and degree of unsaturation were potent inhibitors of the well-characterized, plant-specific isoform of PLD-PLD alpha. It is notable that PLD alpha, unlike other PLD isoforms, has been shown not to catalyze the formation of NAEs from N-acylphosphatidylethanolamine. In general, inhibition of PLD alpha activity by NAEs increased with decreasing acyl chain length and decreasing degree of unsaturation, such that N-lauroylethanolamine and N-myristoylethanolamine were most potent with IC(50)s at submicromolar concentrations for the recombinant castor bean (Ricinus communis) PLD alpha expressed in Escherichia coli and for partially purified cabbage (Brassica oleracea) PLD alpha. NAEs did not inhibit PLD from Streptomyces chromofuscus, and exhibited only moderate, mixed effects for two other recombinant plant PLD isoforms. Consistent with the inhibitory biochemical effects on PLD alpha in vitro, N-lauroylethanolamine, but not lauric acid, selectively inhibited abscisic acid-induced closure of stomata in epidermal peels of tobacco (Nicotiana tabacum cv Xanthi) and Commelina communis at low micromolar concentrations. Together, these results provide a new class of biochemical inhibitors to assist in the evaluation of PLD alpha physiological function(s), and they suggest a novel, lipid mediator role for endogenously produced NAEs in plant cells.

Hexadecylphosphocholine does not influence phospholipase D and sphingomyelinase activity in human leukemia cells

Hexadecylphosphocholine (HePC) is the first representative of the alkylphosphocholines (APC), a new group of biologically active compounds. HePC has pronounced antiproliferative effects on neoplastic cells in vitro and in vivo. The molecular mechanism by which HePC exerts its biological effects is still under investigation. Recently there has been growing evidence that HePC probably interferes with cellular signalling via phospholipases. It has been shown to inhibit both forms of phospholipase C (PLC), the phosphatidylinositol- and the phosphatidylcholine-specific PLC, and phospholipase A2. Here we present data showing that HePC inhibits the activity of phospholipase D in vitro, whereas the action of this enzyme in leukemic cell lines is not affected. Furthermore HePC does not seem to disturbed the activity of sphingomyelinase, another enzyme of phospholipid metabolism which has been shown to play an important role in cellular signalling as well.

1,3-Diacylglycero-2-phosphocholines--synthesis, aggregation behaviour and properties as inhibitors of phospholipase D

A series of 1,3-diacylglycero-2-phosphocholines (1,3-PCs) with acyl chain lengths of C8-C18 were synthesised by chemical introduction of the phosphocholine moiety into the regioisomerically pure 1,3-diacylglycerols, which were obtained from glycerol and the vinyl esters of fatty acid by means of lipase from Rhizomucor mihei. The 1,3-PCs being regioisomers of the natural glycerophospholipids were studied with respect to their aggregation behaviour in the absence and in the presence of sodium dodecylsulfate (SDS) as well as their properties as substrates and inhibitors of phospholipase D (PLD) from cabbage. While the main structures of the pure 1,3-PCs were micelles (C8), liposomes (C10, C12) or planar bilayers (C14, C16, C18), the addition of SDS resulted in the formation of mixed micelles (C8, C10) and mixed liposomes (C12, C14, C16, C18). None of the 1,3-PCs was found to be hydrolysed by PLD, whereas all of them showed inhibitory properties in the standard assay for PLD. The inhibitory power was strongest with 1,3-didecanoylglycero-2-phosphocholine (IC50 = 43 microM).

The effect of hexadecylphosphocholine on the degradation of mitochondrial phospholipids

Hexadecylphosphocholine (HePC) is known as antitumor agent but the mechanism has not yet been understood. In rat liver mitochondria its effect on phospholipid transformation has been studied by quantitative HPTLC and phosphorus determination. From the results it can be concluded that HePC influences the activities of phospholipase A2, phospholipase C, phospholipase D, and lysophospholipase A. The phospholipid transformation as well as the influence of HePC are affected by exogenous calcium ions. In the presence of calcium HePC has been found to inhibit enzyme activities, whereas in the absence of exogenous calcium ions enzymatic phospholipid transformations are activated or inhibited depending on HePC concentrations.

Hexadecylphosphocholine and 2-modified 1,3-diacylglycerols as effectors of phospholipase D

The kinetic behaviour of phospholipase D (PLD) from cabbage has been studied in the presence of several substrate-like compounds such as hexadecylphosphocholine (HPC) and 1,3-didodecanoylglycero-2-phosphatides. 1,3-Didodecanoyl- glycero-2-phosphocholine (1,3-DiC12PC) was found being not cleft by PLD, whereas HPC is hydrolyzed by PLD with small rate. The plot of initial velocity vs. substrate concentration for HPC is more sigmoidal than those for the common substrate phosphatidylcholine (PC)/sodium dodecylsulfate (SDS) (1:0.5) or the short-chain 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DiC6PC). The anionic amphiphiles 1,3-didodecanoylglycero-2-sulfate and 1,3-didodecanoylglycero-2-phosphate act as activators of PLD towards PC similar to SDS. In contrast, 1,3-DiC12PC shows inhibitory properties with an increase in the sigmoidicity of the initial velocity as a function of substrate concentration in the PC/SDS assay. Also HPC inhibits the hydrolysis of PC/SDS, whereas it acts as activator or inhibitor in the hydrolysis of DiC6PC. The results suggest that PLD possesses two substrate-binding sites, where one binds substrate in function of an effector without catalytic activity while the other is the catalytic site. HPC and 1,3-DiC12PC are assumed to compete with the substrate for both binding sites with effects depending on the ratio of concentrations and affinities of substrates and effectors.

Inhibition of phospholipase D by lysophosphatidylethanolamine, a lipid-derived senescence retardant

Phospholipid signaling mediated by lipid-derived second messengers or biologically active lipids is still new and is not well established in plants. We recently have found that lysophosphatidylethanolamine (LPE), a naturally occurring lipid, retards senescence of leaves, flowers, and postharvest fruits. Phospholipase D (PLD) has been suggested as a key enzyme in mediating the degradation of membrane phospholipids during the early stages of plant senescence. Here we report that LPE inhibited the activity of partially purified cabbage PLD in a cell-free system in a highly specific manner. Inhibition of PLD by LPE was dose-dependent and increased with the length and unsaturation of the LPE acyl chain whereas individual molecular components of LPE such as ethanolamine and free fatty acid had no effect on PLD activity. Enzyme-kinetic analysis suggested noncompetitive inhibition of PLD by LPE. In comparison, the related lysophospholipids such as lysophosphatidylcholine, lysophosphatidylglycerol, and lysophosphotidylserine had no significant effect on PLD activity whereas PLD was stimulated by lysophosphatidic acid and inhibited by lysophosphatidylinositol. Membrane-associated and soluble PLD, extracted from cabbage and castor bean leaf tissues, also was inhibited by LPE. Consistent with acyl-specific inhibition of PLD by LPE, senescence of cranberry fruits as measured by ethylene production was more effectively inhibited according to the increasing acyl chain length and unsaturation of LPE. There are no known specific inhibitors of PLD in plants and animals. We demonstrate specific inhibitory regulation of PLD by a lysophospholipid.