Phospholipid synthesis: effects of solvents and catalysts on acylation

Novel cationic polyene glycol phospholipids as DNA transfer reagents--lack of a structure-activity relationship due to uncontrolled self-assembling processes

Cationic glycol phospholipids were synthesized introducing chromophoric, rigid polyenoic C20:5 and C30:9 chains next to saturated flexible alkyl chains of variable lengths C6-20:0. Surface properties and liposome formation of the amphiphilic compounds were determined, the properties of liposome/DNA complexes (lipoplexes) were established using three formulations (no co-lipid, DOPE as a co-lipid, or cholesterol as a co-lipid), and the microstructure of the best transfecting compounds inspected using small angle X-ray diffraction to explore details of the partially ordered structures of the systems that constitute the series. Transfection and cytotoxicity of the lipoplexes were evaluated by DNA delivery to Chinese hamster ovary (CHO-K1) cells using the cationic glycerol phospholipid 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (EPC) as a reference compound. The uncontrollable self-association of the molecules in water resulted in aggregates and liposomes of quite different sizes without a structure-property relationship. Likewise, adding DNA to the liposomes gave rise to unpredictable sized lipoplexes, which, again, transfected without a structure-activity relationship. Nevertheless, one compound among the novel lipids (C30:9 chain paired with a C20:0 chain) exhibited comparable transfection efficiency and toxicity to the control cationic lipid EPC. Thus, the presence of a rigid polyene chain in this best performing achiral glycol lipid did not have an influence on transfection compared with the chiral glycerolipid reference ethyl phosphocholine EPC with two flexible saturated C14 chains.

The use of natural and synthetic phospholipids as pharmaceutical excipients

In pharmaceutical formulations, phospholipids obtained from plant or animal sources and synthetic phospholipids are used. Natural phospholipids are purified from, e.g., soybeans or egg yolk using non-toxic solvent extraction and chromatographic procedures with low consumption of energy and minimum possible waste. Because of the use of validated purification procedures and sourcing of raw materials with consistent quality, the resulting products differing in phosphatidylcholine content possess an excellent batch to batch reproducibility with respect to phospholipid and fatty acid composition. The natural phospholipids are described in pharmacopeias and relevant regulatory guidance documentation of the Food and Drug Administration (FDA) and European Medicines Agency (EMA). Synthetic phospholipids with specific polar head group, fatty acid composition can be manufactured using various synthesis routes. Synthetic phospholipids with the natural stereochemical configuration are preferably synthesized from glycerophosphocholine (GPC), which is obtained from natural phospholipids, using acylation and enzyme catalyzed reactions. Synthetic phospholipids play compared to natural phospholipid (including hydrogenated phospholipids), as derived from the number of drug products containing synthetic phospholipids, a minor role. Only in a few pharmaceutical products synthetic phospholipids are used. Natural phospholipids are used in oral, dermal, and parenteral products including liposomes. Natural phospholipids instead of synthetic phospholipids should be selected as phospholipid excipients for formulation development, whenever possible, because natural phospholipids are derived from renewable sources and produced with more ecologically friendly processes and are available in larger scale at relatively low costs compared to synthetic phospholipids. Practical applications: For selection of phospholipid excipients for pharmaceutical formulations, natural phospholipids are preferred compared to synthetic phospholipids because they are available at large scale with reproducible quality at lower costs of goods. They are well accepted by regulatory authorities and are produced using less chemicals and solvents at higher yields. In order to avoid scale up problems during pharmaceutical development and production, natural phospholipid excipients instead of synthetic phospholipids should be selected whenever possible.

Lipid structure and not membrane structure is the major determinant in the regulation of protein kinase C by phosphatidylserine

This study addresses the molecular basis for protein kinase C's specific activation by phosphatidylserine. Specifically, we ask whether protein kinase C's phospholipid specificity arises from specific protein/lipid interactions or whether it arises from unique membrane-structuring properties of phosphatidylserine. We measured the interaction of protein kinase C betaII to membranes that differed only in being enantiomers to one another: physical properties such as acyl chain composition, membrane fluidity, surface curvature, microdomains, headgroup packing, and H-bonding with water were identical. Binding and activity measurements reveal that protein kinase C specifically recognizes 1, 2-sn-phosphatidyl-L-serine, independently of membrane structure. High-affinity binding and activation are abolished in the presence of enantiomeric membranes containing 2,3-sn-phosphatidyl-L-serine, 2, 3-sn-diacylglycerol, and 2,3-sn-phosphatidylcholine. Our data also show that the stereoselectivity for 1,2-sn-diacylglycerol is not absolute; 2,3-sn-diacylglycerol modestly increases the membrane affinity of protein kinase C provided that 1, 2-sn-phosphatidyl-L-serine is present. We also find that the stereochemistry of the bulk phospholipid, in this case phosphatidylcholine, has no significant influence on protein kinase C's membrane interaction. These data reveal that specific molecular determinants on protein kinase C stereospecifically recognize structural determinants of phosphatidylserine.

On the relation between a stearoyl-specific transacylase from bovine testis membranes and a copurifying acyltransferase

Bovine testis membranes contain a coenzyme A-dependent transacylase that can catalyze the preferential transfer of stearoyl groups from phosphoglycerides to sn-2-acyl molecular species of lysophosphatidic acid and lysophosphatidylinositol [Itabe et al., (1992) J. Biol. Chem. 267, 15319-15325]. We have now purified this enzyme 1000-fold and shown that it copurifies with an acyltransferase. The purified transacylase can use phosphatidic acid, phosphatidylinositol, or phosphatidylinositol-4-phosphate as an acyl donor and catalyzes the transfer of stearoyl groups in preference to palmitoyl groups or oleoyl groups. In contrast, the purified acyltransferase uses acyl-coenzyme A as an acyl donor and shows no such preference for stearoyl group transfer. Furthermore, phosphatidylinositol-4, 5-bisphosphate inhibits the two enzymes to different extents and by different mechanisms. Nevertheless, the enzymes are similar in several respects: they use the same acyl acceptors and, when assayed together, compete for the acyl acceptor, sn-2-oleoyl lysophosphatidic acid; they lose activity in parallel unless stabilized by the addition of an anionic phosphoglyceride or stearoyl-coenzyme A; and they show similar sensitivities to heat and pH. One way to explain these results is to postulate that the transacylase reaction occurs in two successive steps: a stearoyl-specific first step in which a stearoyl group is transferred from an sn-1-stearoyl-2-acyl phosphoglyceride to coenzyme A, and a relatively non-acyl-chain-specific second step in which a stearoyl group is transferred from stearoyl-coenzyme A to an sn-2-acyl lysophosphoglyceride. According to this line of reasoning, the transacylase assay that we have used measures the net effect of both steps, whereas the acyltransferase assay measures only the effect of the second step.

Syntheses of 1,2-di-O-palmitoyl-sn-glycero-3-phosphocholine (DPPC) and analogs with 13C- and 2H-labeled choline head groups

The syntheses of four head group labeled analogs of 1,2-di-O-palmitoyl-sn-glycero-3-phosphocholine (DPPC) (6) by a general method from 1,2-di-O-palmitoyl-sn-glycero-3-phosphatidic acid (5) have been performed. The syntheses of 1,2-di-O-palmitoyl-sn-glycero-3-phospho[alpha-13C]choline (6a) and 1,2-di-O-palmitoyl-sn-glycero-3-phospho[beta-13C]choline (6b) were performed from labeled [1-13C]glycine (1a) in 52% overall yield and from [2-13C]glycine (1b) in 56% overall yield, respectively. 1,2-Di-O-palmitoyl-sn-glycero-3-phospho[N(C2H3)3]choline (9) was prepared from 2-aminoethanol in 39% overall yield. 1,2-Di-O-palmitoyl-sn-glycero-3-phospho[alpha-C2H2]choline (12) was prepared from N,N-dimethylglycine ethyl ester in 50% overall yield.

Competitive inhibition of lipolytic enzymes. XI. Estimation of the interfacial dissociation constants of porcine pancreatic phospholipase A2 for substrate and inhibitor in the absence of detergents

Based on the strong inhibitory properties of (R)-2-decanoylamino-octanol-1-phosphocholine and its phosphoglycol analogue for porcine pancreatic phospholipase A2, the corresponding 2-decanoyloxy derivatives have been synthesised in both enantiomeric forms and their substrate properties for the enzyme were analysed. The high aqueous solubility in the absence of detergents, combined with low critical micelle concentrations of both the amide- and ester phospholipids allowed the estimation of the interfacial dissociation constants of the enzyme-substrate and enzyme-inhibitor complexes by kinetic and direct binding techniques.

Synthesis of 1-palmitoyl-2-hexadecyl-sn-glycero-3-phosphocholine (PHPC)

A general method for the chirospecific synthesis of 1-acyl-2-alkyl-sn-glycero-3-phosphocholines is described. 1-Palmitoyl-2-hexadecyl-sn-glycero-3-phosphocholine (PHPC) was synthesized in 18% overall yield in ten steps via five new synthetic intermediates, and 1-acetyl-2-hexadecyl-sn-glycero-3- phosphocholine (AHPC) was also synthesized. 1-Acyl-2-alkyl-sn-glycero-3-phosphocholines, which have not been found to exist in nature, are ether lipid analogs of 1,2-diacyl-sn-glycero-3-phosphocholines, which are important components of cell membranes. Biophysical studies of hydrated bilayers of PHPC will be of interest in probing the critical importance of the central region of these amphiphilic molecules to the molecular assemblies that are formed.