Effects of ?-tocopherol and its acetate on the hydrolytic activity of phospholipase D in egg yolk phosphatidylcholine bilayers

Molecular associations of vitamin E

To understand how vitamin E fulfills its functions in membranes and lipoproteins, it is necessary to know how it associates with the lipid components of these structures and the effects its presence has on their structure and stability. Studies of model membrane systems containing vitamin E have proved to be an informative approach to address these questions. A review of the way vitamin E interacts with phospholipid bilayers, how it distributes within the structure, its motional diffusion characteristics, and orientation has been undertaken. The effect of vitamin E on membrane stability and permeability has been described. The tendency of vitamin E to form complexes with certain phospholipids is examined as is the way modulation of protein functions takes place. Finally, recent evidence relevant to the putative role of vitamin E in protecting membranes from free radical attack and the consequences of lipid oxidation in lipoproteins and membranes is examined.

Interfacial properties of phosphatidylcholine bilayers containing vitamin E derivatives

alpha-Tocopherol and alpha-tocopheryl succinate are biologically active lipids. The activity of these lipids may be related to how they affect membrane physical-chemical properties. Utilizing fluorescence methods, we have investigated the effect of alpha-tocopherol, alpha-tocopheryl succinate, and alpha-tocopheryl acetate on the properties of model membranes consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. In liquid-crystalline phase phospholipid bilayers, alpha-tocopherol decreased acyl chain mobility and decreased the interfacial polarity, but had no effect on the interfacial surface charge. In contrast, alpha-tocopheryl succinate had little effect on acyl chain motion or interfacial hydration, but increased the interfacial surface charge. alpha-Tocopheryl acetate had very little effect on any of the measurements of these bilayer properties. In a gel phase bilayer, alpha-tocopherol decreased acyl chain order, whereas alpha-tocopheryl succinate and alpha-tocopheryl acetate did not. Each alpha-tocopheryl derivative had a different effect on interfacial polarity, however, only alpha-tocopheryl succinate increased the interfacial surface charge. The acylation of alpha-tocopherol abolishes its antioxidant activity and generates molecules with different membrane physical properties. The non-polar acetate group of alpha-tocopheryl acetate locates this compound in a region of the bilayer where it has little effect on bilayer interfacial properties. The free carboxyl group of alpha-tocopheryl succinate is located in the interfacial region of the bilayer where it increases the membrane surface charge.

Synthesis of phytyl- and chroman-derivatized photoaffinity labels based on alpha-tocopherol

Photoaffinity analogues of alpha-tocopherol have been prepared by substituting photosensitive functional groups at either the terminus of an alkyl chain of varying length mimicking the phytyl tail or on C-3 of the chroman portion of tocopherol. The alkyl chain-modified compounds 2a-d contain a hexyl to nonyl alkyl chain extending from C-2 of the chroman, terminating in a tetrafluoroazidobenzyloxy group. These compounds were prepared starting from the commercially available Trolox acid 4, followed by esterification, protection, and reduction to the silyl-protected Trolox aldehyde 7, which was coupled using Wittig chemistry to different omega-hydroxyphosphonium bromides. Reduction of the alkene product, coupling with p-azidotetrafluorobenzyl bromide, and deprotection of the phenolic silyl group gave compounds 2a-d in excellent yields. Chroman-functionalized photoaffinity labels were synthesized starting from the protected tocopherol chromene 16b which was a key intermediate for preparation of a 3-hydroxy derivative, either by reduction of epoxides produced directly with Jacobsen's catalysts or by treatment with NBS in wet DME to give two stereoisomeric bromohydrins which were cyclized and reduced to give the phenol-protected C-3 alcohols 19a,b. These alcohols were then converted to diazoacetate esters, and the protecting group was removed to give 3-diazoacetoxy alpha-tocopherols 3a,b.

Formation and stability of the dispersed particles composed of retinoic acid, sesame oil and phosphatidylcholine

All trans-retinoic acid (RA) was dispersed by sonication with soybean phosphatidylcholine (PC). The particle size in the dispersion was increased to 240 nm up to the RA mol fraction range (X(RA)) of 0.4. At X(RA)=0.5, the RA/PC mixture was difficult to disperse and the macroscopic oil/water phase separation was observed. On the other hand, by the addition of sesame oil (SO) to RA (molar ratio of RA:SO=1:1), stable aqueous dispersions (diameter: 40-80 nm) were obtained in the mol fraction range RA and SO mixture (X(M)) of 0.1-0.8. In order to clarify these dispersal mechanism, the dispersed particles were characterized and the interaction among RA, SO and PC was investigated using several physicochemical techniques. The trapped aqueous volume inside the RA/PC particles was determined using the aqueous space marker, calcein and it was increased with the addition of RA into small unilamellar vesicles of PC. On the other hand, that of RA/SO/PC particles was decreased remarkably with increase in X(M) and the decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO(4). These results indicate that the interaction of RA with PC bilayers and the structure of RA/PC mixture will be changed by the addition of SO.

Interaction of alpha-Tocopherol and Soybean Oil with Phosphatidylcholine and Their Formation of Small Dispersed Particles

In this study, alpha-tocopherol (alpha-T) was dispersed by cosonication with soybean phosphatidylcholine (PC). The particle size in the dispersion was increased to 250 nm up to the alpha-T mole fraction (XT) 0.4. At XT = 0.5, the alpha-T/PC mixture was difficult to disperse and the macroscopic oil/water phase separation was observed. By the addition of soybean oil (SO) to alpha-T (molar ratio alpha-T:SO = 1:1), stable aqueous dispersions (diameter 50-70 nm) were obtained in the mole fraction range for the alpha-T and SO mixture (XM) 0.1-0.8. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction among alpha-T, SO, and PC was investigated using several physicochemical techniques. The trapped aqueous volume inside the alpha-T/PC particles was determined using the aqueous space marker calcein, and this volume was increased with the addition of alpha-T into small unilamellar vesicles of PC. The trapped aqueous volume of alpha-T/SO/PC particles was decreased remarkably with an increase in XM, and the decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO4. These results indicate that the interaction of alpha-T with PC bilayers and the structure of the alpha-T/PC mixture will be changed by the addition of SO. Copyright 1999 Academic Press.

Interaction of retinol with dipalmitoylphosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of all-trans-retinol (vitamin A, VA) were obtained by sonication with dipalmitoylphos-phatidylcholine (DPPC) in the VA mole fraction range 0.1-0.7. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50-70 nm. A limited amount of VA was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VA/DPPC particles was decreased markedly with the addition of VA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess VA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

The interaction of ubiquinone-3 with phospholipid membranes

The effects of ubiquinone-3 (UQ) on dipalmitoylphosphatidylcholine (DPPC) membrane were studied by surface monolayer, differential scanning calorimetry (DSC) and fluorescence techniques. DPPC and UQ are proved to be freely miscible in the mixed monolayer at an air/water interface, and to be partially miscible in bulk phase, i.e. bilayer and solid phase. There is a condensing interaction between UQ and DPPC in the UQ/DPPC mixed monolayers. The solubility of UQ in the DPPC is about 20 mole% and the solubility of DPPC in UQ is about 10 mole%. The membrane fluidity of DPPC was increased by the addition of UQ and the phase transition temperature was decreased.

Regulation of phospholipase D activity by neutral lipids in egg-yolk phosphatidylcholine small unilamellar vesicles and by calcium ion in aqueous medium

Hydrolysis activity of phospholipase D from Streptomyces chromofuscus (PLD) was studied in small unilamellar vesicles (SUV) of egg yolk phosphatidylcholine (PC). The enzyme was associated with PC-SUV in a Ca(2+)-dependent manner. Both apparent maximum velocity, Vmax(app), and reciprocal of apparent Michaelis constant, i.e., apparent binding constant, 1/Km(app), increased with Ca2+ concentration, and the maximum values of these kinetic parameters were obtained at about 20 microM Ca2+. Incorporation of 1,2-diacylglycerol (DAG), cholesterol (Chol) or alpha-tocopherol (Toc) into PC-SUV induced shift of the antisymmetric PO2- stretching band of PC to lower frequency. The neutral lipids in SUV brought about increase of the Vmax(app) value (Yamamoto et al. (1993) Biochim. Biophys. Acta 1145, 293-297). On the basis of these findings we discussed the regulation of PLD activity in terms of the Ca(2+)-dependent complex formation of PLD with SUV, and the enhancement of susceptibility of the P-O bond in PC molecule by neutral lipids.