Phospholipid head group, effect

Seelig, A., Allegrini, P. R. and Seelig, J. (1988). Partitioning of local anesthetics into membranes surface charge effects monitored by the phospholipid head-group, Biochim. Biophys. Acta, 939, 267-276. 

Scherer, P. G. and Seelig, J. (1989). Electric charge effects on phospholipid head-groups. Phosphatidylcholine in mixtures with cationic and anionic amphiphiles, Biochem., 28, 7720-7728. 

The dansyl (dimethylaminonaphthalenesultonyl) group has been attached to various lipids, most notably at the phospholipid head group, where the probe is sensitive to solvent effects (see below). 

A major effect of cholesterol on the conformation of apoE was revealed by comparing the conformation on DMPC discs, on HDLc, and on spherical artificial microemulsion particles by circular dichroism (Mims et ai, 1990). Conformational differences of apoE on different types of particles also were demonstrated using NMR to probe lysyl microenvironments. When the apoE lysyl residues were labeled by reductive methylation with [ C]formaldehyde to allow detection, the lysyl microenvironments manifested dramatic differences on a discoidal particle compared to spherical particles (S. Lund-Katz et aL, 1993). On spherical particles, two lysine microenvironments were observed, but on discoidal particles eight peaks were observed (apoE has 12 lysyl residues). These results indicate that apoE structure differs significantly on the two lipid surfaces. In a systematic study of the effect of the particle lipid composition on the conformation of apoE, conformation was shown to be affected by a number of parameters (Mims et ai, 1990). The a-helical content was lower when apoE was bound to a spherical particle compared to a discoidal particle. It was concluded that this probably reflects the different ways in which the amphipathic helices interact with phospholipid on the two particles. With discoidal particles the interaction is primarily with phospholipid acyl side chains, whereas with spherical particles the interaction is with polar phospholipid head groups. In addition, the conformation of apoE was influenced by the diameter of the microemulsion particle and possibly by the order/ disorder of the lipid components. 

The effect of different phospholipid head groups on the protein-stimulated transfer by phosphatidylcholine- and phosphatidylinositol-specific proteins has been studied. Contradictory results were obtained for the effect of acidic phospholipids on the transfer of phospholipid by the phosphatidylcholine exchange protein from beef liver. DiCorleto et al. (1977) used small unilamellar vesicle-mitochondria and small unilamellar vesicle-multilamellar vesicles to study the effect of varying amounts of acidic phospholipids incorporated into phosphatidylcholine donor vesicles. Up to 20 mol% phosphatidic acid or phosphatidylinositol in the donor was found to stimulate the transfer of phosphatidylcholine in both assay systems. Wirtz et al. (1979) and Hellings et al. (1974) found different results for the phosphatidylcholine exchange protein with unilamellar and multilamellar vesicles. In these assays, the incorporation of acidic phospholipids (phosphatidic acid or phosphatidylinositol) into the donor particles had an inhibitory effect on the rate of phosphatidylcholine transfer. 

Figure 3. The effect of phospholipid head group on the protonation dynamics of an indicator adsorbed on a surface of neutral micelle. The indicator, bromocresol green, was adsorbed on Brij-58 micelles at a ratio of one per micelle. The pulse protonation, measured at pH 7.3, was initiated by photoexcitation of a water-soluble proton emitter 2-naphthol-3,6-disulfonate (2 mM). The reaction was followed spectrophotometrically. A, control no phospholipids added. B, phosphatidylcholine added to amount of 6 molecules per micelle. C, phosphatidyl-serine added to 6 molecules per micelle. D, phosphatidic acid added to 6 molecules per micelle. For more details see Nachliel and Gutman (ll).

Phosphorus " P is as well as other nuclei, present in biological membranes and has special advantages. Phospholipid head groups contain an isolated 7=1/2 spin system which depends only on chemical-shift anisotropy and dipolar proton-phosphorus interactions. It is therefore a useful probe for structure and motion. The chemical shift of P changes with the orientation of the magnetic field with respect to the nucleus. The observed spectrum can therefore be measured over a wide range of about 100 ppm. As the chemical-shift difference for P is only 4 ppm the chemical-shift anisotropy, because of orientational effects, controls the spectrum. A typical P-NMR spectrum of polymorphic phases of phospholipid bilayers is depicted in Figure 11-6. For details the reader is referred to specific publications [63]. 

G. L. Jendrasiak and J. C. Mendible, The Phospholipid Head-Group Orientation Effect on Hydration and Electrical Conductivity, Biochim. Biophys. Acta 424, 149-158 (1976). 

The effects described here in a model membrane may have some implications for membrane fusion. When a fusogenic lipid is introduced into the asymmetric bilayer structure of an erythrocyte membrane (Zwaal et al., 1973) it may initially interact with choline-containing phospholipids in the outer half of the bilayer (Maggio Lucy, 1975). This will alter the phospholipid head groups and... 

---