Model membrane morphology and crosslinking of oxidized lipids with proteins

Determination of lipid ester ozonides and core aldehydes by high-performance liquid chromatography with on-line mass spectrometry

Unsaturated triacylglycerols (TG) and choline (PC) and ethanolamine (PE) phosphatides of known structure were subjected to ozonization and reduction with triphenylphosphine to yield the corresponding lipid ester core aldehydes. Mono- and di-C9 aldehyde palmitoylglycerols were prepared from oleoyldipalmitoyl and oleoyllinoleoylpalmitoyl glycerols, respectively, while egg yolk PC and PE provided the mono-C5 and mono-C9 aldehydes of palmitoyl-and stearoyl glycerophospholipids. The aldehydes were isolated in the free form and as the dinitrophenylhydrazone (DNPH) derivatives by thin-layer chromatography (TLC). The intermediate ozonides, free aldehydes and hydrazones were identified by reversed phase high performance liquid chromatography (HPLC) with on-line negative ion thermospray and normal phase HPLC with on-line positive ion electrospray mass spectrometry (LC-MS). The synthetic aldehydes were used as carriers during isolation from natural sources and as reference compounds in quantitative analyses.

Lipid polymorphism of model and cellular membranes as revealed by electron microscopy

This review surveys the current state of knowledge relating to lipid polymorphism within both model lipid membrane and cellular membrane systems. Emphasis is placed upon the contribution of data obtained by transmission electron microscopy of freeze-fractured specimens. Some consideration is also given to the other important methods for the study of lipid polymorphism, namely X-ray diffraction and NMR spectroscopy. A detailed account of the different phases found in lipid mixtures within model membranes (bilayer, cubic or tetragonal, tubular or hexagonal) provides the background to the understanding of the factors involved in polymorphic phase transitions. The sequential steps involved in lipid polymorphism are defined from electron microscopical data and are related to the structural changes that can be detected within biological membranes. It is proposed that the fine structural changes detected at the initial stages of polymorphic transition in vivo may be highly relevant in relation to membrane fusion events, to the formation of tight junctions, and even to physiological transport processes. Since the later stages of polymorphic transition generally destroy the permeability barrier of model and cellular membranes, extensive rather than localized phase transition of the lipid bilayer is not at the moment considered to be compatible with cellular viability.

Ultrastructure of reconstituted rat liver microsomal membranes and cytochrome b5- or P-450-containing proteoliposomes

Thin sectioning and freeze-fracture electron microscopy have been used to show that it is possible to obtain topologically closed vesicles by means of reconstitution of rat liver microsomal membrane "ghosts." The reconstitution by 15 hr dialysis resulted in the formation of vesicles with intramembrane particles (IMP) while after 40 hr dialysis no IMP were observed in the membranes. The protein/lipid ratio and functional activity of NADPH- and NADH-linked enzyme systems were similar in both cases. Cytochrome P-450 (LM2) was incorporated into liposomes of different composition (protein: lipid ratio--1:200). IMP were observed only when the incorporation of cytochrome P-450 was performed in the presence of detergent Emulgen 913 as specific additive to the initial protein-lipid-sodium cholate mixture or in the course of incubation of proteoliposomal suspensions at 37 degrees C. After the incorporation of cytochrome b5 into azolectin liposomes vesicular membranes contain IMP if the incorporated membrane protein: lipid ratio is at least 1:50. Pronase-induced splitting off of a 11 kDa heme-containing fragment of cytochrome b5 did not affect IMP content. The conditions of IMP formation in reconstituted membranes and in microsomal ghosts are discussed.