Evaluation of the photooxidation of galactosyl- and lactosylceramide by electrospray ionization mass spectrometry

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Analysis of oxidized glucosylceramide and its effects on altering gene expressions of inflammation induced by LPS in intestinal tract cell models

Glucosylceramide (GlcCer) belongs to sphingolipids and is found naturally in plant foods and other sources that humans consume daily. Our previous studies demonstrated that GlcCer prevents inflammatory bowel disease both in vitro and in vivo, whose patients are increasing alarmingly. Although some lipids are vulnerable to oxidation which changes their structure and activities, it is unknown whether oxidative modification of GlcCer affects its activity. In this research, we oxidized GlcCer in the presence of a photosensitizer, analyzed the oxide by mass spectrometric techniques, and examined its anti-inflammatory activity in lipopolysaccharide (LPS)-treated differentiated Caco-2 cells as in vitro model of intestinal inflammation. The results showed that GlcCer is indeed oxidized, producing GlcCer hydroperoxide (GlcCerOOH) as a primary oxidation product. We also found that oxidized GlcCer preserves beneficial functions of GlcCer, suppressing inflammatory-related gene expressions. These findings suggested that GlcCerOOH may perform as an LPS recognition antagonist to discourage inflammation rather than induce inflammation.

In vitro generation of oxidized standards for lipidomics. Application to major membrane lipid components

Membrane lipids (sphingolipids, glycerophospholipids, cardiolipins, and cholesteryl esters) are critical in cellular functions. Alterations in the levels of oxidized counterparts of some of these lipids have been linked to the onset and development of many pathologies. Unfortunately, the scarce commercial availability of chemically defined oxidized lipids is a limitation for accurate quantitative analysis, characterization of oxidized composition, or testing their biological effects in lipidomic studies. To address this dearth of standards, several approaches rely on in-house prepared mixtures of oxidized species generated under in vitro conditions from different sources - non-oxidized commercial standards, liposomes, micelles, cells, yeasts, and human preparations - and using different oxidant systems - UVA radiation, air exposure, enzymatic or chemical oxidant systems, among others. Moreover, high-throughput analytical techniques such as liquid chromatography coupled to mass spectrometry (LC-MS) have provided evidence of their capabilities to study oxidized lipids both in in vitro models and complex biological samples. In this review, we describe the commercial resources currently available, the in vitro strategies carried out for obtaining oxidized lipids as standards for LC-MS analysis, and their applications in lipidomics studies, specifically for lipids found in cell and mitochondria membranes.

Structural Analysis of Oxidized Cerebrosides from the Extract of Deep-Sea Sponge Aulosaccus sp.: Occurrence of Amide-Linked Allylically Oxygenated Fatty Acids

The structural elucidation of primary and secondary peroxidation products, formed from complex lipids, is a challenge in lipid analysis. In the present study, rare minor oxidized cerebrosides, isolated from the extract of a far eastern deep-sea glass sponge, Aulosaccus sp., were analyzed as constituents of a multi-component RP-HPLC (high-performance liquid chromatography on reversed-phase column) fraction using NMR (nuclear magnetic resonance) spectroscopy, mass spectrometry, GC (gas chromatography), and chemical transformations (including hydrogenation or derivatization with dimethyl disulfide before hydrolysis). Eighteen previously unknown β-D-glucopyranosyl-(1→1)-ceramides (1a-a//, 1b-b//, 2a-a//, 2b-b//, 3c-c//, 3d-d//) were shown to contain phytosphingosine-type backbones (2S,3S,4R,11Z)-2-aminoeicos-11-ene-1,3,4-triol (in 1), (2S,3S,4R,13Z)-2-aminoeicos-13-ene-1,3,4-triol (in 2), and (13S*,14R*)-2-amino-13,14-methylene-eicosane-1,3,4-triol (in 3). These backbones were N-acylated with straight-chain monoenoic (2R)-2-hydroxy acids that had allylic hydroperoxy/hydroxy/keto groups on C-17/ in the 15/E-23:1 chain (a-a//), C-16/ in the 17/E-23:1 (b-b//) and 14/E-22:1 (c-c//) chains, and C-15/ in the 16/E-22:1 chain (d-d//). Utilizing complementary instrumental and chemical methods allowed for the first detailed structural analysis of a complex mixture of glycosphingolipids, containing allylically oxygenated monoenoic acyl chains.

Qualitative and Quantitative Study of Glycosphingolipids in Human Milk and Bovine Milk Using High Performance Liquid Chromatography-Data-Dependent Acquisition-Mass Spectrometry

Cerebrosides (Crb; including glucosylceramide and galactosylceramide) and lactosylceramide (LacCer) are structurally complex lipids found in many eukaryotic cell membranes, where they play important roles in cell growth, apoptosis, cell recognition and signaling. They are also found in mammalian milk as part of the milk fat globule membrane (MFGM), making milk an important dietary component for the rapidly growing infant. This study reports the development of a robust analytical method for the identification and characterization of 44 Crb and 23 LacCer molecular species in milk, using high performance liquid chromatography-tandem mass spectrometry in data-dependent acquisition mode. For the first time, it also compares the distributions of these species in human and bovine milks, a commercial MFGM-enriched dairy ingredient (MFGM Lipid 100) and commercial standards purified from bovine milk. A method for quantifying Crb and LacCer in milk using mass spectrometry in neutral loss scan mode was developed and validated for human milk, bovine milk and MFGM Lipid 100. Human milk was found to contain approximately 9.9-17.4 µg Crb/mL and 1.3-3.0 µg LacCer/mL, whereas bovine milk (pooled milk from a Friesian herd) contained 9.8-12.0 and 14.3-16.2 µg/mL of these lipids, respectively. The process used to produce MFGM Lipid 100 was shown to have enriched these components to 448 and 1036 µg/g, respectively. No significant changes in the concentrations of both Crb and LacCer were observed during lactation.

New Insights on Non-Enzymatic Oxidation of Ganglioside GM1 Using Mass Spectrometry

Gangliosides are acidic glycosphingolipids that are present in cell membranes and lipid raft domains, being particularly abundant in central nervous systems. They participate in modulating cell membrane properties, cell-cell recognition, cell regulation, and signaling. Disturbance in ganglioside metabolism has been correlated with the development of diseases, such as neurodegenerative diseases, and in inflammation. Both conditions are associated with an increased production of reactive oxidation species (ROS) that can induce changes in the structure of biomolecules, including lipids, leading to the loss or modification of their function. Oxidized phospholipids are usually involved in chronic diseases and inflammation. However, knowledge regarding oxidation of gangliosides is scarce. In order to evaluate the effect of ROS in gangliosides, an in vitro biomimetic model system was used to study the susceptibility of GM1 (Neu5Acα2-3(Galβ1-3GalNAcβ1-4)Galβ1-4Glcβ1Cer) to undergo oxidative modifications. Oxidation of GM1 under Fenton reaction conditions was monitored using high resolution electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). Upon oxidation, GM1 underwent oxidative cleavages in the carbohydrate chain, leading to the formation of other gangliosides GM2 (GalNAcβ1-4Gal(Neu5Acα2-3)1-4Glcβ1Cer), GM3 (Neu5Acα2-3Galβ1-4Glcβ1Cer), asialo-GM1 (Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1Cer), asialo-GM2 (GalNAcβ1-4Galβ1-4Glcβ1Cer), of the small glycolipids lactosylceramide (LacCer), glucosylceramide (GlcCer), and of ceramide (Cer). In addition, oxygenated GM1 and GM2 (as keto and hydroxy derivatives), glycans, oxidized glycans, and oxidized ceramides were also identified. Nonenzymatic oxidation of GM1 under oxidative stress contributes to the generation of other gangliosides that may participate in the imbalance of gangliosides metabolism in vivo, through uncontrolled enzymatic pathways and, consequently, play some role in neurodegenerative processes. Graphical Abstract ᅟ.