Analysis of plant phosphatidylcholines by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Effects of fatty acyl chain length, double-bond number and matrix on phosphatidylcholine responses in matrix-assisted laser desorption/ionization on an Orbitrap mass spectrometer

RATIONALE

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is used for the fast qualitative and quantitative analysis of phosphatidylcholines (PC). Fatty acyl chain lengths and the number of double bonds (DB) affect relative responses of PC; hence the determination of correction factors of individual PC is important for the accurate quantitation. The signal intensity in MALDI-MS strongly depends on the matrix; therefore, the following matrices typically used in lipidomics are studied in the present work: 2,5-dihydroxybenzoic acid (DHB), 1,5-diaminonaphthalene (DAN) and 9-aminoacridine (9AA).

METHODS

Series of PC with various fatty acyl chain lengths are synthesized for this study. PC concentrations over two orders of magnitude are studied with MALDI-MS. These experiments provide sets of calibration curves for each of the synthesized PC and the further analysis of parameters of calibration curves is performed.

RESULTS

Correction factors for PC decrease with increasing fatty acyl chain length for all matrices. These dependences are steeper for unsaturated PC than for saturated ones. MALDI matrices also have a significant effect on this dependence. The weakest dependence on fatty acyl chain length is found for saturated PC in 9AA. In the case of the other matrices, the effect of fatty acyl chain length on the response is essential for both saturated and unsaturated PC. Calibration curves and parameters of calibration curves for both saturated and monounsaturated PC are fitted by a linear function with regression coefficients decreasing in the order 9AA > DAN > DHB.

CONCLUSIONS

Differences in relative responses for PC in MALDI-MS measurements must be taken into account for accurate quantitation. Parameters of calibration curves can be used for the determination of PC concentrations using a single internal standard (IS). This method gives good results for the 9AA matrix, but the reproducibility of measurements for the DHB and DAN matrices is lower and the method can be used for a rough estimation only. These matrices are less convenient for the quantitation of PC.

Rapid and Reliable Identification of Phospholipids for Untargeted Metabolomics with LC-ESI-QTOF-MS/MS

Lipids are important components of biological systems, and their role can be currently investigated by the application of untargeted, holistic approaches such as metabolomics and lipidomics. Acquired data are analyzed to find significant signals responsible for the differentiation between the investigated conditions. Subsequently, identification has to be performed to bring biological meaning to the obtained results. Lipid identification seems to be relatively easy due to the known characteristic fragments; however, the large number of structural isomers and the formation of different adducts makes it challenging and at risk of misidentification. The inspection of data, acquired for plasma samples by a standard metabolic fingerprinting method, revealed multisignal formations for phosphatidylcholines, phosphatidylethanolamines, and sphingomyelins by the formation of ions such as [M + H](+), [M + Na](+), and [M + K](+) in positive ionization mode and [M - H](-), [M + HCOO](-), and [M + Cl](-) in negative mode. Moreover, sodium formate cluster formation was found for [M + H·HCOONa](+) and [H-H·HCOONa](-). The MS/MS spectrum obtained for each of the multi-ions revealed significant differences in the fragmentation, which were confirmed by the analysis of the samples in two independent research centers. After the inspection of an acquired spectra, a list of characteristic and diagnostic fragments was proposed that allowed for easy, quick, and robust lipid identification that provides information about the headgroup, formed adduct, and fatty acyl composition. This ensures successful identification, which is of great importance for the contextualization of data and results validation.

Lipidomics in tissues, cells and subcellular compartments

Mass spectrometry (MS) advances in recent years have revolutionized the biochemical analysis of lipids in plants, and made possible new theories about the structural diversity and functional complexity of lipids in plant cells. Approaches have been developed to profile the lipidome of plants with increasing chemical and spatial resolution. Here we highlight a variety of methods for lipidomics analysis at the tissue, cellular and subcellular levels. These procedures allow the simultaneous identification and quantification of hundreds of lipids species in tissue extracts by direct-infusion MS, localization of lipids in tissues and cells by laser desorption/ionization MS, and even profiling of lipids in individual subcellular compartments by direct-organelle MS. Applications of these approaches to achieve improved understanding of plant lipid metabolism, compartmentation and function are discussed.

Direct profiling and imaging of plant metabolites in intact tissues by using colloidal graphite-assisted laser desorption ionization mass spectrometry

Laser desorption/ionization (LDI)-based imaging mass spectrometry (MS) has been applied to several biological systems to obtain information about both the identities of the major chemical species and their localization. Colloidal graphite-assisted LDI (GALDI) MS imaging was introduced for the imaging of small molecules such as phospholipids, cerebrosides, oligosaccharides, flavonoids, and other secondary metabolites with high spatial homogeneity due to finely dispersed particles. Mass profiles and images of Arabidopsis thaliana have been recorded directly from various plant surfaces and cross sections. The main targeted metabolites were flavonoids and cuticular waxes, both of which are important in many aspects of functional genomics, proteomics, and metabolomics. The mass spectral profiles revealed tissue-specific accumulation of flavonoids in flowers and petals. In addition, many other location-specific ions were observed. The location and the degree of light-induced accumulation of flavonoids in stem sections were successfully probed by GALDI MS.

Desorption electrospray ionization (DESI) mass spectrometry and tandem mass spectrometry (MS/MS) of phospholipids and sphingolipids: ionization, adduct formation, and fragmentation

Desorption electrospray ionization (DESI) mass spectrometry was evaluated for the characterization of glycerophospholipid standards, including glycerophosphocholine (GPCho), glycerophosphoglycerol (GPGro), glycerophosphoethanolamine (GPEtn), glycerophosphoserine (GPSer), glycerophosphoinositol (GPIns), cardiolipin (CL), and sphingolipid standards, including sulfatides (ST) and sphingomyelin (SM). Of specific interest were the effects of surface and solvent composition on signal stability and intensity, along with the ions observed in the full scan mode and the fragmentations seen upon collisional activation for each of the above classes. These experiments were performed without the addition of matrix compounds to the sample and were conducted in the free ambient environment at atmospheric pressure. The compounds GPSer, GPGro, GPIns, ST, and CL were best analyzed in the negative ion mode while PE was ionized efficiently in both positive and negative ion modes. SM and GPCho, which typically generate more abundant ions in the positive ion mode, could be analyzed in the negative ion mode by the addition of anionic reagents such as acetate to the spray solvent. Full scan DESI mass spectra and tandem (MS/MS) spectra for this representative set of physiological phospho/sphingolipids are presented. Similarities with other ionization methods in terms of fragmentation behavior were strong, although ambient ionization of untreated samples is only available with DESI. The effect of surface and solvent properties on signal intensity and stability were determined by depositing standard compounds on several different surfaces and analyzing with various proportions of methanol in the aqueous spray. Analysis was extended to complex mixtures of phospholipids and sphingolipids by examining the total lipid extract of porcine brain and by direct analysis of rat brain cryotome sections. These types of mixture analyses and molecular imaging studies are likely to represent major areas of application of DESI.

MALDI seamless postsource decay fragment ion analysis of sodiated and lithiated phospholipids

In this paper, we present the results of a detailed study using MALDI seamless postsource decay (sPSD) fragment ion analysis of all major glycerophospholipid (GPL) classes, cardiolipin (bisphosphatidylglycerol), and sphingomyelin, respectively. The matrix compound 2,4,6-trihydroxyacetophenon recently introduced for lipid analysis (Stübiger, G.; Belgacem O. Anal. Chem. 2007, 79, 3206-3213) was applied in conjunction with a high-resolution curved field reflectron analyzer allowing detection of the fragment ions without stepping the reflectron voltages (seamless PSD). This instrumental feature helps to define in a fast way the polar headgroups of the different GPL classes and gives information about the constituent fatty acid residues dependent on the type of adduct ion used. Our experiments demonstrate that fragment ions specifying the fatty acid composition of the lipid molecules could only be generated from cationized molecular ions (sodiated or lithiated). Additionally, information about the stereospecificity of the fatty acid residues on the glycerol backbone (sn-1, and -2 position) of particular GPLs could be obtained during sPSD analysis. In the case of phosphatidylcholine, significant fragmentation related to the loss of the acyl groups could only be observed from [M + Li](+) ions. Generally, alkali adduction (sodium and lithium) enhanced fragmentation of most lipid classes, particularly favoring fragment ions associated with the polar headgroups.

Analysis of Lipids Using 2,4,6-Trihydroxyacetophenone as a Matrix for MALDI Mass Spectrometry

Lipids exhibit a broad range of chemical properties that make their analysis quite demanding. Today, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) represents a versatile tool in the field of lipid analysis, also offering the possibility for molecular structural identification using novel MALDI tandem time-of-flight (TOF/TOF) instrumentation. In this study, we evaluated 2,4,6-trihydroxyacetophenone (THAP) for the analysis of various lipid classes including neutral storage lipids (triacylglycerols), polar membrane lipids (glycerophospho- and sphingolipids), and glycosphingolipids. THAP proved to be a versatile matrix for the routine analysis of various lipids from biological samples ("lipidomics"). A sample preparation methodology was established using selective alkali salt doping for subsequent MS/MS experiments. Sodiated and lithiated molecules provided superior structural information on lipids (i.e., acyl group identification); thus, following this approach, both selective peak detection with high sensitivity and more reliable structural information were obtained simultaneously.

Isolation of esterified fatty acids bound to serum albumin purified from human plasma and characterised by MALDI mass spectrometry

Human serum albumin (HSA) is the most abundant protein in plasma. It is known to transport drugs as well as endogenous ligands, like free fatty acids (FFA). A mass spectrometry based method was applied to analyze the albumin bound lipid ligands. HSA was isolated from a human plasma pool by cold ethanol fractionation and ion exchange chromatography. HSA was defatted using a solvent extraction method to release the copurified lipids bound to the protein. The extracts were then analyzed by matrix-assisted laser desorption ionisation (MALDI) mass spectrometry (MS). Using this method, phospholipids and acylglycerols were detected. The phospholipids were identified to be lyso-phosphatidylcholine (lyso-PC) with distribution of different fatty acids (palmitic, stearic, oleic, and linoleic acids). An abundant species in the HSA lipid extract was found to be a diacylglycerol, composed of two linoleic and/or oleic acid chains. The identified motifs reflect structures that are known to be present in plasma. The binding of lysophospholipids has already been described but it is the first ever-reported evidence of native diacylglycerol ligands bound to HSA. Besides the native ligands from plasma a triacylglycerol was detected that has been added during the albumin preparation steps.

Metabolite profiling of plant carotenoids using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Although modern MS has facilitated the advent of metabolomics, some natural products such as carotenoids are not readily compatible to detection by MS. In the present article, we describe how matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI/TOF-MS) can be utilized to acquire mass spectra of carotenoids effectively. The procedure is sensitive (pmole range), reduces 'spot to spot' variation and provides high mass accuracy, thus aiding identification. The technique has been applied in vivo to the analysis of carotenoids in isolated plant cells and in vitro to three applications: (i) to show compatibility with purification methods such as LC, TLC and HPLC; (ii) for the rapid identification and quantification (by isotope dilution) of carotenoids present in crude extracts from plant tissues and whole cells; (iii) simultaneous semi-quantitative determination of carotenoids metabolites (m/z values) in crude plant extracts. Multivariate analysis of the recorded m/z values shows the effectiveness of the procedure in distinguishing genotypes from each other. In addition, the utility of the technique has been demonstrated on two mutant tomato populations, to determine alterations in carotenoid content, and a comparison made with traditional HPLC-photodiode array analysis. These data show that MALDI/TOF-MS can be used to rapidly profile, identify and quantify plant carotenoids reproducibly, as well as detecting other metabolites (m/z) in complex biological systems.

Identification of sulfoglycolipids from the alga Porphyridium purpureum by matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight mass spectrometry

Sulfoglycolipids, isolated from different phototrophic organisms, particularly plants and algae, have already been identified as bioactive compounds. In addition to their antiviral activity their influence on the immune response in mammalian cells is the focus of many studies. For the first time it has been possible to investigate purified sulfoquinovosyldiacylglycerols (SQDGs) from the microalga Porphyridium purpureum by matrix-assisted laser desorption/ionisation (MALDI) in the negative ion reflectron mode. Thereby, different solid and ionic liquid matrices have been tested to improve signal intensity during the laser ionisation. By using the MALDI Trap time-of-flight (ToF) multiple-stage (MS(n)) hybrid mass spectrometer the fatty acid compositions of the SQDGs were analysed by MS, and confirmed by MS(2) and MS(3) experiments. Thereby, hexadecanoic acid (C16:0), octadecadienoic acid (C18:2), eicosatetraenoic acid (C20:4), and eicosapentaenoic acid (C20:5) were detected in the purified fraction of SQDGs. The localisation of hexadecanoic acid (C16:0) at the sn-2 position, and unsaturated fatty acids at the sn-1 position of the SQDGs, determined by specific enzymatic hydrolysis, marks a procaryotic biosynthesis of SQDGs in the eucaryotic alga cells.

Lipid composition of membranes of Escherichia coli by liquid chromatography/tandem mass spectrometry using negative electrospray ionization

A liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method using reversed-phase chromatography was developed for the analysis of phospholipids from bacterial extracts of a wild-type strain of Escherichia coli. Product ion mass spectra from [M--H](-) precursor ions allowed an identification of individual phospholipid species that includes both fatty acid composition and fatty acyl location on the glycerol backbone using diagnostic product ions. Thus, complete assignment, including sn-1/sn-2 fatty acyl position, was achieved for this strain of E. coli. In addition, the phospholipids were quantified relative to one another using an internal standard method.

Ionic liquid matrix-induced metastable decay of peptides and oligonucleotides and stabilization of phospholipids in MALDI FTMS analyses

Room-temperature ionic liquid matrices (ILMs) have recently been investigated for use in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) and proven to be advantageous. Literature accounts of ILM performance for biological samples document increased sensitivity and ionization efficiency. These claims have been investigated here, and are supported for MALDI TOF applications to peptides, oligonucleotides, and phospholipids. Peptides and oligonucleotides however, do not behave in the same way when ILMs are used for MALDI FTMS. As reported here, with 3 tesla MALDI FTMS peptides and oligonucleotides fragment readily. These observations contrast with those found for MALDI time-of-flight mass spectrometry. Fragmentation is apparently slower than the time required to accelerate ions in a MALDI TOF mass spectrometer, but is readily observed by MALDI FTMS. Therefore, fragmentation of these molecules must occur on a relatively slow time scale. As trapping time is extended, increased fragmentation of peptides and oligonucleotides is seen. However, phospholipids do not fragment extensively. Furthermore, use of traditional solid matrices causes significant fragmentation for this category of compound but is suppressed by use of ILMs.

A comprehensive and comparative analysis for MALDI FTMS lipid and phospholipid profiles from biological samples

Described here is a computationally automated method for translating complex accurate mass spectra into biologically relevant and meaningful data. Rapid profiling of detailed high resolution mass spectra resulting from direct analysis of whole cells and tissues by matrix-assisted laser desorption/ionization (MALDI) Fourier transform mass spectrometry (FTMS) is discussed. Lipid and phospholipid ions create complex spectra containing multiple m/z values corresponding to the same fundamental chemical species. A computational approach is employed to sort ions, with mass to charge ratios lower than m/z 1000, into groups of similar lipid and phospholipid compositions for comprehensive and rapid analysis. By sorting or binning ions in this manner, variations in the degree of cation exchange can be avoided, thus increasing the comparability of the data. The result is displayed as a histogram that is easily interpretable and comparable with similar analyses and is particularly useful for direct comparison of similar tissues. Spectra of leaves from a healthy Prunus persica (peach) tree are compared with those from leaves infected by the fungus Taphrina deformans. Although the infection can be seen as a difference in leaf structure and by visual inspection of the mass spectra, the method described here details the chemical difference in phospholipid compositions and their relative abundances.

Radiation-induced free-radical transformation of phospholipids: MALDI-TOF MS study

Under the action of free-radical reaction initiators on membrane phospholipids, complex processes are taking place in both hydrophobic and hydrophilic parts of the phospholipids. Realization of these processes results in a mixture consisting of the initial lipids and their peroxidation and fragmentation products. Identification of compounds in such mixtures requires analytical methods of high sensitivity, reproducibility and accuracy to be applied. These properties are characteristic of the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) method. In the studies of radiation-induced free-radical transformations of phosphatidylglycerol, the MALDI-TOF MS in combination with thin layer chromatography (TLC) has been shown to be able to detect and identify products of free-radical transformations taking place in both hydrophilic and hydrophobic parts of the phospholipid. Thus, the MALDI-TOF MS can serve as a suitable analytical tool to investigate free-radical transformations of lipids.

Strategies and data analysis techniques for lipid and phospholipid chemistry elucidation by intact cell MALDI-FTMS

Ions attributed to lipids and phospholipids are directly observed by desorption from whole bacteria using intact cell (IC) matrix-assisted laser desorption-ionization (MALDI) Fourier transform mass spectrometry (FTMS). Saccharomyces cerevisiae are grown in rich media broth, concentrated, and applied directly to the MALDI surface without lysis or chemical treatment. FTMS of MALDI ions gives excellent signal to noise ratios with typical resolving powers of 90,000 and mass precision better than 0.002 Da. Use of accurate mass measurements and a simple set of rules allow assignment of major peaks into one of twelve expected lipid classes. Subsequently, fractional mass versus whole number mass plots are employed to enhance visual interpretation of the high-resolution data and to facilitate detection of related ions such as those representing homologous series or different degrees of unsaturation. This approach, coupled with rules based on bacterial biochemistry, is used to classify ions with m/z up to about 1000. Major spectral peaks in the range m/z 200-1000 are assigned as lipids and phospholipids. In this study, it is assumed that biologically-derived ions with m/z values lower than 1000 are lipids. This is not unreasonable in view of the facts that molecular weights of lipids are almost always less than 1000 Da, that the copy numbers for lipids in a cell are higher than those for any single protein or other component, and that lipids are generally collections of distinct homologous partners, unlike proteins or other cell components. This paper presents a new rapid lipid-profiling method based on IC MALDI-FTMS.

Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry in lipid and phospholipid research

The interest in the analysis of lipids and phospholipids is continuously increasing due to the importance of these molecules in biochemistry (e.g. in the context of biomembranes and lipid second messengers) as well as in industry. Unfortunately, commonly used methods of lipid analysis are often time-consuming and tedious because they include previous separation and/or derivatization steps. With the development of "soft-ionization techniques" like electrospray ionization (ESI) or matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF), mass spectrometry became also applicable to lipid analysis. The aim of this review is to summarize so far available experiences in MALDI-TOF mass spectrometric analysis of lipids. It will be shown that MALDI-TOF MS can be applied to all known lipid classes and the characteristics of individual lipids will be discussed. Additionally, some selected applications in medicine and biology, e.g. mixture analysis, cell and tissue analysis and the determination of enzyme activities will be described. Advantages and disadvantages of MALDI-TOF MS in comparison to other established lipid analysis methods will be also discussed.

Electrospray mass spectrometry of phospholipids

Phospholipids play a central role in the biochemistry of all living cells. These molecules constitute the lipid bilayer defining the outer confines of a cell, but also serve as the structural entities which confine subcellular components. Mass spectrometry has emerged as a powerful tool useful for the qualitative and quantitative analysis of complex phospholipids, including glycerophospholipids and the sphingolipid, sphingomyelin. Collision induced decomposition of both positive and negative molecular ion species yield rich information as to the polar head group of the phospholipid and the fatty-acyl substituents esterified to the glycerophospholipid backbone. This review presents the current level of understanding of the mechanisms involved in the formation of various product ions following collisional activation of molecular ion species generated by electrospray ionization of the common glycerophospholipids, including phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol, phosphatidylserine, cardiolipin, and sphingomyelin. Recent advances in the application of matrix assisted laser desorption ionization is also considered. Several applications of mass spectrometry applied to phospholipid analysis are presented as they apply to physiology as well as pathophysiology.

Structural analysis of phosphatidylcholines by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The utility of post-source decay (PSD) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was investigated for the structural analysis of phosphatidylcholine (PC). PC did not produce detectable negative molecular ion from MALDI, but positive ions were observed as both [PC+H](+) and [PC+Na](+). The PSD spectra of the protonated PC species contained only one fragment corresponding to the head group (m/z 184), while the sodiated precursors produced many fragment ions, including those derived from the loss of fatty acids. The loss of fatty acid from the C-1 position (sn-1) of the glycerol backbone was favored over the loss of fatty acid from the C-2 position (sn-2). Ions emanating from the fragmentation of the head group (phosphocholine) included [PC+Na-59](+), [PC+Na-183](+) and [PC+Na-205](+), which corresponded to the loss of trimethylamine (TMA), non-sodiated choline phosphate and sodiated choline phosphate, respectively. Other fragments reflecting the structure of the head group were observed at m/z 183, 146 and 86. The difference in the fragmentation patterns for the PSD of [PC+Na](+) compared to [PC+H](+) is attributed to difference in the binding of Na(+) and H(+). While the proton binds to a negatively charged oxygen of the phosphate group, the sodium ion can be associated with several regions of the PC molecule. Hence, in the sodiated PC, intermolecular interaction of the negatively charged oxygen of the phosphate group, along with sodium association at multiple sites, can lead to a complex and characteristic ion fragmentation pattern. The preferential loss of sn-1 fatty acid group could be explained by the formation of an energetically favorable six-member ring intermediate, as apposed to the five-member ring intermediate formed prior to the loss of sn-2 fatty acid group.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of lipids: ionization and prompt fragmentation patterns

Ionization and prompt fragmentation patterns of triacylglycerols, phospholipids (PLs) and galactolipids were investigated using matrix-assisted laser desorption/ionization (MALDI). Positive ions of non-nitrogen-containing lipids appeared only in the sodiated form, while nitrogen-containing lipids were detected as both sodiated and protonated adducts. Lipids containing acidic hydroxyls were detected as multiple sodium adducts or deprotonated ions in the positive and negative modes, respectively, with the exception of phosphatidylcholines. The positive MALDI spectra of triacylglycerols contained prompt fragments equivalent to the loss of RCOO(-) from the neutral molecules. Prompt fragment ions [PL-polar head](+) were observed in the positive MALDI spectra of all phospholipids except phosphatidylcholines. The phosphatidylcholines produced only a minor positive fragment corresponding to the head group itself (m/z 184). Galactolipids did not undergo prompt fragmentation. Post-source decay (PSD) was used to examine the source of prompt fragments. PSD fragment patterns indicated that the lipid prompt fragment ions did not originate from the observed molecular ions (sodiated or protonated), and suggested that the prompt fragmentation followed the formation of highly unstable, probably protonated, precursor ions. Pathways leading to the formation of prompt fragment ions are proposed.

Oxidative metabolism and the physiological age of seed potatoes are affected by increased alpha-linolenate content

The effects of high alpha-linolenate content on lipid peroxidation, oxidative stress and loss of plant growth potential during ageing of potato (Solanum tuberosum L.) seed-tubers was examined. Endoplasmic reticulum (FAD3) and plastidal (FAD7) 18:2 fatty acid desaturases were upregulated in potato (cv. Desiree), resulting in a 2-fold average increase in mol percentage 18:3 in the total lipid fraction across all transgenic clones. In double-transformed (FAD3+7) tubers, high alpha-linolenate phenotype effected accelerated ageing, resulting in growth responses characteristic of older seed-tubers. Although respiration rates of wild-type (WT) and FAD3+7 tubers were equal at 7 months of storage, rates had increased by 23% and 50% in WT and FAD3+7 tubers, respectively, by 19 months of storage. Electrolyte leakage of tissue from 19-month-old FAD3+7 tubers was significantly greater than that from WT tubers of the same age, indicating that the high alpha-linolenate phenotype was detrimental to membrane integrity during long-term storage. On average, indices of lipid peroxidation (malondialdehyde, ethane, C-6 aldehydes) were higher in older FAD3+7 tubers, relative to WT tubers. Activities of glucose-6-phosphate dehydrogenase, peroxidase, glutathione reductase, ascorbate peroxidase and monodehydroascorbate reductase increased in tubers with advancing age and were higher, on average, in FAD3+7 tubers. Dehydroascorbate reductase activity decreased with age, with no difference between transgenic and WT lines. Collectively, these results indicate that FAD3+7 tubers underwent a higher degree of oxidative stress during ageing. The age-induced increase in respiration of FAD3+7 tubers was at least partly a response to fuel increased free radical scavenging through the ascorbate-glutathione antioxidant pathway. By affecting the susceptibility of lipids to peroxidation, the degree of fatty acid unsaturation influenced the development of oxidative stress and the overall rate at which growth potential was lost from seed-tubers during ageing. Thus, oxidative stress plays an integral role in modulating the ageing process to affect growth potential from potato seed-tubers.

Application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for monitoring the digestion of phosphatidylcholine by pancreatic phospholipase A(2)

Different methods were established for monitoring the phospholipase A(2)(PLA(2)) activity but all of them are rather cumbersome and time consuming. In this paper we have investigated the suitability of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the determination of the PLA(2) activity. Phosphatidylcholine (PC) was digested with pancreatic PLA(2) under different conditions, i.e., various Ca(2+), PC, and PLA(2) concentrations. The digestion products were analyzed by MALDI-TOF MS and the concentration of lysophosphatidylcholine (LPC)-generated upon PLA(2) digestion-was determined by the application of an internal standard (known concentration) and by a comparison of their signal-to-noise ratios. The results clearly demonstrate that the LPC concentration determined from the MALDI-TOF mass spectra correlates directly with the activity of the applied enzyme. Additionally, LPC concentration increased with an increase in Ca(2+), as well as in the PC concentration. A single MALDI-TOF mass spectrum provides immediate information on the digestion products as well as on the residual substrate without requirements for any previous derivatization. MALDI-TOF MS can be easily and simply applied for monitoring the PLA(2) activity and we assume that this method might also be useful for other types of phospholipases.

Monitoring neurotensin[8-13] degradation in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A method was developed to quantify neurotensin (NT) fragment [8-13] and a novel NT[8-13] derivative, KK1, in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The method allows for simultaneous quantification of the major NT[8-13] metabolite, NT[9-13] (according to molecular mass), and detection of the major KK1 metabolite, KK1M (according to molecular mass). The degradation rates of NT[8-13] and KK1 were calculated to be 24.1+/-1.0 and 193+/-8min in human serum and 5.90+/-0.22 and 153+/-4min in rat serum, respectively. The method utilizes a novel sample drying technique and spectrum acquisition protocol. In addition, an internal standard dissimilar in structure to the analytes was used. This method may be broadly applicable to the quantification of NT[8-13] and other peptide analogues of varying structure.

Lipid metabolism during aging of high-alpha-linolenate-phenotype potato tubers

Previous studies demonstrated that high levels of alpha-linolenate in cell membranes of potato tubers (achieved by overexpressing fatty acid desaturases) enhances lipid peroxidation, oxidative stress, and tuber metabolic rate, effectively accelerating the physiological age of tubers. This study details the changes in lipid molecular species of microsomal and mitochondrial membranes from wild-type (WT) and high-alpha-linolenate tubers during aging. The microsomal and mitochondrial polar lipids of high-alpha-linolenate tubers were dominated by 18:3/18:3 and 16:0/18:3 molecular species. Relative to WT tubers, high-alpha-linolenate tubers had a substantially higher 16:0/18:n to 18:n/18:n molecular species ratio in mitochondria and microsomes, potentially reflecting a compensatory response to maintain membrane biophysical properties in the face of increased unsaturation. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) accounted for 53 and 37% of polar lipids, respectively, in mitochondria from younger WT and high-alpha-linolenate tubers. The relative proportions of these phospholipids (PL) did not change during aging of WT tubers. In contrast, PE increased to dominate the PL pool of mitochondria during aging of high-alpha-linolenate tubers. While aging effected an increase in mitochondrial 18:3-bearing PCs and PEs in WT tubers, the concentration of 18:3-bearing PCs fell with a concomitant increase in 18:3-bearing PEs during aging of high-alpha-linolenate tubers. These age- and high-alpha-linolenate-induced changes had no effect on the respiration rate and functional integrity of isolated mitochondria. Differential increases in the respiration rates of WT and high-alpha-linolenate tubers during aging were therefore a consequence of unsaturation-dependent alterations in the microenvironments of cells. Microsomal 18:3-bearing PCs, PEs, digalactosyldiacylglycerols (DGDG), and monogalactosyldiacylglycerols all increased in WT tubers during aging. In contrast, a selective loss of 18:3-bearing PCs and DGDGs from microsomes of high-alpha-linolenate tubers likely reflects a greater susceptibility of membranes to peroxidative catabolism during aging. Aging resulted in an increase in sterol/PL ratio in microsomes from WT tubers, due primarily to a decline in PL. In high-alpha-linolenate tubers, the increase in sterol/PL ratio during aging was due to increases in Delta 5-avenasterol and stigmasterol, indicating membrane rigidification and likely contributing to increased membrane permeability. Age-induced changes in 18:3-bearing lipids in membranes of transformed tubers are discussed relative to the development of oxidative stress and accelerated aging.

Changes in lipid molecular species and sterols of microsomal membranes during aging of potato (Solanum tuberosum L.) seed-tubers

Changes in sterols and the molecular species composition of polar lipids from microsomal membranes were characterized as a prerequisite to determining how lipid chemistry affects membrane susceptibility to peroxidation during aging of potato tubers. Polar lipid content of the microsomal fraction fell 17% (protein basis) as tubers aged from 22 to 38 mon at 4 degrees C. In younger seed-tubers, PC concentration (protein basis) was the highest, followed by digalactosyldiacylglycerol (DGDG), PE, monogalactosyldiacylglycerol (MGDG), and PI. PC and PE increased 14 and 27%, respectively, whereas glycolipids fell 64 and PI 43% with advancing age. These changes resulted in PC and PE dominating the microsomal membrane lipids of 38-mon-old tubers. Nonpositional analysis of lipid molecular species across lipid pools showed an increase in 16:0/18:3, 18:3/18:3, and 18:2/18:3 (PC and PE only), and a decline in 18:2/18:2 and 16:0/18:2 (except for MGDG) with advancing tuber age. The increase in 18:3-bearing species effected a linear increase in double-bond index (DBI) of PC and PE during aging. The DBI of DGDG did not change with age; however, it fell 65% for MGDG, resulting in an overall decrease in average microsomal DBI. In addition, A5-avenasterol and stigmasterol concentrations increased 1.6- and 3.3-fold, respectively, effecting a significant increase in the sterol/phospholipid ratio with advancing tuber age. The increase in sterol/phospholipid ratio and the possibility that the increased unsaturation of microsomal membranes reflects a compensatory response to maintain optimal membrane function in light of the age-induced loss of galactolipid and PI are discussed.

CsCl as an auxiliary reagent for the analysis of phosphatidylcholine mixtures by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS)

Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) is mainly used for protein and peptide analysis. However, there is growing evidence that also phospholipids like phosphatidylcholines (PC) can be easily analyzed by MALDI-TOF MS. In MALDI-TOF methodology, the sample is cationized by the addition of inorganic ions. This process is strongly dependent on the corresponding ion concentration. In biological samples various cations are present (mainly H+, Na+ and K+) and, therefore, a mixture of different adducts is formed. Since phospholipids exhibit a wide distribution of different fatty acid residues a considerable peak overlap may occur. This is a major problem since the peak assignment in a mixture will be often unclear. In this paper we demonstrate that this problem can be easily overcome by mixing the analyte with caesium chloride (CsCl). This yields naturally non-occurring Cs+ adducts that are apparent due to the large shift of the molecular mass. The proposed method facilitates the clear assignment of most peaks. Besides that, we will show that CsCl can also be used for the determination of the relative fatty acid composition of a given PC sample. For this purpose naturally occurring mixtures of PCs as well as organic extracts of human lipoproteins-that are mainly composed of PC and sphingomyeline-are used.