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Lipid Matters - Archive of Older Blogs - 2018

This Blog is an occasional series of notes on publications or other items dealing with lipid science that seem to be of particular interest to the editor Bill Christie. Inevitably, the selection is highly personal and subjective. In this web page, the blogs for 2018 are archived, while those for other years can be accessed from the foot of the current blog page.

February 28th, 2018

Scottish thistleWhen I do my weekly literature search, I have to scan lists of around 400 references to select those that are useful to me or should appear in my literature survey section, and then transfer them to my data base in an appropriate format. I can only spare about 20 minutes for the selection phase and I have to rely on the clarity of the titles, so inevitably I miss some important papers; also, the search algorithm I use is less than perfect. I am therefore grateful when correspondents point out items that I have missed. For example, two publications from 2016 dealing with novel methodology for the determination of trans fatty acids involving gas chromatography-vacuum ultraviolet spectroscopy have just been drawn to my attention, and they are listed in my current monthly analysis list (and will appear subsequently in that for 2016).

The Lipid Maps "Lipid Of the month" for February (see their home page) was another novelty for me, i.e. the elovanoids - C32 and C34 analogues of the protectins, which have been found in retinal cells. I presumably missed these because my weekly search did not contain suitable key words. Again, I have gone back to the Web of Science, to recover the relevant references which are now in my new monthly list for "Lipid essentials". What intrigued me especially, when I read the original papers was the fact that the protectins per se are derived from DHA in position sn-2 of phospholipids, while the precursors of the elovanoids are in position sn-1. This implies that the enzymes involved in the release of the two classes of fatty acid substrates are very different and so must be the stimulatory and regulatory mechanisms. There is a comparable difference for eicosanoid and anandamide biosynthesis; the arachidonic acid for the former comes from position sn-2 of phospholipids, while that for the latter comes from position sn-1.

This brings me back to an old theme of mine that in the world of lipidomics positional distributions are not given sufficient weight in comparison to molecular species analyses. I am sure that positional distributions obtained by mass spectrometry offer nothing near the precision of the older methods using selective lipases, but I would like to see an experimental comparison.

February 21st, 2018

As I have mentioned from time to time in this blog, I am an armchair analyst these days so I am reluctant to endorse what seems novel methodology. I do read and I can comment, however, and others can correct me if they wish. For example, a major problem with HPLC of all polar lipids has always been adsorption on columns. It is usually necessary to add ionic species to the mobile phase to get sharp peaks, and these can cause detection problems or degrade stationary phases. I never attempted to analyse CoA esters in my research days, but I am aware of some of the technical difficulties and I found a new publication had some interesting ideas to eliminate adsorption effects (Abranko et al. Comprehensive quantitative analysis of fatty-acyl-Coenzyme A species in biological samples by ultra-high performance liquid chromatography-tandem mass spectrometry harmonizing hydrophilic interaction and reversed phase chromatography. J. Chromatogr. A, 1534, 111-122 (2018);   DOI); the paper is open access. The authors tested various ammonium salts in the mobile phase and found that ammonium bicarbonate (10 mM adjusted to pH 8.5) was much the best over the whole range of chain-lengths, aided appreciably by the incorporation of a 0.1% phosphoric acid wash step between injections. Of course, the nature of the stationary phase is also important, and I have complained in the blog from time to time of the use of the term 'HILIC chromatography', which tells us nothing about the mode of interaction with analytes, adsorption, ion-exchange, etc. In this work, a Waters BEH HILIC column was used in part; it took me an age to find what 'BEH' implied, although I am little further forward in my understanding.

An interesting article by a young scientist was published in the Guardian newspaper last week complaining about the inappropriate use of metrics to evaluate scientific research. During much of my career, the pressures were different and I did not have to pay too much attention to this aspect of publication; my usual philosophy in selecting a particular journal for a new publication was whether it would reach the intended audience. The important objective was to see that any new information I had uncovered was disseminated - not how. It is a different world now and I recognize that young scientists starting on their careers face real difficulties. Many of the best papers I read have multiple authors (often into double figures) because multiple technologies and expertise may be required to provide answers. At the start of their careers, scientists working on their own can only hope to make incremental progress in their chosen fields, and I would hope that this would be recognized by scientific administrators. In my later years of research, my pet hate was projected 'milestones' - if I knew where the results were going to lead 2-3 years ahead, the work was not worth doing.

What really worries me now having just reviewed the above comments, is that I am beginning to sound like my father (no disrespect intended) in his later years - "when I was a boy, etc., etc.!" It is bad enough that I now look like he did. I will have to try to resurrect the youthful inner me.

February 14th, 2018

In plants,glycerolipid biosynthesis in chloroplasts has long been termed the "prokaryotic pathway", and it produces galactosyldiacylglycerols and phosphatidylglycerol with C18 acids in position sn-1 and C16 acids in position sn-2. This pattern is seen in cyanobacteria and it has been assumed that the similarity was a consequence of endosymbiosis during evolution. A new study demonstrates that this is not true (Sato, N. and Awai, K. "Prokaryotic Pathway" is not prokaryotic: noncyanobacterial origin of the chloroplast lipid biosynthetic pathway revealed by comprehensive phylogenomic analysis. Genome Biol. Evolution, 9, 3162-3178 (2017);  DOI); the paper is open access. It is now evident that the two steps of acylation in cyanobacteria and chloroplasts utilize enzymes that have no phylogenetic relationship. The structural differences in the diacylglycerol moieties of galactolipids from various species of algae and higher plants originate in fact in compartmentalization of the biosynthetic pathways or precursors in cells, especially between the chloroplasts and endoplasmic reticulum, each compartment having its own distinctive enzymes with characteristic specificities (see my web page on galactosyldiacylglycerols for further discussion).

My prize for the most unusual new lipid that I have encountered this year so far goes to a novel arsenolipid. The unicellular marine alga Dunaliella tertiolecta contains phytyl 5‑dimethylarsinoyl-2-O-methyl-ribofuranoside as 35 to 65% of the total arsenolipids. Apart from its arsenic content, this lipid is unique in containing both an ether-linked phytyl group and a 2‑O‑methylriboside of a type normally found only in RNA. (Glabonjat, R.A. et al. A 2-O-methylriboside unknown outside the RNA world contains arsenic. Angew. Chem.-Int. Ed., 56, 11963–11965 (2017);  DOI.

Formula of phytyl 5-dimethylarsinoyl-2-O-methyl-ribofuranoside

There is an interesting article in Nature on the importance of scientific blogs. Regretfully, they missed this one!

February 7th, 2018

In my blog of two weeks ago, I discussed the therapeutic potential of nitro fatty acids and in particular how subcutaneous injection of nitro-oleic acid suppressed allergic contact dermatitis in mice. Alas, a new publication from the same research group reports that topical applications have the opposite effect (Mathers, A.R. et al. Topical electrophilic nitro-fatty acids potentiate cutaneous inflammation. Free Rad. Biol. Med., 115, 31-42 (2018);  DOI). The problem appears to lie in the finding that nitro-conjugated linoleate is formed naturally "in the skin microenvironment as products of cutaneous inflammatory responses and, in high local concentrations, may exacerbate inflammatory skin diseases".

I couldn't have put better the philosophy implied in the title to an editorial contribution to a Nature journal (Marx, V. Fats add structure, they signal, they interact. In the lab, lipids are tough to work with but worth the challenge. Nature Methods, 15, 35-38 (2018);  DOI).

I always enjoy reading the personal reminiscences of lipid scientists and that by Dennis E. Vance was no exception, and it has a title that recalls my previous paragraph (From masochistic enzymology to mechanistic physiology and disease. J. Biol. Chem., 292, 17169-17177 (2017);  DOI). However, it also served to remind me that a new edition of one of the most valued books in my personal library was now available, but with new editors (Ridgway, N.D. and McLeod, R.S. Biochemistry of Lipids, Lipoproteins and Membranes, 6th Edition. (Elsevier, Amsterdam) (2016) - see Science Direct) - and prompted recourse to Amazon. It covers much the same range of topics as in previous editions, but often with replacement authors, and I expect to make good use of it. As expected with modern textbooks it is very well produced from a technical standpoint. Comparison with the book by Gurr et al. (Lipids: Biochemistry, Biotechnology and Health, 6th Edition), which I reviewed some months ago is probably unfair, as they probably aim at different audiences. My superficial first impression is that the new book goes into the subjects in greater depth but the coverage is not so exhaustive. For example, it was of great help to me in updating my web page on proteolipids, but there is no mention of "endocannabinoids" or "lipid sulfates" in the index.

January 31st, 2018

Scottish thistleIt has been a puzzle to me how the endocannabinoids, anandamide and 2-arachidonoylglycerol, can have so many different functions in a given tissue while operating through a single G protein receptor. A new review illustrates how this happens using brain as the model (Busquets-Garcia, A. et al. CB1 receptor signaling in the brain: extracting specificity from ubiquity. Neuropsychopharmacology, 43, 4-20 (2018);  DOI). It seems that endocannabinoid receptors exert their wide variety of different cellular effects by specific interactions with many other G proteins and that this is dependent on such factors as cell type, subcellular location and cellular functional state. Within a given tissue, expression of a receptor can vary between different locations. For example, in brain some types of neuron contain very high levels of CB1 receptor protein, whereas others have much lower levels; there are even lower levels in some regions of the hypothalamus and astroglial cells. In turn, variation in the location of CB1-interacting proteins may have a role in the cell-specific modulation of endocannabinoid signalling. This paper is part of a special issues on "Cannabinoids and endocannabinoids" and the only one to be open access. However, most of the other contributions appear to be more suitable for specialists.

The number of different molecular species of lipids in a given cell is astonishing (at least 1,000). Yet it is surprising how often only a single molecular species of a given lipid is required for optimum activity in a specific function. Cardiolipin in heart muscle is a prime example. If it had say 10 different fatty acid components, as is usual in most glycerolipids, together with the four different positions for acylation, there could in theory be 104 different molecular species. Instead, as is well known, linoleic acid is by far the most abundant fatty acid (as much as 80%) and the tetralinoleoyl species amounts to at least half the total. Many have speculated as to why this should be so, but a new publication demonstrates that if linoleate is displaced by docosahexaenoate in mitochondrial cardiolipin in the rat then the enzyme activities of the respiratory complexes are greatly reduced, apparently by preventing the formation of phospholipid domains that regulate enzyme activity (Sullivan, E.M. et al. Docosahexaenoic acid lowers cardiac mitochondrial enzyme activity by replacing linoleic acid in the phospholipidome. J. Biol. Chem., 293, 466-483 (2013);  DOI). I wonder what would be the result if they tried to displace linoleate with palmitate as this is the most abundant fatty acid in testis cardiolipin and has a much less mobile conformation than DHA.

January 24th, 2018

Nitro fatty acids are fascinating 21st Century molecules with important anti-inflammatory properties (they were first recognized as natural lipid components in 1999). In tissues, they occur in the free form, bound reversibly to thiol-containing proteins and glutathione, and as esters in triacylglycerols and phospholipids. In human serum in addition to non-covalent binding with albumin, nitro-conjugated linoleic acid has been shown to form covalent adducts at Cys-34 (Michael reaction), suggesting that this may be a means of systemic distribution. The effective concentrations of nitro fatty acids in tissues are reduced by this means, but a mechanism for the reversal of the reaction has been revealed under conditions of oxidative stress in vitro in plants at least. Reactive oxygen and nitrogen species, as represented by hydrogen peroxide and peroxynitrite, respectively, have the ability to oxidize cysteine-adducted nitro fatty acids with the release of free nitroalkenes, which can presumably then exert their anti-inflammatory effects (Padilla, M.N. et al. In vitro nitro-fatty acid release from Cys-NO2-fatty acid adducts under nitro-oxidative conditions. Nitric Oxide, Biol. Chem., 68, 14-22 (2017); DOI). Incidentally, nitro fatty acids are present in olives and virgin olive oil at concentrations that may be significant biologically, and it has been argued that they could be one reason for the beneficial effects of the Mediterranean diet. Their clinical potential is under active investigation, for example to inhibit cutaneous inflammation (Mathers, A.R. et al. Electrophilic nitro-fatty acids suppress allergic contact dermatitis in mice. Allergy, 72, 656-664 (2017); DOI).

The concept of lipid rafts in membranes, i.e. laterally segregated domains usually enriched in sphingolipids and cholesterol that provide platforms for signalling proteins, is now well established in the scientific literature. The terms termed 'membrane rafts', 'nanodomains' and 'microdomains' tend to be used interchangeably, but a new review suggests that in plants at least, different types of domain exist with differing compositions and functions so more precise definitions are required (Ott, T. Membrane nanodomains and microdomains in plant-microbe interactions. Curr. Opinion Plant. Biol., 40, 82-88 (2017); DOI).

Two substantial review articles deal with the methodology of lipidomics (Rustam, Y.H. and Reid, G.E. Analytical challenges and recent advances in mass spectrometry based lipidomics. Anal. Chem., 90, 374-397 (2018); DOI. And - Hu, T. and Zhang, J.L. Mass-spectrometry-based lipidomics. J. Sep. Sci., 41, 351-372 (2018); DOI). The second of these is open access.

January 17th, 2018

In recent years, I have written to several journals, most recently two months ago, to point out major errors in the interpretation of mass spectra in published papers. On most occasions, the journal has replied promptly and has promised to publish a correction - I still have to see one. Usually, the problem lies in computerized identification of methyl esters of fatty acids, as computers don't recognize that they cannot distinguish between positional and geometrical isomers of monoenoic or dienoic fatty acids, but often the authors (and reviewers) have simply not done their homework. One example, was a report of iso-methyl branched fatty acids in higher plants on the basis of a large ion equivalent to m/z = M-43; it has been known since the 1950s that this is common to mass spectra of all straight-chain fatty acids as a result of a complex rearrangement involving expulsion of carbons 2 to 4.

Problems also arise with interpretation of mass spectra of dimethyloxazoline derivatives of fatty acids. The first authors described a simple rule for how to locate double bonds. However, lacking appropriate standards, they were not aware that the rule did not apply when the double bonds were close to either end of the molecule. In addition, 3-isomers isomerize to 2-isomers on derivatization. The answer is to compare with authentic spectra, such as those on this website. Unfortunately, in spite of my best efforts, there are many flawed publications out there.

The ISI Web of Science seems to be just catching up after the holiday, and my weekly literature seach has given me three special review volumes to digest. I will list selected individual papers in my next literature update, but many of you will wish to consult the original journals for the full list. The December issue of Current Opinion in Plant Biology (Volume 40, Pages 1-168 (2017)) is devoted to the topic of "Cell biology: Membrane dynamics - being at the right place at the right time" (edited by Eugenia Russinova and Karin Schumacher) and includes a number of reviews of interest to plant lipid biochemists. The open access journal Antioxidants (Volume 6, issue 4 (2017)) contains three review articles dealing with tocopherols, including their biosynthesis in plants and their metabolism in humans. The journal Prostaglandins & Other Lipid Mediators (Volume 133, November (2017)) is a special issue from the "6th European Workshop on Lipid Mediators" (edited by Bannenberg, G. et al.) with a number of review articles on polyunsaturated fatty acids and oxylipins (eicosanoids and docosanoids).

January 10th, 2018

Although the most important aspect of any new publication is its content, the manner of presentation can make a big difference to how well the message gets across. Text books have lead the way in this regard, but the ability to use colour in diagrams now afforded by many journals has been a considerable step forward, and as more journals go online only I am sure that this facility will be used increasingly. I am also rather envious of those authors who have access to design departments who produce figures and diagrams that are works of art. One new article that meets all these quality criteria deals with phosphoinositides (Choy, C.H. et al. Phosphoinositide diversity, distribution, and effector function: stepping out of the box. Bioessays, 39, 1700121 (2017); DOI). I found this review to be of considerable value in updating my web page here on the topic.

Some years ago, I was rather pleased with myself when my cholesterol level was measured and found to be in the bottom quartile for my age group, but a friend brought me down to earth by telling me that all this meant was that rather than having a heart attack I would probably die of cancer. The editors' selection (and therefore open access) in the latest issue of JBC explains how two isoforms of phospholipase A regulate the nature of the eicosanoids produced during a heart attack and thence the damage done. In non-failing human hearts, one isoform channels arachidonic acid into protective epoxyeicosatrienoic acids (EETs), whereas in failing hearts, activation of a second isoform channels arachidonic acid into toxic hydroxyeicosatetraenoic acids (HETEs) (Moon, S.H. et al. Heart failure-induced activation of phospholipase iPLA2γ generates hydroxyeicosatetraenoic acids opening the mitochondrial permeability transition pore. J. Biol. Chem., 293, 115-129 (2018); DOI). One way or another your lipids probably get you in the end!

January 3rd, 2018

As a New Year gift to lipid analysts, I draw your attention to a 72 page open access publication reviewing NMR spectroscopy (1H, 13C and 31P) of lipids (Alexandri, E. et al. High resolution NMR spectroscopy as a structural and analytical tool for unsaturated lipids in solution. Molecules, 22, 1663 (2017); DOI). It is a large file at 34Mb, so you may need a fast broadband connection.

The lipid A (endotoxin) component of bacterial lipopolysaccharides is a fascinating complex molecule that serves to protect the organism from attack from external agencies, including antibiotics, but is a major reason for the virulence of pathogenic bacteria. Many factors are involved, including the number and nature of the fatty acid constituents, but the general mechanism of the immune response is usually considered to be a binding to a large hydrophobic pocket in a receptor such as toll-like receptor 4 (TLR4) via the lipid chains, while the phosphate groups can interact directly with the receptor leading to formation of a heterodimer complex that is active in immune signalling. However, a new publication demonstrates that the TLR4 receptor does not recognize the endotoxin of a rather nasty pathogen, Francisella novicida, which is thus able to evade the host innate immune system. Instead, this stimulates the cyclooxygenase-2-dependent inflammatory pathway and is responsible for the lethality of such infections through overproduction of proinflammatory effectors such as prostaglandin E2 (Scott, A.J. et al. Host-based lipid inflammation drives pathogenesis in Francisella infection. PNAS, 114, 12596-12601 (2017); DOI).

The regulation of cholesterol levels in animal tissues is a complicated topic involving innumerable factors, and I struggle to come to grips with it. One novel feature that has just come to light is that the first 100 amino acid in a key enzyme in cholesterol biosynthesis, i.e. squalene monooxygenase, is a proteasomal degradation signal or 'degron'. This sequence attaches reversibly to the ER membrane, and in the presence of excessive cholesterol levels, it is ejected and unravels to expose a hydrophobic patch, which then says "eat me" (Chua, N.K. et al. A conserved degron containing an amphipathic helix regulates the cholesterol-mediated turnover of human squalene monooxygenase, a rate-limiting enzyme in cholesterol synthesis. J. Biol. Chem., 292, 19959-19973 (2017); DOI).

Blogs for the previous year (2017) can be located here..

Author: William W. Christie Updated: July 4th, 2018 Credits/disclaimer LipidWeb logo