There has been a recent explosion in research concerning novel bioactive sphingolipids (SPLs) such as ceramide (Cer), sphingosine (Sph), and sphingosine 1-phosphate (Sph-1P) and this has necessitated the development of accurate and user-friendly methodology for analyzing and quantitating the endogenous levels of these molecules. ESI/MS/MS methodology provides a universal tool used for detecting and monitoring changes in SPL levels and composition from biological materials. Simultaneous ESI/MS/MS analysis of sphingoid bases (SBs), sphingoid base 1-phosphates (SB-1Ps), ceramides (Cers), ceramide 1-phosphates (Cer-1P), glucosyl/galactosyl-ceramides (Glu-Cers), and sphingomyelins (SMs) is performed on a Thermo Fisher Scientific triple quadrupole mass spectrometer operating in a multiple reaction monitoring (MRM) positive ionization mode. Biological materials (cells, tissues, or physiological fluids) are fortified with internal standards (ISs), extracted into a one-phase neutral organic solvent system, and analyzed by a LC/MS/MS system. Qualitative analysis (identification) of SPLs is performed by a Parent Ion scan of a common fragment ion characteristic for a particular class of SPLs. Quantitative analysis is based on calibration curves generated by spiking an artificial matrix with known amounts of target analyte, synthetic standards, and an equal amount of IS. The calibration curves are constructed by plotting the peak area ratios of analyte to the respective IS against concentration, using a linear regression model. This robust analytical procedure can determine the composition of endogenous sphingolipids (ESPLs) in varied biological materials and achieve a detection limit of subpicomole level. This methodology constitutes a "Lipidomic" approach to study the SPLs metabolism, defining a function of distinct subspecies of individual bioactive SPL classes.

S phingolipid (SPL) metabolism ( Fig. 1) serves a key role in the complex mechanisms regulating cellular stress responses to environment. Several SPL metabolites, especially ceramide (Cer), sphingosine (Sph) and sphingosine1-phosphate (S1P) act as key bioactive molecules governing cell growth and programmed cell death (Fig. 2). Perturbations in sphingolipids of one type may enhance or interfere with the action of another. To monitor changes in SPL composition therefore, reliable analytical methods are necessary.Here we present the liquid chromatography tandem mass spectrometry (LC-MS/MS) approach for simultaneous qualitative and quantitative monitoring of SPL components (classes and molecular species) in biological material as an effective tool to study sphingolipid signaling events. The LC-MS/MS methodology is the only available technique that provides high specificity and sensitivity, along with a wealth of structural identification information.

There is an urgent need to identify cellular/molecular mechanisms responsible for severe COVID-19 progressing to mortality. We initially performed untargeted/targeted lipidomics and focused biochemistry on 127 plasma samples and found elevated metabolites associated with secreted phospholipase A2 (sPLA2) activity and mitochondrial dysfunction in severe COVID-19 patients.Deceased COVID-19 patients had higher levels of circulating, catalytically active sPLA2 Group IIA (sPLA2-IIA), with a median value 9.6-fold higher than mild patients and 5.0-fold higher than severe COVID-19 survivors. Elevated sPLA2-IIA levels paralleled several indices of COVID-19 disease severity (e.g., kidney dysfunction, hypoxia, multiple organ dysfunction). A decision tree generated by machine learning identified sPLA2-IIA levels as a central node in stratifying patients that succumbed to COVID-19. Random forest analysis and LASSO-based regression analysis additionally identified sPLA2-IIA and blood urea nitrogen (BUN) as the key variables among 80 clinical indices in predicting COVID-19 mortality. The combined PLA-BUN index performed significantly better than either alone. An independent cohort (n=154) confirmed higher plasma sPLA2-IIA levels in deceased patients vs. severe or mild COVID-19, with the PLA-BUN indexbased decision tree satisfactorily stratifying mild, severe, and deceased COVID-19 patients. With clinically tested inhibitors available, this study supports sPLA2-IIA as a therapeutic target to reduce COVID-19 mortality.

This study revealed a novel mechanism linking dietary fat composition to IR. Given the emerging roles for ER stress in non-alcoholic liver disease, we conclude that dietary fat composition rather than total amount may mediate hepatic pathology associated with obesity.

Background/aims Leukodystrophies due to abnormal production of myelin cause extensive morbidity in early life; their genetic background is still largely unknown. We aimed at reaching a molecular diagnosis in Ashkenazi-Jewish patients who suffered from developmental regression at 6–13 months, leukodystrophy and peripheral neuropathy. Methods Exome analysis, determination of alkaline ceramidase activity catalysing the conversion of C18:1-ceramide to sphingosine and D-ribo-C12-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) (NBD)-phytoceramide to NBD-C12-fatty acid using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and thin layer chromatography, respectively, and sphingolipid analysis in patients’ blood by LC-MS/MS. Results The patients were homozygous for p.E33G in the ACER3, which encodes a C18:1-alkaline ceramidase and C20:1-alkaline ceramidase. The mutation abolished ACER3 catalytic activity in the patients’ cells and failed to restore alkaline ceramidase activity in yeast mutant strain. The levels of ACER3 substrates, C18:1-ceramides and dihydroceramides and C20:1-ceramides and dihydroceramides and other long-chain ceramides and dihydroceramides were markedly increased in the patients’ plasma, along with that of complex sphingolipids, including monohexosylceramides and lactosylceramides. Conclusions Homozygosity for the p.E33G mutation in the ACER3 gene results in inactivation of ACER3, leading to the accumulation of various sphingolipids in blood and probably in brain, likely accounting for this new form of childhood leukodystrophy.

Sphingolipids constitute a dynamic metabolic network that interconnects several bioactive molecules, including ceramide (Cer), sphingosine (Sph), sphingosine 1-phosphate (S1P), and ceramide 1-phosphate (C1P). The interconversion of these metabolites is controlled by a cohort of at least 40 enzymes, many of which respond to endogenous or exogenous stimuli. Typical probing of the sphingolipid pathway relies on sphingolipid mass levels or determination of the activity of individual enzymes. Either approach is unable to provide a complete analysis of flux through sphingolipid metabolism, which, given the interconnectivity of the sphingolipid pathway, is critical information to identify nodes of regulation. Here, we present a onestep in situ assay which comprehensively probes the flux through de novo sphingolipid synthesis, post serine palmitoyltransferase (SPT), by monitoring the incorporation and metabolism of the 17 carbon dihydrosphingosine (d17dhSph, precursor) precursor with LC/MS. Pulse labeling and analysis of precursor metabolism identified sequential, well defined phases of sphingolipid synthesis, corresponding to the activity of different enzymes in the pathway, further confirmed by the use of specific inhibitors and modulators of sphingolipid metabolism. This work establishes precursor pulse labeling as a practical tool for comprehensively studying metabolic flux through de novo sphingolipid synthesis and complex sphingolipid generation.

Sphingolipids (SLs) such as ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) are important bioactive lipids, and they are involved in numerous cellular biologies, including cell proliferation, migration, inflammation, and apoptosis (1, 2). In cancer cells, Cer functions as a tumor-suppressor lipid, whereas S1P acts as a tumorpromoter lipid, and this has led to increased interest in modulating SL levels for cancer therapeutics (3-6). However, studies translating the effects on SL metabolism into signaling components have been hindered by the interconnected nature of the SL network. Indeed, SL metabolism represents a dynamic network in which a single stimulus can result in diverse responses due to the metabolic interconversion of the various bioactive SLs, leading to multiple potential biologies. For example, Cer, the central hub of SL metabolism, can be generated in response to chemotherapeutics, cytokines, growth factors, and UV radiation, and once generated, Cer then functions to enhance/ mediate apoptosis, senescence, and/or cell-cycle arrest (7). However, the catabolism of Cer leads to Sph, which is subsequently phosphorylated to S1P, a potent prosurvival signal. Cer can also be phosphorylated to ceramide-1-phosphate, which has been associated with breast cancer invasiveness (8). Similarly, the activation of glucosylceramide synthase (GCS) can neutralize Cer accumulation and reduce cell death (9). Moreover, one stimulus can result in the Abstract Sphingolipids (SLs) have been implicated in numerous important cellular biologies; however, their study has been hindered by the complexities of SL metabolism. Furthermore, enzymes of SL metabolism represent a dynamic and interconnected network in which one metabolite can be transformed into other bioactive SLs through further metabolism, resulting in diverse cellular responses. Here we explore the effects of both lethal and sublethal doses of doxorubicin (Dox) in MCF-7 cells. The two concentrations of Dox resulted in the regulation of SLs, including accumulations in sphingosine, sphingosine-1-phosphate, dihydroceramide, and ceramide, as well as reduced levels of hexosylceramide. To further define the effects of Dox on SLs, metabolic flux experiments utilizing a d17 dihydrosphingosine probe were conducted. Results indicated the regulation of ceramidases and sphingomyelin synthase components specifically in response to the cytostatic dose. The results also unexpectedly demonstrated dose-dependent inhibition of dihydroceramide desaturase and glucosylceramide synthase in response to Dox. Taken together, this study uncovers novel targets in the SL network for the action of Dox, and the results reveal the significant complexity of SL response to even a single agent. This approach helps to define the role of specific SL enzymes, their metabolic products, and the resulting biologies in response to chemotherapeutics and other stimuli.-Snider,

Serine palmitoyltransferase (SPT) catalyzes the rate-limiting step of condensation of L-serine and palmitoyl-CoA to form 3-ketodihydrosphingosine (3KDS). Here, we report a HPLC-ESI-MS/MS method to directly quantify 3KDS generated by SPT. With this technique, we were able to detect 3KDS at a level comparable to that of dihydrosphingosine in yeast An in vitro SPT assay measuring the incorporation of deuterated serine into deuterated 3KDS was developed. The results show that SPT kinetics in response to palmitoyl-CoA fit into an allosteric sigmoidal model, suggesting the existence of more than one palmitoyl-CoA binding site on yeast SPT and positive cooperativity between them. Myriocin inhibition of yeast SPT activity was also investigated and we report here, for the first time, an estimated myriocin for yeast SPT of approximately 10 nM. Lastly, we investigated the fate of serine α-proton during SPT reaction. We provide additional evidence to support the proposed mechanism of SPT catalytic activity in regard to proton exchange between the intermediate NH base formed on the active Lys residue with surrounding water. These findings establish the current method as a powerful tool with significant resolution and quantitative power to study SPT activity.