Identification of nitrite treated tuna fish meat via the determination of nitrous oxide by head space-gas chromatography/mass spectrometry.

Tuna fish meat is an expensive and highly perishable sea food. Fresh meat has a bright red colour which soon turns into an unsightly brown during storage. To prolong the aspect of freshness, the red colour is stabilised or even enhanced e.g. with carbon monoxide or nitric oxide, the product of a nitrite / ascorbic acid treatment, which bind as a ligand to myoglobin. These procedures are illegal. Here we present a method for identifying tuna meat samples, which have undergone fraudulent wet salting with nitrite. The method uses headspace-gas chromatography/mass spectrometry (GC/MS) for the determination of nitrous oxide, which is formed as the final product of the two-step reduction nitrite (added agent) to nitric oxide (ligand) to nitrous oxide (target compound). Complex bound nitric oxide is set free with sulfuric acid, which also promotes the reduction to nitrous oxide. The method was validated using 15N labelled nitrite as well as treated and untreated reference fish samples. A survey of 13 samples taken from the Swiss market in 2019 showed that 45 % of all samples were illegally treated with nitrite.

Revisiting Hansen Solubility Parameters by Including Thermodynamics.

The Hansen solubility parameter approach is revisited by implementing the thermodynamics of dissolution and mixing. Hansen's pragmatic approach has earned its spurs in predicting solvents for polymer solutions, but for molecular solutes improvements are needed. By going into the details of entropy and enthalpy, several corrections are suggested that make the methodology thermodynamically sound without losing its ease of use. The most important corrections include accounting for the solvent molecules' size, the destruction of the solid's crystal structure, and the specificity of hydrogen-bonding interactions, as well as opportunities to predict the solubility at extrapolated temperatures. Testing the original and the improved methods on a large industrial dataset including solvent blends, fit qualities improved from 0.89 to 0.97 and the percentage of correct predictions rose from 54 % to 78 %. Full Matlab scripts are included in the Supporting Information, allowing readers to implement these improvements on their own datasets.

Determination of phosphine in plant materials: method optimization and validation in interlaboratory comparison tests.

The optimization and validation of a method for the determination of phosphine in plant materials are described. The method is based on headspace sampling over the sample heated in 5% sulfuric acid. Critical factors such as sample amount, equilibration conditions, method of quantitation, and matrix effects are discussed, and validation data are presented. Grinding of coarse samples does not lead to lower results and is a prerequisite for standard addition experiments, which present the most reliable approach for quantitation because of notable matrix effects. Two interlaboratory comparisons showed that results varied considerably and that an uncertainty of measurement of about 50% has to be assessed. Flame photometric and mass spectrometric detection gave similar results. The proposed method is well reproducible within one laboratory, and results from the authors' laboratories using different injection and detection techniques are very close to each other. The considerable variation in the interlaboratory comparison shows that this analysis is still challenging in practice and further proficiency testing is needed.

PBN derived amphiphilic spin-traps. II/Study of their antioxidant properties in biomimetic membranes.

The work reported herein deals with the evaluation of the antioxidant properties of bitailed amphiphilic α-phenyl-N-tert-butylnitrone derivatives (BPBNs) towards oxidation of an unsaturated lipid, the 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLoPC). Oxidation was induced either by UV light irradiation or radical initiators, i.e. the water soluble AAPH and the Fenton reaction, and the antioxidant evaluation was carried out using two biomimetic systems, namely Langmuir monolayers and large unilamellar vesicles. Measurement of the molecular area and the membrane fluidity of pure nitrone monolayers before and after UV-irradiation demonstrated the better stability and antioxidant properties of B17PBN, the derivative with two C17H35 alkyl chains, compared to its analogue B11PBN with two C11H23 alkyl chains. At only 5% molar ratio of nitrone in mixed DLoPC/nitrone monolayers, a complete inhibition of the molecular area decrease was observed for B17PBN whereas B11PBN showed lower protection. The oxidation of mixed DLoPC/nitrones large unilamellar vesicles in the presence of free radicals arising from AAPH decomposition or Fenton reaction was assessed by measuring lipid conjugated dienes and thiobarbituric acid reactive substances on the whole series of nitrone, i.e. C11-, C13-, C15- and C17-based compounds. Compared to the saturated 1,2-dimyristoyl-sn-glycero-3-phosphocholine, all bitailed amphiphilic nitrones were able to decrease conjugated dienes and TBARS in both oxidative paradigms, demonstrating therefore antioxidant property. The inhibition of phospholipids oxidation was increased when increasing the concentration of nitrone with the two B11PBN and B13PBN derivatives exhibiting higher potency. This study underlines the importance in the choice of a model membrane system when evaluating the potency of antioxidants against lipid oxidation.

A thermodynamic study of GPI-anchored and soluble form of alkaline phosphatase films at the air-water interface.

Glycosylphosphatidyl inositol (GPI) anchored proteins are localized and clustered on the outer layer of the plasma membranes forming microdomains. Among them, mammalian alkaline phosphatases (AP-GPI) are widely distributed enzymes. They can also exist as soluble proteins without anchor (APs). Using the Langmuir film technique, we study the thermodynamic properties of monolayers for both protein forms at the air-buffer interface. The enzymatic activity is maintained at the interface but the adsorption of the two forms of AP is very different. AP-GPI presents a higher surface activity and a larger molecular area than the soluble form. The molecular area deduced for high surface pressures suggests a different organization of the monolayers for these two forms. APs molecules seem to adsorb as a multilayer at the interface while AP-GPI appear to be orientated with the major axis parallel to the interface. This orientation allows the accessibility of AP-GPI enzymatic sites that are turned in direction of the subphase as in vivo where the active sites must be turned outside of the membrane.

Measurement of the anchorage force between GPI-anchored alkaline phosphatase and supported membranes by AFM force spectroscopy.

Mammalian alkaline phosphatases (AP) are glycosylphosphatidylinositol (GPI) anchored proteins that are localized on the outer layer of the plasma membrane. The GPI anchors are covalently attached to the C-termini of proteins and consist of a glycan chain bonded to phosphatidylinositol with two acyl chains anchored into the membrane bilayer. Force spectroscopy, based on atomic force microscope (AFM) technology, was used to determine the adhesion of alkaline phosphatase in the absence and presence of anchors. The GPI anchors increase markedly the adhesion frequency (i.e., the protein affinity for the membrane). An adhesion force of 350 +/- 200 pN is measured between GPI-anchored AP (AP(GPI)) and supported phospholipid bilayers of dipalmitoylphosphatidylcholine (DPPC) presenting structural defects (holes). In the absence of defects, the adhesion force (103 +/- 17 pN) and the adhesion frequency are reduced. These results indicate that AP(GPI) poorly spontaneously insert into membranes in vivo and open new perspectives for the characterization of the interactions between GPI proteins and membranes.

Adsorption of a phospholipid-hydroperoxide glutathione peroxidase into phospholipid monolayers at the air-water interface.

The interfacial behavior differences of two glutathione peroxidase isoforms have been investigated. The first isoform is the phospholipid-hydroperoxide glutathione peroxidase (EC 1.11.1.12) (GPx-4) isolated from rat testes and the second one is the cytosolic glutathione peroxidase (EC 1.11.1.9) (GPx-1) from bovine erythrocytes. Injected in the subphase buffer of a Langmuir trough, GPx-4 was able to adsorb quickly at the air-water interface whereas the GPx-1 was not. Then, the protein interaction with phospholipid monolayers was explored. Indeed, a monolayer of phospholipids containing a different number of polyunsaturated fatty acyl chains was prepared at the air-water interface. Under each kind of monolayer, the protein solution was injected and its adsorption was visualized by the measurement of successive pressure-area isotherms. We have, then, determined the molecular area increase due to the protein adsorption. It was found that the GPx-4 is adsorbed in each kind of monolayer tested whereas no molecular area increase was detected with the GPx-1. This indicates that the GPx-4 has a higher affinity for the interface, recovered or not by lipids, than the GPx-1. Moreover, the GPx-4 presents a different affinity for the phospholipid monolayers depending on the number of polyunsaturated fatty acyl chains.

Insertion of GPI-anchored alkaline phosphatase into supported membranes: a combined AFM and fluorescence microscopy study.

A new method based on combined atomic force microscopy (AFM) and fluorescence microscopy observations, is proposed to visualize the insertion of glycosylphosphatidyl inositol (GPI) anchored alkaline phosphatase from buffer solutions into supported phospholipid bilayers. The technique involves the use of 27 nm diameter fluorescent latex beads covalently coupled to the amine groups of proteins. Fluorescence microscopy allows the estimation of the relative protein coverage into the membrane and also introduces a height amplification for the detection of protein/bead complexes with the AFM. The coupling of the beads with the amine groups is not specific; this new and simple approach opens up new ways to investigate proteins into supported membrane systems.

Streptomyces chromofuscus phospholipase D interaction with lipidic activators at the air-water interface.

The phospholipase D from Streptomyces chromofuscus (PLDSc) is a soluble enzyme that interacts with membranes to catalyse phosphatidylcholine (PC) transformation. In this work, we focused on the interaction between PLDSc and two lipid activators: a neutral lipid, diacylglycerol (DAG), and an anionic one, phosphatidic acid (PA). DAG is a naturally occurring alcohol, so it is a potent nucleophile for the transphosphatidylation reaction catalysed by PLD. Concerning PA, it is a widely described activator of PLDSc-catalysed hydrolysis of PC. The monolayer technique allowed us to define PLDSc interaction with DAG and PA. In the case of DAG, the results suggest an insertion of PLDSc within the acyl chains of the lipid with an exclusion pressure of approximately 45 mN/m. PLDSc-DAG interaction seemed to occur preferentially with the lipid in the liquid-expanded (LE) phase. PLDSc interaction with PA was found to be more effective at high surface pressures. The overall results obtained with PA show a preferential interaction of the protein with condensed PA domains. No exclusion pressure could be found for PLDSc-PA interaction indicating only superficial interaction with the polar head of this lipid. Brewster angle microscopy (BAM) images were acquired in order to confirm these results and to visualise the patterns induced by PLDSc adsorption.

Transphosphatidylation activity of Streptomyces chromofuscus phospholipase D in biomimetic membranes.

The phospholipase D (PLD) from Streptomyces chromofuscus belongs to the superfamily of PLDs. All the enzymes included in this superfamily are able to catalyze both hydrolysis and transphosphatidylation activities. However, S. chromofuscus PLD is calcium dependent and is often described as an enzyme with weak transphosphatidylation activity. S. chromofuscus PLD-catalyzed hydrolysis of phospholipids in aqueous medium leads to the formation of phosphatidic acid. Previous studies have shown that phosphatidic acid-calcium complexes are activators for the hydrolysis activity of this bacterial PLD. In this work, we investigated the influence of diacylglycerols (naturally occurring alcohols) as candidates for the transphosphatidylation reaction. Our results indicate that the transphosphatidylation reaction may occur using diacylglycerols as a substrate and that the phosphatidylalcohol produced can be directly hydrolyzed by PLD. We also focused on the surface pressure dependency of PLD-catalyzed hydrolysis of phospholipids. These experiments provided new information about PLD activity at a water-lipid interface. Our findings showed that classical phospholipid hydrolysis is influenced by surface pressure. In contrast, phosphatidylalcohol hydrolysis was found to be independent of surface pressure. This latter result was thought to be related to headgroup hydrophobicity. This work also highlights the physiological significance of phosphatidylalcohol production for bacterial infection of eukaryotic cells.

Plasmalogens protect unsaturated lipids against UV-induced oxidation in monolayer.

Oxidative stress results from the attack by free radicals of several cellular targets (proteins, DNA and lipids). The cell equilibrium is a direct consequence of the pro-/antioxidant balance. In order to understand the physiological processes involved in oxidative stress, we followed oxidation of unsaturated lipids using a biomimetic system: Langmuir monolayers. The oxidation mode chosen was UV-irradiation and the lipid model was a polyunsaturated phospholipid: 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC). The monomolecular film technique was used to measure membrane rheology before and after UV-irradiation. We showed that the UV-irradiation of a DLPC monomolecular film led to a molecular area and surface elasticity modulus decrease that attests to the apparition of new molecular species at the air-water interface. The antioxidant effect of a synthetic plasmalogen (1-O-(1'-(Z)-hexadecenyl)-2-O-oleoyl-sn-glycero-3-phosphocholine or P(PLM)OPE) was tested on the oxidation of DLPC. Indeed, for about 25% mol P(PLM)OPE in mixed DLPC/P(PLM)OPE monolayers, a complete inhibition of the molecular area and the surface elasticity modulus decreases was observed in our experimental conditions. Lower P(PLM)OPE quantities delayed but did not prevent the DLPC oxidation in mixed monolayers.

Chick embryo anchored alkaline phosphatase and mineralization process in vitro.

Bone alkaline phosphatase with glycolipid anchor (GPI-bALP) from chick embryo femurs in a medium without exogenous inorganic phosphate, but containing calcium and GPI-bALP substrates, served as in vitro model of mineral formation. The mineralization process was initiated by the formation of inorganic phosphate, arising from the hydrolysis of a substrate by GPI-bALP. Several mineralization media containing different substrates were analysed after an incubation time ranging from 1.5 h to 144 h. The measurements of Ca/Pi ratio and infrared spectra permitted us to follow the presence of amorphous and noncrystalline structures, while the analysis of X-ray diffraction data allowed us to obtain the stoichiometry of crystals. The hydrolysis of phosphocreatine, glucose 1-phosphate, glucose 6-phosphate, glucose 1,6-bisphosphate by GPI-bALP produced hydroxyapatite in a manner similar to that of beta-glycerophosphate. Several distinct steps in the mineral formation were observed. Amorphous calcium phosphate was present at the onset of the mineral formation, then poorly formed hydroxyapatite crystalline structures were observed, followed by the presence of hydroxyapatite crystals after 6-12 h incubation time. However, the hydrolysis of either ATP or ADP, catalysed by GPI-bALP in calcium-containing medium, did not lead to the formation of any hydroxyapatite crystals, even after 144 h incubation time, when hydrolysis of both nucleotides was completed. In contrast, the hydrolysis of AMP by GPI-bALP led to the appearance of hydroxyapatite crystals after 12 h incubation time. The hydroxyapatite formation depends not only on the ability of GPI-bALP to hydrolyze the organic phosphate but also on the nature of substrates affecting the nucleation process or producing inhibitors of the mineralization.

Protein and lipid analysis of detergent-resistant membranes isolated from bovine kidney.

Detergent-resistant membranes (DRM) were prepared from bovine kidney cortex. The criterion used to test their purification was the increase in the activity of a GPI membrane-anchored protein, the alkaline phosphatase. Its association with specific proteins and lipids was tested. Two successive Triton X-100 treatments followed by purification on sucrose gradient at 4 degrees C were necessary to obtain DRM with a maximum of alkaline phosphatase activity and a typical protein pattern. A third Triton treatment did not alter this DRM composition. Among the enriched protein, we identified, by mass spectrometry, a microsomal dipeptidase, which was GPI membrane-anchored. Protein-kinase activities, mainly serine-kinase, were enriched during the DRM purification. Using the typical FTIR olefinic =C-H bands of the acyl chains, a global decrease in the unsaturation level of DRM lipids was observed as compared with total membranes. Three main phospholipids were identified in DRM. Their fatty acid compositions were determined by gas chromatography and compared with those of total membranes. The most enriched saturated fatty acid was palmitic acid (+44% for phosphatidylethanolamine, +52% for phosphatidylcholine and +49% for sphingomyelin), agreeing with a selection of specific phospholipids among the saturated ones during the DRM purification.

Cholesterol-dependent insertion of glycosylphosphatidylinositol-anchored enzyme.

Evidence is now accumulating that the plasma membrane is organized in different lipid and protein subdomains. Thus, glycosylphosphatidylinositol (GPI)-anchored proteins are proposed to be clustered in membrane microdomains enriched in cholesterol and sphingolipids, called rafts. By a detergent-mediated method, alkaline phosphatase, a GPI-anchored enzyme, was efficiently inserted into the membrane of sphingolipids- and cholesterol-rich liposomes as demonstrated by flotation in sucrose gradients. We have determined the enzyme extraluminal orientation. Using defined lipid components to assess the possible requirements for GPI-anchored protein insertion, we have demonstrated that insertion into membranes was cholesterol-dependent as the cholesterol addition increased the enzyme incorporation in simple phosphatidylcholine liposomes.

Spontaneous insertion and partitioning of alkaline phosphatase into model lipid rafts.

Several cell surface eukaryotic proteins have a glycosylphosphatidylinositol (GPI) modification at the C-terminal end that serves as an anchor to the plasma membrane and could be responsible for the presence of GPI proteins in rafts, a type of functionally important membrane microdomain enriched in sphingolipids and cholesterol. In order to understand better how GPI proteins partition into rafts, the insertion of the GPI-anchored alkaline phosphatase (AP) was studied in real-time using atomic force microscopy. Supported phospholipid bilayers made of a mixture of sphingomyelin-dioleoylphosphatidylcholine containing cholesterol (Chl+) or not (Chl-) were used to mimic the fluid-ordered lipid phase separation in biological membranes. Spontaneous insertion of AP through its GPI anchor was observed inside both Chl+ and Chl- lipid ordered domains, but AP insertion was markedly increased by the presence of cholesterol.

Behavior of a GPI-anchored protein in phospholipid monolayers at the air-water interface.

The interaction between alkaline phosphatase (AP), a glycosylphosphatidylinositol (GPI)-anchored protein (AP-GPI), and phospholipids was monitored using Langmuir isotherms and PM-IRRAS spectroscopy. AP-GPI was injected under C16 phospholipid monolayers with either a neutral polar head (1,2-dipalmitoyl-sn-glycero-3-phosphocholine monohydrate (DPPC)) or an anionic polar head (1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS)). The increase in molecular area due to the injection of protein depended on the surface pressure and the type of phospholipid. At all surface pressures, it was highest in the case of DPPS monolayers. The surface elasticity coefficient E, determined from the pi-A diagrams, allowed to deduct that the AP-GPI-phospholipid mixtures presented a molecular arrangement less condensed than the corresponding pure phospholipid films. PM-IRRAS spectra suggested different protein-lipid interactions as a function of the nature of the lipids. AP-GPI modified the organization of the DPPS deuterated chains whereas AP-GPI affected only the polar group of DPPC at low surface pressure (8 mN/m). Different protein hydration layers between the DPPC and DPPS monolayers were suggested to explain these results. PM-IRRAS spectra of AP-GPI in the presence of lipids showed a shape similar to those collected for pure AP-GPI, indicating a similar orientation of AP-GPI in the presence or absence of phospholipids, where the active sites of the enzyme are turned outside of the membrane.

An infrared study of the thermal and pH stabilities of the GPI-alkaline phosphatase from bovine intestine.

Alkaline phosphatase (EC 3.1.3.1) from bovine intestine mucosa (BIAP) is a homodimeric metalloenzyme, which hydrolyses nonspecifically phosphate monoesters at alkaline pH with release of inorganic phosphate and alcohol. BIAP is either soluble (sBIAP) or membrane-anchored by a glycosylphosphatidylinositol moiety (GPI-BIAP). This anchor might have some contribution in the stabilization of the GPI-linked protein structure. Our purpose was to study the role of the anchor by using two parameters, the enzymatic activity and the protein conformation, which was analyzed by using FTIR spectroscopy. We determined that the two forms of BIAP show some similarities with the previously described structure of alkaline phosphatase isolated from Escherichia coli and human placenta. Meanwhile GPI-BIAP and sBIAP exhibit similar specific activities, the presence of the anchor increases the thermal and pH stabilities of the enzyme activity and conformation.

Role of calcium and membrane organization on phospholipase D localization and activity. Competition between a soluble and insoluble substrate.

The phospholipase D (PLD) from Streptomyces chromofuscus is a soluble enzyme known to be activated by the phosphatidic acid-calcium complexes. PLD-catalyzed hydrolysis of phospholipids in aqueous medium leads to the formation of phosphatidic acid (PA). Previous studies concluded on an allosteric activation of PLD by the PA-calcium complexes. In this work, the role of PA and calcium was investigated in terms of membrane structure and dynamics. The role of calcium in PLD partitioning between the soluble phase and the water-lipid interface was tested. The monomolecular film technique was used to measure both membrane dynamics and PLD activity. These experiments provided information on PLD activity at a water-lipid interface. Moreover, the ability of PA to enhance PLD activity toward phosphatidylcholine was correlated to the physical properties of PA itself, affecting the rheology of the membrane. The effect of calcium was investigated on PLD binding to lipids and on the catalytic process by competition experiments between a soluble and a vesicular substrate. These experiments confirmed the absolute PLD requirement for calcium and pointed out the importance of calcium for PLD catalytic process and for the enzyme location at the water-lipid interface.