Charge-dependent insertion of beta-lactoglobulin into monoglyceride monolayers

Effect of the formulation and structure of monoglyceride-based gels on the viability of probiotic Lactobacillus rhamnosus upon in vitro digestion

This research was conducted to evaluate the potential use of saturated monoglyceride (MG)-based gels in the protection of probiotics upon in vitro digestion. For this purpose, a Lactobacillus rhamnosus strain was inoculated into binary and ternary systems, containing MGs, a water phase composed of an aqueous solution at controlled pH or UHT skimmed milk, and in ternary gels, sunflower oil. Gel structure characterization was initially performed just after preparation and after 14 days of storage at 4 °C by rheological, mechanical, thermal, and microscopy analyses. Afterwards, probiotic viability upon in vitro digestion was evaluated. The results highlighted that all freshly prepared samples showed good capability to protect L. rhamnosus with the exception of the binary system containing milk. However, the digestion of samples after 14 days of storage showed that the ternary system containing skimmed milk exhibited the best protection performance ensuring a L. rhamnosus viability of almost 10⁶ CFU g^-1 at the end of the gastrointestinal passage. Confocal microscopy results demonstrated that bacterial cells were located prevalently within the aqueous domain near the monoglycerides and protein aggregates. Under these conditions, they can simultaneously achieve physical protection and find nutrients to survive environmental stresses. These findings suggest that MG-based gels can be proposed as efficient carriers of probiotic bacteria not only during food processing and storage but also upon digestion.

Improved Bioavailability of Encapsulated Bioactive Nutrients Delivered through Monoglyceride-Structured O/W Emulsions

Effects of monoglyceride (MG) on the properties of WPI-stabilized emulsions and the bioavailability of encapsulated β-carotene were investigated. MG-structured emulsions showed reduced surface charge, higher viscosity, and better creaming stability than an emulsion without MG. Exposure of emulsions to GIT digestion led to significant changes in droplet size and interfacial properties. In vitro bioavailability of β-carotene in 1% MG (63.9%) and 2% MG (77.1%) structured emulsions were higher than that in emulsion without MG (53.4%) (p < 0.05). All MG emulsions demonstrated a better cellular uptake of β-carotene by Caco-2 cells than the emulsion without MG (p < 0.05). A significant increase in the cellular uptake of β-carotene with increasing MG content was observed, increasing from 0.109 μg/well for the 0.5% MG emulsion and up to 0.138 μg/well for 2% MG emulsion. The findings in this study confirm the potential of the MG-structured emulsions as novel carriers for lipophilic nutrients with improved stability and bioavailability.

Effect of pectin, starch, and locust bean gum on the interfacial activity of monostearin and β-lactoglobulin

The behavior of some hydrocolloids widely used as stabilizers of low-oil-content water emulsions (starch, pectins, and a locust bean gum-pectin blend) at the air-water and model oil-water interface is analyzed. Their influence on the surface and interface activity of typical food emulsifiers, such as β-lactoglobulin and monostearin, is also considered. It is demonstrated that the greatest interfacial activity is provided by one of the commercial pectins studied. It is capable of modifying the characteristics of monostearin and β-lactoglobulin interfacial films in a different way depending on both the nature of the oil phase and the type of surfactant used.

PRACTICAL APPLICATION

This research may contribute not only to enhance the final-consumer life quality by optimizing low-oil-content food emulsion formulations which contain "natural" stabilizers, but also to increase the added value of by-products of some fruit juices as well as of sugar factories since pectin can be manufactured not only from citrus and apple peels but also from sugar beet pulps.

Conformational changes of beta-lactoglobulin induced by anionic phospholipid

Conformational changes of beta-lactoglobulin (beta-LG) induced by anionic phospholipid (dimyristoylphosphatidylglycerol, DMPG) at physiological conditions (pH 7.0) have been investigated by UV-VIS, circular dichroism (CD) and fluorescence spectra. The experimental results suggest that beta-LG-DMPG interactions cause beta-LG a structural reorganization of the secondary structure elements accompanied by an increase in alpha-helical content, and a loosening of the protein tertiary structure. The interaction forces between beta-LG and DMPG are further evaluated by fluorescence spectra. The fluorescence spectral data show that conformational changes in the protein are driven by electrostatic interaction at first, then by hydrophobic interaction between a protein with a negative net charge and a negatively charged phospholipid.

C11-BODIPY(581/591), an oxidation-sensitive fluorescent lipid peroxidation probe: (micro)spectroscopic characterization and validation of methodology

C11-BODIPY(581/591) is a fluorescent radio-probe for indexing lipid peroxidation and antioxidant efficacy in model membrane systems and living cells, with excellent characteristics: (i) emission in the visible range of the electromagnetic spectrum, with good spectral separation of the nonoxidized (595 nm) and oxidized (520 nm) forms; (ii) has a high quantum yield and because of this, low labeling concentrations can be used, ensuring minimal perturbation of the membrane whilst retaining favorable signal to noise ratios; (iii) has a good photo-stability and displays very few fluorescence artifacts; (iv) is virtually insensitive to environmental changes, i.e., pH or solvent polarity; (v) is lipophilic and as such easily enters membranes; (vi) once oxidized, C11-BODIPY(581/591) remains lipophilic and does not spontaneously leave the lipid bilayer; (vii) C11-BODIPY(581/591) localizes in two distinct pools within the lipid bilayer, a shallow pool at 18 A and a deep pool at < 7.5 A from the center of the bilayer; (viii) is not cytotoxic to rat-1 fibroblasts up to 50 microM; (ix) is sensitive to a variety of oxy-radicals and peroxynitrite, but not to superoxide, nitric oxide, transition metal ions, and hydroperoxides per se; (x) its sensitivity to oxidation is comparable to that of endogenous fatty acyl moieties.

The effect of protein stability on protein-monoglyceride interactions

We have previously shown that proteins such as beta-lactoglobulin and lysozyme insert into monoglyceride monolayers and are able to induce an L(beta) to coagel phase transition in monoglyceride bilayers. These studies gave a first indication that protein stability could be an important factor for these interactions. This study therefore aims at further investigating the potential role of protein stability on protein-monoglyceride interactions. To this end we studied the interaction of stable and destabilized alpha-lactalbumin with monostearoylglycerol. Our results show that protein stability is important for the insertion of proteins into a monostearoylglycerol monolayer, such that the lower the stability of the protein the better the protein inserts. In marked contrast to beta-lactoglobulin and lysozyme we found that destabilized alpha-lactalbumin does not induce the L(beta) to coagel phase transition in monoglyceride bilayers. We propose that this is due to an increased surface coverage by the protein which could result from the unfolding of the protein upon binding to the interface.

Limited conformational change of beta-lactoglobulin when adsorbed at the air-water interface

Detailed insight can be obtained from proteins at and near the air-water interface using external reflection IR and circular dichroism techniques. Besides information on local protein concentrations and surface layer thickness, it is shown that beta-lactoglobulin displays a limited unfolding at the interface. The conformational change is comparable to that observed upon heat-induced aggregation of the protein and can be understood in view of the high surface concentration of the protein (approximately 40% volume fraction). The layer thickness and the conformational properties of the protein do not depend on the bulk concentration. After adsorption of beta-lactoglobulin to a preformed lipid monomolecular layer a similar conformational change is induced, suggesting that the folding properties of the protein itself determine the extent of conformational changes at the interfaces.

Thermotropic phase behavior of monoglyceride-dicetylphosphate dispersions and interactions with proteins: a (2)H and (31)P NMR study

The phase behavior of a 1-[(2)H(35)]-stearoyl-rac-glycerol ([(2)H(35)]-MSG)/dicetylphosphate (DCP) mixture and its interaction with beta-lactoglobulin and lysozyme were studied by (2)H and (31)P nuclear magnetic resonance (NMR). The behavior of the lipids was monitored by using deuterium-labeled [(2)H(35)]-MSG as a selective probe for (2)H NMR and DCP for (31)P NMR. Both (2)H and (31)P NMR spectra exhibit characteristic features representative of different phases. In the lamellar phases, (31)P NMR spectra of DCP are different from the spectra of natural phospholipids, which is attributable to differences in the intramolecular motions and the orientation of the shielding tensor of DCP compared with phospholipids. The presence of the negatively charged amphiphile DCP has a large effect on the phase behavior of [(2)H(35)]-MSG. At low temperature, the presence of DCP inhibits crystallization of the gel phase into the coagel. Upon increasing the temperature, the gel phase of [(2)H(35)]-MSG transforms in the liquid-crystalline lamellar phase. In the presence of DCP, the gel phase directly transforms into an isotropic phase. The negatively charged beta-lactoglobulin and the positively charged lysozyme completely neutralize the destabilizing effect of DCP on the monoglyceride liquid-crystalline phase and they even stabilize this phase. Without DCP the proteins do not seem to interact with the monoglyceride. These results suggest that interaction is facilitated by electrostatic interactions between the negatively charged DCP and positively charged residues in the proteins. In addition, the nonbilayer-forming DCP creates insertion sites for proteins in the bilayer.

The specificity of monoglyceride-protein interactions and mechanism of the protein induced L(beta) to coagel phase transition

This study aims at gaining insight into the specificity and molecular mechanism of monoglyceride-protein interactions. We used beta-lactoglobulin (beta-LG) and lysozyme as model proteins and both monostearoylglycerol and monopalmitoylglycerol as defined gel phase monoglycerides. The monoglycerides were used in different combinations with the two negatively charged amphiphiles dicetylphosphate and distearylphosphate. The interactions were characterized using the monolayer technique, isothermal titration calorimetry, (2)H-nuclear magnetic resonance (NMR) using deuterium labelled monoglycerides and freeze fracture electron microscopy (EM). Our results show that lysozyme inserts efficiently into all monolayers tested, including pure monoglyceride layers. The insertion of beta-LG depends on the lipid composition of the monolayer and is promoted when the acylchains of the negatively charged amphiphile are shorter than that of the monoglyceride. The binding parameters found for the interaction of beta-LG and lysozyme with monoglyceride bilayers were generally similar. Moreover, in all cases a large exothermic binding enthalpy was observed which was found to depend on the nature of the monoglycerides but not of the proteins. (2)H-NMR and freeze fracture EM showed that this large enthalpy results from a protein mediated catalysis of the monoglyceride L(beta) to coagel phase transition. The mechanism of this phase transition consists of two steps, an initial protein mediated vesicle aggregation step which is followed by stacking and probably fusion of the bilayers.

Lipid organization and dynamics of the monostearoylglycerol-water system. A 2H NMR study

Deuterium labeled monostearoylglycerols with fully ([2H(35)]-MSG) and selectively ([11-(2)H(2)]-MSG) deuterated chains have been synthesized and used as a probe for 2H NMR. At low temperature monoglyceride-water systems form the coagel or crystalline phase, which transforms with increasing temperature subsequently into the gel, liquid crystalline and cubic phase. The 2H NMR spectra exhibit characteristic features representative of these phases. The gel phase is metastable and gradually transforms into the coagel at temperatures below 40 degrees C. The undercooled cubic phase transforms into the liquid crystalline phase during days. In the liquid crystalline phase, the chain order profile indicates an increase of the chain flexibility towards the methyl group. In the liquid crystalline phase, bilayers spontaneously align in a magnetic field with their normal perpendicular to the field. The results demonstrate that 2H NMR can serve as a convenient tool to study both structure and dynamics of different monoglyceride-water phases.

Interaction mode specific reorganization of gel phase monoglyceride bilayers by beta-lactoglobulin

The interaction between beta-lactoglobulin and sonicated aqueous dispersions of the gel phase forming monoglyceride monostearoylglycerol were studied using isothermal titration calorimetry, direct binding experiments, differential scanning calorimetry, leakage of a fluorescent dye and solid-state (31)P- and (2)H-NMR. In the absence of a charged amphiphile, monostearoylglycerol forms a precipitate. Under these conditions, no interaction with beta-lactoglobulin was observed. In the presence of the negatively charged amphiphile dicetylphosphate, the gel phase monostearoylglycerol formed stable and closed, probably unilamellar, vesicles with an average diameter of 465 nm. beta-Lactoglobulin interacts with these bilayer structures at pH 4, where the protein is positively charged, as well as at pH 7 where the protein is negatively charged. Under both conditions of pH, the binding affinity of beta-lactoglobulin is in the micromolar range as observed with ITC and the direct binding assay. At pH 4, two binding modes were found, one of which is determined with ITC while the direct binding assay determines the net result of both. The first binding mode is observed with ITC and is characterized by a large binding enthalpy, a decreased enthalpy of the MSG L(beta) to L(alpha) phase transition and leakage of a fluorescent dye. These characteristics are explained by a beta-lactoglobulin induced partial L(beta) to coagel phase transition that results from a specific electrostatic interaction between the protein and the charged amphiphile. This explanation is confirmed by solid-state (2)H-NMR using 1-monostearoylglycerol with a fully deuterated acyl chain. Upon interaction with beta-lactoglobulin, the isotropic signal in the (2)H-NMR spectrum of the monostearoylglycerol-dicetylphosphate mixture partially transforms into a broad anisotropic signal which could be assigned to coagel formation. The second binding mode probably results from an aspecific electrostatic attraction between the negatively charged bilayer and the positively charged protein and causes the precipitation of the dispersion. At pH 7, only the first binding mode is observed.

The insertion of human apolipoprotein H into phospholipid membranes: a monolayer study

Apolipoprotein H (ApoH) is a plasma glycoprotein isolated from human serum. The interactions of ApoH with lipid membrane were reported to be essential for its physiological and pathogenic roles. In this paper we studied the ability of ApoH to insert into phospholipid membranes using the monolayer approach. The results show that ApoH is surface active and can insert into the lipid monolayers. The insertion ability of ApoH is stronger when a higher content of negatively charged lipids is present in the membrane. The acidic-pH and low-ionic-strength conditions will also enhance ApoH insertion, but these factors may not have much influence on the final insertion ability of ApoH, suggesting that, in the mechanism of ApoH insertion, not only electrostatic forces, but also hydrophobic interactions, are evidently involved. Modification by heat inactivation and reduction/alkylation does not change the critical insertion pressure (pic) of ApoH, suggesting a stable domain, maybe a linear sequence motif, but not the native three-dimensional structure of ApoH, is responsible for its insertion. The extent to which insertion of ApoH into phospholipid membranes may facilitate the 'immune cleaning' of plasma liposomes is discussed.