40 research outputs found
Oil structuring: polymer bridging mechanism for structuring soft materials using natural emulsions as templates
No full textUsing a bridging flocculation mechanism in the design of oleogels materials constitutes an alternative framework to achieve desired rheological properties of oil-in-water emulsions. Aggregation by polymer bridging generates a droplet network linked together by firmly bound polymer bridges. In this dissertation, we used negatively charged polysaccharides, sodium alginate, xanthan gum, and ι-carrageenan as the structuring agents and soybean oleosomes as the template. Bridging flocculation between polysaccharides and oleosomes was induced by mixing and adjusting the pH to values where both are oppositely charged, leading to electrostatic-driven interactions. Our results indicate that polysaccharides with flexible polymer chains, such as sodium alginate and ι-carrageenan, are effective bridging flocculants.
In contrast, polysaccharides with a more rigid backbone, such as xanthan gum, resulted in depletion flocculation characterized by phase separation between oleosome droplets and xanthan molecules. Bridging flocculation is more effective at an optimum dosage between polysaccharides and oleosomes, expressed as a mass ratio (g polysaccharide/g oleosome) or as an equivalent per droplet surface area (mg/m2). Sodium alginate presented a higher bridging ability than ι-carrageenan, with its optimum bridging ratio at 0.005 g/g and ι-carrageenan at 0.01 g/g. This was confirmed by quantitative analysis of oleosome content upon centrifugation recovery, where sodium alginate yielded more compact and concentrated gels than ι-carrageenan. Differences in structural conformations between sodium alginate and ι-carrageenan account for the difference in bridging ability. Sodium alginate presents a co-block arrangement of alternating charged and uncharged parts. The negatively charged blocks adsorb strongly onto the oleosome interface at several charged units. At the same time, the uncharged parts impart a high degree of flexibility, allowing the polymer chains to bridge several droplets together.
On the other hand, ι-carrageenan is less flexible than alginate, making the individual carrageenan chains more effective for oleosome surface coating but less effective for bridging neighboring droplets. This difference in bridging ability between sodium alginate and ι-carrageenan will influence the structure of the aggregated network and, as a result, will be responsible for the mechanical behavior in rheological measurements. Sodium alginate produced more heterogeneous and interconnected structures, while ι-carrageenan produced smaller and less interconnected clusters. This difference in microstructure and the effect of the structural conformations in the polysaccharide chains becomes relevant at medium and large deformations in amplitude sweeps oscillatory rheology. At deformations between 3- 100%, sodium alginate presented steeper slopes in the moduli G’, indicating sudden microstructure fracture.
In contrast, ι-carrageenan presented less steep slopes indicating yielding rather than fracture behavior in the decrease of the moduli G’. At deformations between 200- 300%, the moduli presented an overshoot indicating a “cage effect” where individual droplets are immobilized due to crowding by surrounding droplets. This effect was more clearly prominent in conditions leading to the densest structures, such as in the compacted gels upon centrifugation performed at the optimum bridging ratios for sodium alginate (0.005 g/g) and ι-carrageenan (0.01 g/g). This study offers many perspectives on how to construct the macroscopic functional properties of oleogels in accordance with their application using the molecular architecture of polysaccharides
Cholesterol as stabilizer of the oxytocin receptor
No full textAbstractThe function of the oxytocin receptor system is strongly dependent on steroids as demonstrated by several physiological studies. One key element of this dependence on steroids may be the interaction of cholesterol and the oxytocin receptor. In this study, we show that cholesterol stabilizes the solubilized human oxytocin receptor against thermal inactivation and proteolytic degradation. In the absence of additional cholesterol, the soluble receptor inactivates within minutes. Maximal stabilization of the oxytocin receptor requires a continuous supply with cholesterol from a cholesterol-rich environment. A structure–activity analysis of various cholesterol analogues and their effect on the thermal stability of the oxytocin receptor showed that the stabilizing function of cholesterol was highly specific. The structural requirements of a potent stabilizing steroid are very similar to those necessary to support the high-affinity state of the receptor. Moreover, in the presence of cholesterol, the oxytocin receptor is significantly more stable against alterations of pH value (pH 4–12). The results show that cholesterol acts as a general stabilizer of the oxytocin receptor
Human oxytocin receptors in cholesterol‐rich vs. cholesterol‐poor microdomains of the plasma membrane
No full text
Specification of the cholesterol interaction with the oxytocin receptor using a chimeric receptor approach
No full textThe Oxytocin Receptor System: Structure, Function, and Regulation
No full textThe neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via Gqproteins to phospholipase C-β. The high-affinity receptor state requires both Mg2+and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF2α. Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF2α. Thus, to initiate labor, it might be essential to generate sufficient PGF2αto overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.</jats:p
