Paraproteins and electrolyte assays: exclusion effect and effect of paraprotein elimination.

Paraproteins are a potential source of error for electrolyte analyses. The exclusion effect itself causes a discrepancy between direct and indirect ion selective electrode assays (dISE and iISE, respectively). We tested the applicability of different pretreatment methods and the difference of dISE and iISE with paraprotein-rich samples. We analysed chloride (Cl-), potassium (K+), and sodium (Na+) on 46 samples with paraproteins up to 73 g/L. We compared pretreatment methods of preheating, precipitation, and filtration to the native sample. All induced a statistically significant difference (p-value <0.05). Clinically significant difference was induced by precipitation for all analytes, and filtration for Cl- and Na+, but for none by preheating. The difference in electrolyte measurements with either dISE or iISE on native samples was explained by total protein concentration (TP). There was a statistically significant difference in all electrolyte measurements. On average, there was a clinically significant difference in Na + but not in Cl- and K + measurements. Paraprotein concentration (PP) or heavy chain class did not induce a statistically significant effect. The regression analysis and comparison to the theoretical exclusion effect supported the conclusion that TP is the only explanatory factor in the difference between dISE and iISE. We conclude that preheating is a suitable pretreatment method for all the studied analytes. Precipitation is not valid for any of them, and filtration can be considered only for K+. Because the difference between dISE and iISE was explained by the exclusion effect caused by TP, dISE is the more suitable method to analyse paraprotein-rich samples.

Role of neutral lipids in tear fluid lipid layer: coarse-grained simulation study.

Tear fluid lipid layer (TFLL) residing at the air-water interface of tears has been recognized to play an important role in the development of dry eye syndrome. Yet, the composition, structure, and mechanical properties of TFLL are only partly known. Here, we report results of coarse-grained simulations of a lipid layer comprising phospholipids, free fatty acids, cholesteryl esters, and triglycerides at the air-water interface to shed light on the properties of TFLL. We consider structural as well as dynamical properties of the lipid layer as a function of surface pressure. Simulations revealed that neutral lipids reside heterogeneously between phospholipids at relatively low pressures but form a separate hydrophobic phase with increasing surface pressure, transforming the initial lipid monolayer to a two-layered structure. When the model of TFLL was compared to a one-component phospholipid monolayer system, we found drastic differences in both structural and dynamical properties that explain the prominent role of neutral lipids as stabilizers of the TFLL. Based on our results, we suggest that neutral lipids are able to increase the stability of the TFLL by modulating its dynamical and structural behavior, which is important for the proper function of tear film.

Molecular organization of the tear fluid lipid layer.

The tear fluid protects the corneal epithelium from drying out as well as from invasion by pathogens. It also provides cell nutrients. Similarly to lung surfactant, it is composed of an aqueous phase covered by a lipid layer. Here we describe the molecular organization of the anterior lipid layer of the tear film. Artificial tear fluid lipid layers (ATFLLs) composed of egg yolk phosphatidylcholine (60 mol %), free fatty acids (20 mol %), cholesteryl oleate (10 mol %), and triglycerides (10 mol %) were deposited on the air-water interface and their physico-chemical behavior was compared to egg-yolk phosphatidylcholine monolayers by using Langmuir-film balance techniques, x-ray diffraction, and imaging techniques as well as in silico molecular level simulations. At low surface pressures, ATFLLs were organized at the air-water interface as heterogeneous monomolecular films. Upon compression the ATFLLs collapsed toward the air phase and formed hemispherelike lipid aggregates. This transition was reversible upon relaxation. These results were confirmed by molecular-level simulations of ATFLL, which further provided molecular-scale insight into the molecular distributions inside and dynamics of the tear film. Similar type of behavior is observed in lung surfactant but the folding takes place toward the aqueous phase. The results provide novel information of the function of lipids in the tear fluid.

Liposome electrokinetic capillary chromatography in the study of analyte-phospholipid membrane interactions. Application to pesticides and related compounds.

Interactions between low-molar mass analytes and phospholipid membranes were studied by liposome electrokinetic capillary chromatography (LEKC). The analytes were pesticides, some degradation products, and compounds associated with the manufacture of pesticides. Negatively charged liposome dispersions with different zwitterionic lipids (PC) were applied to the determination of retention factors (k) of 15 charged and uncharged compounds. The liposome dispersions consisted of 80:20 mol% of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/POPS, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/POPS. Retention factors were calculated from the effective electrophoretic mobilities of the analytes under LEKC and CZE conditions and from the effective electrophoretic mobilities of the liposomes, determined by CZE with a polyacrylamide-coated capillary. Determining the liposome mobilities in this way proved to be a good alternative to the conventional method employing a liposome marker compound. The log k values of the analytes for the different liposome dispersed phases were correlated with one another. In addition, correlation curves were determined between log k and calculated octanol-water partition coefficients. The results showed that the zwitterionic phospholipid in the liposome has a major impact on the interactions between the tested compounds and the lipid membranes.

Surface properties of artificial tear film lipid layers: effects of wax esters.

PURPOSE: The tear film lipid layer is believed to stabilize the tear film and to retard evaporation. Based on previous simple in vitro studies, the evidence for the latter property is scarce. In this study, we used complex lipid mixtures including various wax esters to study their physical properties and evaporation retarding effect. METHODS: Twelve samples of artificial tear film lipid layer mixtures composed of (L-α)-phosphatidylcholine, cholesterol oleate, and triglycerides were mixed with wax esters. A Langmuir balance was used to analyze the compressibility and rheological properties of these mixtures. In addition, a custom-built system was used for the evaporation studies used at 35°C. Lipid films were imaged with Brewster angle microscopy. RESULTS: None of the studied lipid mixtures decreased the evaporation rate. All lipid mixtures had similar compression isotherms and viscoelastic properties regardless of the wax ester species or its concentration. The results suggest that the overall properties of these mixtures are independent of individual lipid species and that these films are very cooperative and showed minor variation depending on the wax ester species. Brewster angle microscopy images revealed that the lipid films assembled into multiple layers. CONCLUSIONS: Wax ester-containing lipid mixtures resembling the tear film lipid layer are organized in a layered fashion so that amphiphilic lipids are adjacent to the aqueous phase and the nonpolar lipids are layered on top of these. This organization does not retard evaporation and raises overall questions about the role of lipids in the tear film.