Detection of lipid phase transitions by surface tensiometry

Comparative cellular toxicity of hydrophilic and hydrophobic microcystins on Caco-2 cells

Microcystins (MC), cyanobacterial peptide hepatotoxins, comprise more than 100 different variants. They are rather polar molecules but some variants contain hydrophobic amino acid residues in the highly variable parts of the molecule. In MC-LF and MC-LW, the more hydrophobic phenylalanine (F) and tryptophan (W), respectively, have replaced arginine (R) in MC-LR. Depending on the structure, microcystins are expected to have different in vivo toxicity and bioavailability, but only a few studies have considered the toxic properties of the more hydrophobic variants. The present study shows that MC-LF and MC-LW have more pronounced cytotoxic effects on Caco-2 cells as compared to those of MC-LR. Treatment of Caco-2 cells with MC-LW and especially MC-LF showed clear apoptotic features including shrinkage and blebbing, and the cell–cell adhesion was lost. An obvious reduction of cell proliferation and viability, assessed as the activity of mitochondrial dehydrogenases, was observed with MC-LF, followed by MC-LW and MC-LR. Cytotoxicity was quantified by measuring lactate dehydrogenase leakage. The more hydrophobic MC-LW and MC-LF induced markedly enhanced lactate dehydrogenase leakage compared to controls and MC-LR, indicating that the plasma membrane was damaged. All of the three toxins examined inhibited protein phosphatase 1, with MC-LF and MC-LW to a weaker extent compared to MC-LR. The higher toxic potential of the more hydrophobic microcystins could not be explained by the biophysical experiments performed. Taken together, our data show that the more hydrophobic microcystin variants induce higher toxicity in Caco-2 cells.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

An alternative view of phospholipid phase behavior at the air-water interface. Microscope and film balance studies

Pure-lipid films at the water interface have surface-pressures vs. area isotherms that are often interpreted as involving first-order phase transitions from a condensed region to a liquid-expanded region. Two phases are presumed to coexist in the intermediate part of the isotherm. We constructed a film balance that could be placed on the stage of an epifluorescence microscope. A dipalmitoyl phosphatidylcholine film containing a low concentration of a fluorescent lipid probe showed an inhomogeneous fluorescence distribution in the so-called liquid-expanded region of the isotherm. Only the intermediate and condensed regions could be prepared so as to be optically homogeneous below 25 degrees C. We investigated membrane flow and lateral lipid diffusion in the membrane on the trough. The isotherms and isochores were measured. The results require, at least, a modified description of the monolayer structure in various regions of the isotherms. The solid-condensed region corresponds to a gel phase of the lipids where there is no flow in the membrane, lateral diffusion is low, the compressibility is low, and the membrane is optically homogeneous. The "liquid-condensed/liquid-expanded" region appears to be a homogeneous membrane where lateral diffusion and membrane flow are both rapid. This is a region of high compressibility. The "liquid-expanded" region is not homogeneous as seen under the microscope, and the flow of the surface layer can be very fast.