The study of biomembrane lipids and signaling events has been an exciting area of research in recent ten years. Lipids are no longer thought to be just structural entities of biomembranes, but are recognized as dynamic species whose specific distribution and metabolism are the result of highly regulated processes and can lead to a variety of important responses. Enzymes involving lipid signaling include phospholipases and lipid kinases·phosphatases.

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Ion channels are fundamental molecular devices in biological membrane and can be expressed by synthetic molecules mimicking their functions, which include rapid ionic flow across the membrane with ion selectivities as well as flux-control. Here reviewed are supramolecular as well as molecular artificial ion channels from totally synthetic sources. Amphiphilic ion pairs consisting of oligoether and hydrophobic units have been assembled in lipid bilayer membranes to afford single channel currents. Existence of several conductivity states reflected different assembly numbers. However, only one conductivity state was observed, when supramolecular channels with a controlled assembly number of ion pairs or macrocyclic molecular channels were introduced. All the artificial channels obtained hitherto gave preferences for cation permeation with respect to the anion. Macrocyclic resorcin [4] arene channel and oligoether supramolecules gave K⁺ selectivity over Na⁺. The flux control has been successful by introducing charges in the membrane phase to afford voltage dependence of open and closed times. Furthermore, the ionic currents were successfully controlled by photochemical isomerization of azo functionality in the supramolecular channel components.

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Electrophysiological methods have elucidated essential properties of ion channels. Basic principles of ion permeation through channel pore were reviewed. To investigate the physical picture of the pore, open channel blockers have been useful tools, providing cut-off size of the pore, location of the binding sites and geometry of the pore entrance. Examples are shown for the inward rectifying potassium channel, where the potential profile for Ba²⁺ blocking was characterized. Permeating properties of pores were described by either the discrete model or the continuum model.

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Recently membrane proteins which permeate water have been idnetified and named aquaporins (AQP). AQPs belong to a large membrane transport protein family called the MIP family. Members of MIP/AQP family distribute widely in almost all organisms. Their main function or role is speculated to be water transport, but rigous demonstration is lacking for most AQPs. 9 mammalian AQPs cloned to date are summarized, and current understanding of the structure of AQP is reviewed.

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Light driven proton translocating activity of bacteriorhodopsin was examined as a function of a pH gradient (ΔpH) and a membrane potential difference (ΔΨ) using planar lipid bilayer method. Variation of ΔΨ exerted a considerably larger effect on the rate of proton translocation than energetically equivalent magnitude of ΔpH. These features are consistent with the structural data, particularly in view of an asymmetric environment provided by the key amino acid residues with different pKa. The relatively small effect of ΔpH is explained in terms of the proton uptake residue, Asp96, and the proton ejecting residue, Asp85, whose pKa's are known to be about 10 and 3 in the unphotolyzed state, respectively. On the other hand, proton transfer from Asp96 to the Schiff base during the decay of the M intermediate can account for the large effect of ΔΨ on the rate of proton translocation. With these experimental data and explanations in mind, I further propose a simplified model based on an asymmetric potential profile of a multi ion channel. In this model, the uni-directional translocation is induced by a stochastic transition between two states of the multi ion channel. Thus, the active pumping can be explained in the theoretical framework of an asymmetric multi ion channel which undergoes stochastic non-equilibrium transitions between two states. A simple numerical simulation could qualitatively reproduce the experimental data. The simple principle for proton pumping described here may be applied in development of an artificial ion pump molecule.

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The present paper reviews the reversed micellar extraction of protein on the basis of the author's research work on the occasion to be accorded the 1997 young investigator's award by the Membrane Society of Japan. The extraction of protein using AOT (di-2-ethylhexyl sodium sulfosuccinate) /isooctane reversed micellar system was investigated from the viewpoint of the solubilization of protein and water by amphiphilic molecules. The minimal AOT concentration required for 100% forward extraction of protein was found. At the minimal AOT concentration, the hydrophilic surroundings, i.e., the molar ratio of water to extracted protein in the organic phase, were independent of the protein concentration. The hydrophilic surroundings were related with Fisher's polarity ratio as an index of the hydrophobicity of protein. Using this relation, a procedure to estimate the sufficient AOT concentration for the protein extraction was proposed. In the cases of cytochrome c and lysozyme extraction, the water concentration was larger than that in the protein-free system in spite of the same AOT concentration. On the contrary, in the case of ribonuclease A, this large water uptake was not observed. These differences of water uptake were discussed in relation to the location of the protein in the AOT reversed micelles. The forward extraction was accompanied by a conformational change of protein, whereas the backward extracted protein consequently refolded in the native conformation in the aqueous phase. Therefore, the relative reactivity of recovered cytochrome c and ribonuclease A after forward and backward extractions showed almost 100%. The recovered activity yield showed a maximum with respect to AOT concentration because of the combined effect of both decreases in forward and backward extraction yields at low and high AOT concentrations, respectively.

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Plasma triglyceride (TG) -rich lipoproteins, chylomicrons and VLDL, vary in lipid composition during their metabolism : specifically their cholesterol content in the surface and cholesteryl oleate (CO) in the core. Using lipid emulsions of defined composition as model particles, we investigated the effects of the lipid composition on the surface and core structures and the binding of apolipoproteins to the emulsion surface. We have demonstrated that an increase in surface cholesterol of emulsions causes the acyl chain condensation of surface phospholipids, resulting in the significant decrease in the binding of apolipoproteins such as apoA-I and C-II. We have also shown that the physical states of not only surface but also core lipids affect the binding behavior of apolipoproteins, such that replacing core TG with CO significantly decreases the mobility of core lipids and the binding of apoE. In addition, replacing core TG with CO is found to cause the relocation of cholesterol in the emulsion surface layers and thereby modulates the surface properties. As a result, exchangeable apolipoproteins such as apoC-II and apoE will dissociate from the cholesterol-enriched, CO-core emulsions that represent the lipid composition of LDL. These results suggest that the binding behavior of exchangeable apolipoproteins may closely correlate with the location of surface cholesterol and the mobility of core lipids.

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Polyether-block amides (PEBA) membranes incorporating polyester substrates, textile or nonwoven fabrics which can suppress the swelling were prepared by melt pressurization. Swelling and permeation of phenol in the membranes were investigated using phenol-water mixtures. The fabrics in the membranes were not wetted by PEBA before and after swelling tests. The sorption isotherm of phenol is linear up to an external concentration of 5×10⁴ mg/kg. Water sorption was not influenced by the presence of phenol for phenol concentrations of less than about 5×10⁴ mg/kg. The permeation rate could be correlated to the swelling of the membrane. Phenol concentration in the swollen PEBA membrane became higher compared with that in the feed solution and decreased in the permeate. The experiments confirmed that the separation was performed by selective sorption of phenol in the swollen PEBA membrane.
In comparison with textiles, nonwoven fabrics suppressed the swelling in the PEBA membranes and the membranes showed high phenol/water selectivity.

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The constitutive relation of power coupling between two power processes is expressed by an interaction between one flow and the other force. This causal relation can be represented by a circuit with a transformer type transducer and two resistors in parallel and in series. The bond graphs represent the coupled volume and solute flows and the thermo-osmosis in membrane transport systems. Ordinary transport equations of membrane transport are deduced from the bond graphs.

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Microgrooves (width 4-8 μm. depth 4.5 μm. length 10-100 μm. number 2600-8736 in parallel) formed on the surface of a single-crystal silicon substrate were converted to leak-proof microchannels by tightly covering them with an optically flat glass plate. The microchannels were used as a model of physiological capillaries to measure erythrocyte deformability, leukocyte adhesiveness, and platelet aggregability. An apparatus named MCFAN (Micro Channel array Flow Analyzer) is now commersially available.

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