Biological membranes pores

Simulations of water in synthetic and biological membranes are often performed by modeling the pore as an approximately cylindrical tube of infinite length (thus employing periodic boundary conditions in one direction only). Such a system contains one (curved) interface between the aqueous phase and the pore surface. If the entrance region of the channel is important, or if the pore is to be simulated in equilibrium with a bulk-like phase, a scheme like the one in Fig. 2 can be used. In such a system there are two planar interfaces (with a hole representing the channel entrance) in addition to the curved interface of interest. Periodic boundary conditions can be applied again in all three directions of space. 

Although for the moment this model is only partially supported by experimental data it offers the opportunity to design new experiments which will help to understand the mechanisms of pardaxin insertion and pore formation in lipid bilayers and biological membranes which at a molecular level are the events leading to shark repellency and toxicity of this marine toxin. 

AK Soloman. Characterization of biological membranes by equivalent pores. I Gen Physiol 51 335s-364s, 1968. 

Compounds can cross biological membranes by two passive processes, transcellu-lar and paracellular mechanisms. For transcellular diffusion two potential mechanisms exist. The compound can distribute into the lipid core of the membrane and diffuse within the membrane to the basolateral side. Alternatively, the solute may diffuse across the apical cell membrane and enter the cytoplasm before exiting across the basolateral membrane. Because both processes involve diffusion through the lipid core of the membrane the physicochemistry of the compound is important. Paracellular absorption involves the passage of the compound through the aqueous-filled pores. Clearly in principle many compounds can be absorbed by this route but the process is invariably slower than the transcellular route (surface area of pores versus surface area of the membrane) and is very dependent on molecular size due to the finite dimensions of the aqueous pores. 

Aquaporins help water to pass through biological membranes. They form hydrophilic pores that allow H2O molecules, but not hydrated ions or larger molecules, to pass through. Aquaporins are particularly important in the kidney, where they promote the reuptake of water (see p. 328). Aquaporin-2 in the renal collecting ducts is regulated by antidiuretic hormone (ADH, vasopressin), which via cAMP leads to shifting of the channels from the ER into the plasma membrane. 

It strikes me that in biological membranes, at least in eucaryotic cells, the transport mode almost universally chosen is the channel, or pore, mode, and not the mobile carrier mode. Surely there must be reasons for this, and it would seem appropriate to me if either Professor Simon or Professor Eisenman could start this discussion with a description of the respective merits of the two transport modes, with respect to selectivity, efficiency, and other parameters. 

As a rule, biological membranes are impermeable to polar molecules or ions. The nuclear membrane and the outer mitochondrial membrane do have pores that admit relatively large molecules (see fig. 17.2). More commonly, however, an ion or polar molecule can cross a biological membrane only if the membrane has a protein that is a specific transporter for that molecule. 

Figure 7.4 Membrane pore diameter from bubble point measurements versus Bacillus prodigiosus concentration [1], Reprinted from W.J. Elford, The Principles of Ultrafiltration as Applied in Biological Studies, Proc. R. Soc. London, Ser. B 112, 384 (1933) with permission from The Royal Society, London, UK...

Although a-LTX is able to insert into pure lipid membranes (Finkelstein et al. 1976), reconstituted receptors greatly enhance the rate of insertion (Scheer et al. 1986). Biological membranes seem even more refractive to the toxin when cells do not possess a-LTX receptors, no pore formation can be detected (Hlubek et al. 2000 Van Renterghem et al. 2000 Volynski et al. 2000), whereas expression of exogenous receptors allows abundant a-LTX insertion and concomitant channel... 

Model systems have been developed for many of these ion-transport mechanisms in the context of bioorganic chemistry. Examples are the cyclic peptides, described by M. R. Ghadiri et al., that have antibiotic activity similar to that of ionophores, a property that is most probably caused by the ability of these peptides to self-assemble inside biological membranes into channels [1], Other compounds able to induce the formation of membrane pores are the bouquet-molecules introduced by J.-M. Lehn [2]. Artificial / -barrels have been developed by S. Matile s group [3]. Many host molecules used in bioorganic chemistry can serve as carriers for ions across membranes and have even made possible the development of systems with which active ion transport can be achieved [4]. 

As stated, biological membranes are normally arranged as bilayers. It has, however, been observed that some lipid components of biological membranes spontaneously form non-lamellar phases, including the inverted hexagonal form (Figure 1.9) and cubic phases [101]. The tendency to form such non-lamellar phases is influenced by the type of phospholipid as well as by inserted proteins and peptides. An example of this is the formation of non-lamellar inverted phases by the polypeptide antibiotic Nisin in unsaturated phosphatidylethanolamines [102]. Non-lamellar inverted phase formation can affect the stability of membranes, pore formation, and fusion processes. So-called lipid polymorphism and protein-lipid interactions have been discussed in detail by Epand [103]. 

Many factors including partition characteristics, degree of ionization, molecular size etc. influence the transport of drugs across biological membranes. Permeation of intact mucosa may also involve passive diffusion, intercellular movement, transport through pores or other mechanisms. The objective of the studies reported here was to employ the dog model to investigate these factors in a systematic and experimentally well-controlled fashion. The non-steriodal anti-inflammatory drug, diclofenac sodium, was selected as a test compound in this evaluation process. 

Emphasis is placed here on features of the biological membranes which are implicated in substrate transport. The lipid bilayer in the "gel" state, in the absence of additives, forms an effective barrier against polar ions and water soluble substrates. Changing the fluidity, by phase transition (induced by temperature changes and/or by the addition of foreign ions or molecules) or by the incorporation of additives (cholesterol, for example), profoundly influences the structure and, hence, the transport properties of membranes. This, and the presence of channel or pore forming peptides or proteins, opens the door to a number of transport mechanisms which will be summarized in the following section. 

There are several mechanisms for explaining how biological membranes can transport charged or uncharged substrates against their thermodynamic forces. It is widely accepted that cross-transports by a protein are discrete events. Biomembranes contain enzymes, pores, charges or membrane potentials, and catalytic activities associated with the transport of substrates. It is well established that the electrostatic interactions between the membrane and a charged... 

As previously discussed, electron, light, and confocal microscopy techniques may be used to visualize the position of electron-dense precipitates, radioactive substances, and fluorescent probes, respectively, in the sample tissue. However, none of these techniques possess the capability both to visualize and to selectively measure the flux of a molecule across the skin. SECM, however, permits the measurement and subsequent imaging of the local flux of an electroactive species across biological membranes. Scott et al. [3] used SECM to investigate the effect of pretreatment of the penetration enhancer sodium dodecyl sulfate (SDS), on the ion transport rate and transport pathways of Fe(CN) across hairless mouse skin. Increasing the time of SDS exposure from 10 min to 30 min increased the overall (porous and nonporous) transport of Fe(CN) by 17-fold. More specifically, the SDS-induced increase in Fe(CN)g transport was found to be associated with nonporous (i.e., intercellular) transport routes, while transport via porous routes was significantly reduced. The fraction of Fe(CN)g transport through pores, as measured by... 

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