Transport through biomembranes

Castelli, F., S. Caruso, and N. Giuffrida. 1999. Different effects of two structurally similar carotenoids, lutein and beta-carotene, on the thermotropic behaviour of phosphatidylcholine liposomes. Calorimetric evidence of their hindered transport through biomembranes. Thermochim. Acta 327 125-131. 

It is well known that the extraction of the coenzyme Q by isooctane abolishes the activity of the electron transport chain. On the contrary introduction of the coenzyme Q into lipid bilayers makes it possible to model the electron transport through biomembranes.32-36... 

Keywords Ion selective micropipettes Metabolic rate Reactive oxygen species Respiratory activity Scanning Electrochemical Microscopy Surviving rate Transport through biomembranes Voltammetric ultra-microelectrodes... 

Thus, the presence of a silatrane group in the phytohormone molecule prolongs its effect or facilitates the auxin transport through the biomembranes. 

The volume of distribution of a peptide or protein drug is determined largely by its physico-chemical properties (e. g., charge, lipophilicity), protein binding, and dependency on active transport processes. Due to their large size - and therefore limited mobility through biomembranes - most therapeutic proteins have small volumes of distribution, typically limited to the volumes of the extracellular space [26, 51]. 

FIGURE 19.4 Types of ion transport across a biomembrane. (a) Diffusion without transport mediator carrier-mediated transport using (b) the traveling or (c) the hopping mode (d) transport through a transmembrane channel. 

Mammalian skin is perhaps the most formidable transport barrier found in nature. The lipids of the stratum comeum, the outermost skin layer, form the primary barrier to transport of many compounds of therapeutic interest (Scheuplein, 1965,1978 Potts and Guy, 1992 Blank and Scheuplein, 1969 Elias, 1983,1987,1991). AsshownschematicallyinFig. 2, these lipids form broad multilamellar arrays in the extracellular space surrounding the remains of epidermal cells known as comeocytes. The lipids have a unique composition (fatty acids, cholesterol, and ceramides no phospholipids are present) and form the only continuous domain within the stratum comeum (Elias, 1983, 1987, 1991). Despite profound differences between stratum comeum lipids and those of the phospholipid bilayers more commonly found in other biomembranes, direct comparison of passive transport through each suggests a common mechanism involving Ifee-volume fluctuations in the lipid alkyl chains. Transport within the lipid hydrocarbon domain substantially restricts the permeability of large mol-... 

Things are more difficult with the theoretical description of transport through channels. Yet it is this transport mechanism that is of the greatest interest from the biological point of view, since the transport systems of biomembranes apparently operate on the channel principle. What... 

It has been recognised for some time (see for example reference 1), that surfactants can increase the rate and extent of transport of solute molecules through biological membranes by fluidisation of the membrane. It is only recently, however, that sufficient work has been carried out to allow some analysis of structure-action relationships. In this overview an attempt is made, by reference to our own work and to work in the literature, to define those structural features in polyoxyethylene alkyl and aryl ethers which give rise to biological activity, especially as it is manifested in interactions with biomembranes and subsequent increase in the transport of drug molecules. 

Therefore, these drug molecules must cross the multiple layers of cellular membranes between the site of administration (the blood circulation in the case of intravenous injection) and their intracellular receptors. Small molecules (MW < 400 Da) that are soluble in both water and octanol readily diffuse through the biomembranes. Many hydrophilic small-molecule drugs such as [3-lactam antibiotics12 and classical antifolates13 require specific transporter proteins on the plasma membrane to enter the cytoplasm of the host cells. 

The biomimetic membranes represent a special group of carrier membranes. They are artificial membranes based on biomembrane mimicking, i.e., imitation of the essential features bio membranes use for separation. Nitrocellulose filters impregnated with fatty acids, their esters, and other lipid-like substances may be used— in other words, an imitation of many nonspecific barrier properties of biomembranes. The transport of gas through these membranes will essentially be according to facilitated transport (see Section 4.2). Biomimetic membranes for CO2 capture will transport the gas as HCO3. Development of these materials may be expected for selected applications. 

The permeation of most drugs through cellular membranes is by the process of passive diffusion, a nonsaturable process that follows first-order kinetics. Concentration gradient and lipid solubility of the drug are important determinants of the rate of diffusion. Only a few drug molecules are substrates for active transport processes (eg, tubular secretion of beta-lactam antibiotics) these are saturable at high concentrations. Only very small ions (eg, Li+) or drugs (eg, ethanol) may penetrate biomembranes via aqueous pores. 

---