Diffusivity of membranes

Methods for quantifying both the transcellular diffusion and concurrent metabolism of drugs and the unusual transcellular diffusion of membrane-interactive molecules coupled with the influence of protein binding are described in detail. To demonstrate the utility of cultured cell monolayers as a tool for basic science investigations, a subsection is devoted to the elucidation of rate-determining steps and factors in the passive diffusion of peptides across biological membranes. The chapter concludes with a discussion on the judicious use of in vitro cell monolayer results to predict in vivo results. 

Kasumi A, Sako Y, Yamamoto M (1993) Confined lateral diffusion of membrane receptors as studied by single particle tracking (nanovid microscopy. Effects of calcium-induced differentiation in cultured epithelial cells. Biophys J 65 2021-2040... 

Alkyl chain heterogeneities cause cell membrane bilayers to remain in the fluid state over a broad temperature range. This permits rapid lateral diffusion of membrane lipids and proteins within the plane of the bilayer. The lateral diffusion rate for an unconstrained phospholipid in a bilayer is of the order of 1 mm2 s 1 an integral membrane protein such as rhodopsin would diffuse 40nm2 s 1. 

Retention of a protein or protein activity after 105,000y, 1 hr Chromatography on gel filtration columns with large pore sizes Electron microscopy—however, sample preparation may partially reconstitute membranes Decrease in solution turbidity, which may be detected by a diminution in light scattering or an enhancement in light transmission Diffusion of membrane lipids as assayed by nuclear magnetic resonance and electron spin resonance... 

A very brief description of biological membrane models, and model membranes, is given. Studies of lateral diffusion in model membranes (phospholipid bilayers) and biological membranes are described, emphasizing magnetic resonance methods. The relationship of the rates of lateral diffusion to lipid phase equilibria is discussed. Experiments are reported in which a membrane-dependent immunochemical reaction, complement fixation, is shown to depend on the rates of diffusion of membrane-bound molecules. It is pointed out that the lateral mobilities and distributions of membrane-bound molecules may be important for cell surface recognition. 

Rates of lateral diffusion of membrane components have also been determined using optical methods. The early experiments of Frye and Ediden16 demonstrated lateral motion of fluorescent-labeled surface antigens in heterokaryons of mouse and human cells. They observed intermixing of fluorescent-labeled antibodies against mouse cell and human cell antigens. Optical methods may also be characterized as either transient or steady state. The use of fluorescence correlation spectroscopy as... 

Looks at the role of caveolins in the formation of membrane caveolae Covers the investigation of hop diffusion of membrane lipids using FRAP (fluorescence recovery after photobleaching)... 

Frye and Edidin s experiment demonstrating lateral diffusion of membrane proteins, (a) Proteins on the plasma membranes of human and mouse cells were labeled with dyes that fluoresced at different wavelengths. (b) The two populations of cells were mixed and infected with a virus that causes cells to fuse. At short times after mixing, red and green fluorescence from the original cells was seen in separate parts of the membranes of the fused cells, (c) Within about 30 min, the two populations of proteins had become intermingled over the entire surface. 

Phospholipid molecules in the plasma membrane diffuse rapidly enough to go from one end of an average-sized animal cell to the other in a few minutes. In a bacterial cell, such a trip would take only a few seconds. Integral membrane proteins move more slowly than phospholipids, as we expect in view of their greater mass. Diffusion of membrane proteins plays essential roles in many biochemical processes, including the cellular uptake of lipoproteins (chapter 18), responses of cells to hormones (chapter 24), immunological reactions (supplement 3), vision (supplement 2), and the transport of nutrients and ions. As we see in a later section, however, some membrane proteins cannot move about rapidly because they are attached to cytoskeletal scaffolds. 

Chen Y, Lagerholm BC, Yang B, Jacobson K. Methods to measure the lateral diffusion of membrane lipids and proteins. Methods 2006 39 147-153. 

Indeed the heart, brain, kidney, liver, and other systems that receive a lot of blood get major portions of the drug shortly after absorption. Other parts of the body that receive less blood flow, such as muscle, viscera, and fat, may take considerably longer to receive the drug. Besides blood flow, the diffusibility of membranes and tissues affects distribution the more diffusible tissues receive the drug more rapidly. 

Transformed and tumorigenic cells are different from normal cell lines in that they are not usually anchorage dependent. They exhibit a spherical shape, increased life span and lateral diffusivity of membrane proteins, decreased cell receptors and membrane proteins, and a different cytoskeletal structure. The decrease in the concentration of the cell adhesion molecules in the cell membrane of these cells causes the anchorage independence. Transformed cell lines also do not assemble a normal ECM. It is important to note that some cell lines (e.g., lymphocytes) that are normally anchorage dependent can be induced and then adapted to become anchorage independent. This is of tremendous importance to recombinant protein production as discussed later, because the scale-up of suspension cultures is easier than that of anchorage-dependent cell lines. 

We and other workers have exploited the triplet lifetime to observe the motion of molecules too large to be studied by singlet techniques. Examples are the rotational motion of RNA polymerase (Austin et al., 1983), the rotational diffusion of membrane-bound components (Cherry et al., 1976 Vazetal., 1979 Jovinetal., 1981) and the motion of the DNA helix... 

Because the lipid bilayer is fluid, there can be rapid lateral diffusion of membrane proteins through the lipid bilayer but membrane proteins, like membrane lipids, do not "flip-flop" across the membrane or turn in the membrane like a revolving door of a department store. 

Measurements of the mobility of membrane-associated components, particularly those associating with the cytoplasmic side of the lipid bilayer and positioned to interact with the intracellular domain of the receptor, are few (Bruckert et al., 1992 Kwon and Neubig, 1992 Kwon, 1992). Measurements of the diffusivity of membrane-associated effectors (see Fig. 2), for example, will be important in determining the role of diffusion in receptor/effector coupling and signal transduction. 

Zhang, F., Crise, B., Su, B., Rose, J. K., Bothwell, A., and Jacobson, K., Lateral diffusion of membrane-spanning glycosylphosphatidylinositol-linked proteins Towards establishing rules governing the lateral mobility of membrane proteins. J. Cell Biol. 115, 75 (1991). 

A lateral penetration of the entire lipo-peptide molecule. i.e. including the lipid tail, is difficult to rationalize as the helix boundle of the receptor represents a tight assembly which precludes diffusion of membrane lipids into its core structure in order to maintain its three-dimensional assembly Consequently, it should therefore preclude also a penetration of the lipo-tail of DM-gastrin and DM-CCK, and the lipo-peptides should approach the receptor with the tail inserted Into the lipid bilayer and then protrude into the binding deft across the extracellular loops. 

NH a, phosphorescence, delayaed fluorescence, diffusion of membrane proteins... 

For cell membranes to be effective permeability barriers, they must be flexible and allow relatively free motion of proteins that are embedded in or linked to them. Integral membrane proteins often diffuse laterally, and many receptor-mediated solute-uptake pathways involve endocytosis that entails phospholipid rearrangement in the membrane. Hormone secretion and other protein trafficking processes involve exo-cytosis and it is usual for membrane vesicles to fuse with each other in a process that also involves the lateral diffusion of membrane constituents. The activity of some receptors is strongly linked to the extent of fluidity of the membrane around them. 

The technological development of Pd-based membrane is crucial for a wide diffusion of membrane reactors. R D efforts have to be focused mainly on increasing membrane temperature threshold, on improving the adherence between the active layer and the support by new deposition and welding techniques, and on improvement of membrane reliability to produce objects suitable for industrial applications. 

Cherry, R. J. (1992). Rotational diffusion of membrane proteins studies of band 3 in the human erythrocyte membrane using triplet probes. In Structural and Dynamic Properties of Lipids and Membranes, P. J. Quinn R. J. 

Cherry, R.J., 1979, Rotational and lateral diffusion of membrane proteins, Biochim. Biophys. Acta, 559 289. 

---