Active membranes

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

The Ionic Basis of Membrane Activity. Almost all living cells maintain specific internal chemical environments that ate different from their external environments. In cardiac cells the principal ions involved in maintaining membrane activity are sodium, Na" potassium, K" chloride, CU and calcium, Ca ". The internal (i) and external (o) concentrations of these ions are Na" = 140 mM, Na" = 30 mM = 4 mM, = 140 mM Cl ... 

Urry, D. W. Nuclear magnetic resonance and the conformation of membrane-active peptides. In Enzymes of Biological Membranes, Vol. 1, (ed. Martonosi, A ), p. 31, Plenum Publishing Corp., New York 1976... 

Calcium channels in the plasma membrane activated after receptor-mediated calcium release from intracellular stores. Diese channels are present in many cellular types and play pivotal roles in a multitude of cell functions. It was recently shown that Orai proteins are the pore-forming subunit of CRAC channels. They are activated by STIM proteins that sense the Ca2+ content of the endoplasmic reticulum. 

Koumanov F, Jin B, Yang J et al (2005) Insulin signaling meets vesicle traffic of GLUT4 at a plasma-membrane-activated fusion step. Cell Metab 2 179-189... 

The growth requirement for EGF is a good example in this regard. EGF stimulates the growth of many different types of animal cells in culture. In order to initiate the growth response, EGF interacts with specific EGF receptors localized in the plasma membrane, activating a tyrosine-specific protein kinase, which is an intrinsic part of the receptor (Figure 12). As a consequence, specific proteins are phosphorylated at tyrosine residues, and some of these proteins (which are also... 

TBT and TFT are membrane-active molecules, and their mechanism of action appears to be strongly dependent on organotin(IV) lipophilicity. They function as ionophores and produce hemolysis, release Ca(II) from sarcoplasmic reticulum, alter phosphatodylseiine-induced histamine release, alter mitochondrial membrane permeability and perturb membrane enzymes. Organotin(IV) compounds have been shown to affect cell signaling they activate protein kinase and increase free arachidonic acid through the activation of phospholipase... 

Wyn Jones, R.G., Storey, R., Leigh, R.A., Ahamad, N. Pollard, A. (1977). A hypothesis of cytoplasmic osmoregulation. In Regulation of Cell Membrane Activities in Plants, ed. E. Marre and O. Ciferri, pp. 121-36. Amsterdam Elsevier/North Holland Biomedical Press. 

V-acetyl-3-0-1 -carboxyethyl- membrane-active agents 178 anaphylaxis 135... 

Amphipathic peptides contain amino acid sequences that allow them to adopt membrane active conformations [219]. Usually amphipathic peptides contain a sequence with both hydrophobic amino acids (e.g., isoleucine, valine) and hydrophilic amino acids (e.g., glutamic acid, aspartic acid). These sequences allow the peptide to interact with lipid bilayer. Depending on the peptide sequence these peptides may form a-helix or j6-sheet conformation [219]. They may also interact with different parts of the bilayer. Importantly, these interactions result in a leaky lipid bilayer and, therefore, these features are quite interesting for drug delivery application. Obviously, many of these peptides are toxic due to their strong membrane interactions. 

The intracellular processes which precede membrane activation appear to differ from those of MOE neurones, in that cyclic nucleotide gating may not occur. The transduction process which induces current flow in snake VN neurones, utilises as a putative second-messenger the modulator compound inositol triphosphate — Ins. (1,4,5) P3 = IP3 (Liu et al, 1999 Taniguichi et al, 2000). The proposed channel component associated with the microvillous membrane is one of the transient receptor potential family (TRPC-2 Heading Fig., pp. 94), the p-splice... 

Ovchinnikov, Yu. A., V. I. Ivanov, and M. M. Shkrob, Membrane Active Complexones, Elsevier, Amsterdam, 1974. 

DiVittorio KM, Leevy WM, O Neil EJ, Johnson JR, Vakulenko S, Morris JD, Rosek KD, Serazin N, Hilkert S, Hurley S, Marquez M, Smith BD (2008) Zinc(II) coordination complexes as membrane-active fluorescent probes and antibiotics. Chembiochem 9 286-293... 

Membrane activities IP3-receptor Certain kinases (calmodulin) C-2 domains... 

In this case, a loss of FRET is observed when the transferable lipid moves to an acceptor membrane. The most common FRET pair used for these studies is the NBD/rhodamine pair [160, 161], Some other FRET pairs have been used for the study of membrane sideness conservation [162] and intermembrane lipid transfer [163], Distinguishably, a study of membrane activation by molecular selective recognition was performed using FRET [97],... 

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

As outlined in previous sections, escape of polyplexes from endosomes to the cytosol can be a major bottleneck in delivery. Membrane-active polymer domains or other conjugated molecules can help to overcome this barrier (see Sect. 2.3), but they may trigger cytotoxicity when acting extracellularly or at the cell surface. Therefore membrane-crossing agents either have to be inherently specific for endo-somal compartments (for example by pH-specificity), or they have to be modified to be activated in endosomes. For example, the reducing stimulus of intracellular vesicles has been used to activate formulations containing less active disulfide precursors of LLO [163] or Mel [170]. 

Wagner E (1998) Effects of membrane-active agents in gene delivery. J Control Release 53 155-158... 

Rozema DB, Ekena K, Lewis DL, Loomis AG, Wolff JA (2003) Endosomolysis by masking of a membrane-active agent (EMMA) for cytoplasmic release of macromolecules. Bioconjug Chem 14 51-57... 

Plank C, Zauner W, Wagner E (1998) Application of membrane-active peptides for drug and gene delivery across cellular membranes. Adv Drug Deliv Rev 34 21-35... 

Enzyme containing Nation membranes prepared according to the proposed protocol have shown high specific activity and stability of immobilized glucose oxidase. As expected, the simplicity of preparation provided high reproducibility. When the same casting solution is used, the maximum deviation in membrane activity is <2%. This, however, is also the precision limit for kinetic investigations. 

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