Neutral carrier lipid

Thus, the system comprising membrane, solution 2 of constant composition (internal filling solution), and electrode 2 (internal reference electrode) constitutes an ion selective electrode. The electrically neutral carrier antibiotics of the valinomycin group and related lipid-soluble compounds can serve as the active components of highly selective liquid... 

Independent of the assumptions A to C the cation selectivity of the membranes in the equilibrium domain is therefore controlled by the ratio of the complex formation constants (6) and should therefore be identical for different types of neutral carrier membranes.18 Figure 2 indicates that there is indeed a close parallelism between the selectivities of solvent polymeric membranes (SPM) and bilayer lipid membranes (BLM) modified with valinomycin 1, nonactin 2, trinactin 5, and tetranac-tin 6 (see also Ref. 18). This is in good agreement with findings from Eisenman s45 and Lev s15 research groups. 

In the case of cationic lipid/DNA complexes, the lipid markers that have been used are often the same as those used for conventional liposomes (i.e., CHE). This may not be appropriate. Therefore, we believe it is best to use a radiolabeled form of the lipids used to prepare the carrier system (133). Ideally, carrier cationic lipid and plasmid DNA quantification should be performed on the same tissues within in the same experiment. It may also be useful to analyze the fate of the neutral lipid components of the carrier. Radiolabeled DOPE is available, for example. It may also be useful, in certain types of analyses, such as pharmacokinetic and biodistribution studies, to label both the cationic and neutral components of the lipid carrier. Without some kind of tag, however, the process of detection may be more complex because of the need to efficiently extract the carrier lipid(s) from cells or tissues prior to analysis. The presence of endogenous lipids may make this difficult. For cationic lipids, such as DOTAP or DODAC, for example, it may be recommended that quantification of tissue levels, such as by HPLC analysis, be performed by those specializing in lipid analysis (Northern Lipids, Vancouver, BC, Canada). 

The above results indicate that in order to maintain the high rate of transmembrane electron transfer, it is necessary to provide efficient neutralization of the arising polarization. For this purpose lipophilic ions and proton carriers were successfully used (see Table 1). These compounds are known to act as the uncouplers of the mitochondrial oxidative phosphorylation and are able to remove the gradients of electric fields across lipid membranes. 

Several carrier systems have been shown to be present in the brain endothelium, allowing for the selective transport of a group of common substrates (Table 13.1). The most common system is the one that mediates the transport of glucose, which provides the brain with virtually all its energy. Carrier-mediated mechanisms are also responsible for the absorption of two other energy sources ketone bodies, which are derived from lipids, and lactic acid, a by-product of sugar metabolism. Carrier-mediated transport systems are also involved in the uptake of amino acids by the brain. The brain can manufacture its own small neutral and acidic amino acids however, large neutral and basic amino acids are obtained from the bloodstream. 

Plasmid/cationic carrier complexes have been proposed to internalize into the endosome and initiate the destabilization of endosomal membranes. This destabilization would induce diffusion of anionic lipids from the external layer of the endosomal membrane into the complexes and form charge neutralized ion pairs with the cationic lipids. Destabilization and/or fusion of the complex with the plasma membrane would permit the same anionic lipids to diffuse to the surface, as would fusion with the endosomal membrane. Release of the condensed DNA from the cationic lipid in the endosome is likely to generate a mechanical or osmotic stress that raptures the endosomal bilayer and releases DNA into the cytoplasm. In contrast, DNA release from complexes on the cell surface might be unable to stress the membrane to a degree sufficient to rapture. 

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