Activation of Lipid Components

Cholesterol s presence in liposome membranes has the effect of decreasing or even abolishing (at high cholesterol concentrations) the phase transition from the gel state to the fluid or liquid crystal state that occurs with increasing temperature. It also can modulate the permeability and fluidity of the associated membrane—increasing both parameters at temperatures below the phase transition point and decreasing both above the phase transition temperature. Most liposomal recipes include cholesterol as an integral component in membrane construction. 

Two approaches for the activation of lipid components may be used to create reactive groups in liposomes. A purified lipid may be activated prior to incorporation into the bilayer construction 

Phosphatidyl Glycerol Phosphatidyl Inositol Cerebrosides Gangliosides 

Two approaches for the activation of lipid components may be used to create reactive groups in liposomes. A purified lipid component may be activated prior to incorporation into the bilayer construction or the activation step may occur after formation of the intact liposome. Either way, the goal of the activation process is to provide a reactive species that can be used to couple with selected target functional groups on 

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The most common type of heterobifunctional reagent used for the activation of lipid components includes the amine- and sulfhydryl-reactive crosslinkers containing an N-hydroxysuccinim-ide (NHS) ester group on one end and either a maleimide, iodoacetyl, or pyridyl disulfide group on the other end (Chapter 5, Section 1). Principle reagents used to effect this activation process include SMCC (Chapter 5, Section 1.3), MBS (Chapter 5, Section 1.4), SMPB (Chapter 5, Section 1.6), SIAB (Chapter 5, Section 1.5), and SPDP (Chapter 5, Section 1.1). Other... 

This nontoxic lipid may represent a potential candidate for use in the immunotherapy of human cancer. In addition, it will be interesting to determine to what extent this nontoxic lipid A can replace the toxic components in eliciting the numerous other biological activities of lipid A. 

The chemistry and biological activities of lipid A are reviewed in earlier chapters of this volume. As noted there, the components of the lipid A complexes from Salmonella species and j5. coli are derivatives of a disaccharide comprised of two D-glucosamine units linked B,l->-6. The disaccharide is substituted at positions 1 and 4 by phosphate functions and on the amino nitrogens by B-hydroxymyristoyl groups. A variable number (up to 5) of ester-linked fatty acyl residues is also present. In the simplest components the phosphates are present as monoester groups. 

Newman, M.A., Daniels, M.J., Dow, J.M. The activity of lipid A and core components of bacterial lipopolysaccharides in the prevention of the hypersensitive response in pepper. Mol Plant-Microbe Interact 10 (1997) 926-928. 

Synaptic vesicles, isolated from rat brain cortex, and cholinergic vesicles, isolated from the electric organ of Torpedo nobiliana, were broken down by a phospholipase A from cobra venom. The breakdown was accompanied by a release of acetylcholine. Morphological analysis revealed membrane fragments about one-third the size of the vesicle circumference. Subcellular fractions enriched in nerve ending membranes showed some phospholipase A activity. On the basis of these findings models of transmitter release, involving specific alterations of lipid components in the vesicular membrane, are discussed. 

The lipid peroxidation inhibitory activities of EOs are assessed by the P-carotene bleaching tests (Yadegarinia et al., 2006). In this method, the ability to minimize the coupled oxidation of P -carotene and linoleic acid is measured with a photospectrometer. The reaction with radicals shows a change in this orange color. The P-carotene bleaching test shows better results than the DPPH assay because it is more specialized in lipophilic compounds. The test is important in the food industry because the test medium is an emulsion, which is near to the situation in food, therefore allowable alternatives to synthetic antioxidants can be found. An only qualitative assertion uses the TLC procedure. A sample of the EOs is applied onto a TLC plate and is sprayed with P-carotene and linoleic acid. Afterwards, the plate is abandoned to the daylight for 45 min. Zones with constant yellow colors show an antioxidative activity of the component (Guerrini et al., 2006). 

One model that has found applicability in the lipids area is the regular solution theory developed by Hildebrand and Scott [9] and Scatchard [10]. Incorporating a partial molar entropy of mixing term [11-14] into the regular solution theory yields the following expression for the activity of a component in a liquid mixture ... 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

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