Lipid thiols

The strong chemical interaction of a lipid thiol monolayer developed on a gold disk electrode yields a structure resembling a biological membrane. The blocking ability of various substances was tested for the cyclic voltametry redox reaction of [Fe(CN)g]3-ions, upon adsorption on the thiol monolayer. This was applied to measure the threshold concentrations for detection of various odor substances. The order of response shown in series 30 was the same as the one followed by the human olfactory sense, pointing to... 

Antioxidant free radicals (A H) produced in this way are relatively stable, but they may react with another lipid free radical to form a quinone [15.3] and [15.4]. Quinones can react with amine or thiol groups of proteins, forming polymerisable yellow or red coloured compounds (Pokorny, 1987). Antioxidant free radicals may also react with either another antioxidant free radical or a lipid free radical, forming dimers [15.5] or copolymers [15.6] and [15.7], respectively. Oligomeric or condensated products of antioxidant free radicals usually possess moderate antioxidant activities too (Pokorny et al, 1974) ... 

Organic peroxides such as cumene hydroperoxide and t-butyl hydroperoxide have extensively been used as experimental agents. They provoke lipid peroxidation in hepatocytes, probably by the generation of alkoxyl and peroxyl radical intermediates after reaction with cytochrome P450. Other cytotoxic mechanisms are probably involved including protein thiol and non-protein thiol oxidation and deranged calcium homeostasis (Jewell et al., 1986). In fact, the addition of cumene hydroperoxide to isolated bUe duct cells, devoid of cytochrome P450 activity, still results in cell death but lipid peroxidation is not detectable (Parola et al., 1990). 

Jewell, S.A., DiMonte, D., Richelmi, P., Bellomo, G. and Orrenius, S. (1986). tert-Butylhydroperoxide-induced toxicity in isolated hepatocytes contribution of thiol oxidation and lipid peroxidation. J. Biochem. Toxicol. 1, 13-22. 

Thiols are also important protection against lipid peroxidation. Glutathione (7-Glu-Cys-Gly) is used by several glutathione-dependent enzymes such as free-radical reductase (converts vitamin E radical to vitamin E), glutathione peroxidase (reduces hydrogen peroxide and lipid hydroperoxides to water and to the lipid alcohol, respectively), and others. In addition, the thiol group of many proteins is essential for function. Oxidation of the thiol of calcium ATPases impairs function and leads to increased intracellular calcium. Thiol derivatives such as the ovothiols (l-methyl-4-mercaptohistidines) (Shapiro, 1991) have been explored as therapeutics. 

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

As well as fluorescence-based assays, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In a commercial BIA-core system [231] a hydrophobic SPR sensor with an alkane thiol surface was incubated with vesicles of defined size distribution generating a hybrid membrane by fusion of the lipid vesicles with the alkane thiol layer [232]. If the vesicles contain biotinylated lipopeptides their membrane anchoring can be analyzed by incubation with streptavidine. Accordingly, experiments with lipopeptides representing the C-terminal sequence of N-Ras show clear differences between single and double hydrophobic modified peptides in their ability to persist in the lipid layer [233]. 

Fig. 19. Lipopeptides with the aminoacid sequence of the Ras C-terminus and the natural or artificial lipid-modifications can be coupled with C-terminally truncated Ras via a maleimi-docaproyl linker. This electrophile reacts with free thiol groups (here, a C-terminal cysteine at the Ras moiety)...

The results summarized above were obtained by using fluorescence based assays employing phospholipid vesicles and fluorescent labeled lipopeptides. Recently, surface plasmon resonance (SPR) was developed as new a technique for the study of membrane association of lipidated peptides. Thus, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In SPR (surface plasmon resonance) systemsI713bl changes of the refractive index (RI) in the proximity of the sensor layer are monitored. In a commercial BIAcore system1341 the resonance signal is proportional to the mass of macromolecules bound to the membrane and allows analysis with a time resolution of seconds. Vesicles of defined size distribution were prepared from mixtures of lipids and biotinylated lipopeptides by extruder technique and fused with a alkane thiol surface of a hydrophobic SPR sensor. 

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

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