Hydrophilic protective systems

The kinetics of Reaction (26) can only be discussed on a case-by-case basis. Consumption of GS via Reaction (27) should be in competition with irreversible dimerization or with reversible addition of oxygen (yielding GSOO ). Since mitochondrial respiration keeps oxygen tension within the low micromolar range, while intracellular GSH is in the millimolar range, Reaction (27) will 

Protective enzymes involve dismutases, which are not costly from an energetic point of view, and reductases, which consume reducing cofactors. The main dismutases are the ubiquitous superoxide dismutases (SOD) which 

The bimolecular rate constants associated with the reaction of superoxide with Cu/Zn- and Mn-SOD are 2.4x 109M 1 s 1 and 1.5x 109M 1 s 1, respectively [201,202]. Cu/Zn-SOD is found in the cytosol and in mitochondria, whereas Mn-SOD, a remnant of its bacterial ancestor, is only present in mitochondrial periplasm [200,203,204], 

As previously mentioned, when hydrophilic free radicals are trapped by GSH, superoxide will act as a free radical sink and SOD should then behave as a terminal antioxidant [189]. The simultaneous presence of the two antioxidants will result in an optimal sparing effect on both GSH and oxygen consumptions in superoxide- or GS -dependent chain oxidations of metabolites or xenobiotics. 

In this process, an equivalent of glutathione disulfide is made and recycling of GSH through NADPH-dependent glutathione reductase [217] must be achieved for the protection to be maintained. Glutathione reductase is a flavin-containing enzyme which catalyzes the reduction of GSSG by NADPH as follows  

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To prevent an overload in free radicals and peroxides, aerobic organisms use a sophisticated defense system which operates both in the intra- and extracellular aqueous phases and in membranes. A detailed description of the extracellular protections is outside the scope of this chapter. Cell protections may be divided into hydrophobic and hydrophilic molecular systems. 

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. 

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

In addition to lowering surface tension, surface-active agents contribute to emulsion stability by oriented adsorption at the interface and by formation of a protective film around the droplets. Apparently, the first molecules of a surfactant introduced into a two-phase system act to form a monolayer additional surfactant molecules tend to associate with each other, forming micelles, which stabilize the system by hydrophilic-lipophilic arrangements. This behavior has been depicted by Stutz et al. ( ) and is shown in Figures 1-5. 

The simultaneous HPLC-UV determination of sulfamonomethoxine (SMM), miloxacin (MLX), and OXO in serum and muscle of cultured fish was developed (153). A sample of muscle was extracted with MeCN-THF (95 5) after centrifugation, the supernatant was injected into the HPLC system. A Hisep column, used in this study, is packed with restricted-access materials (RAMs) consisting of the polymeric hydrophilic/hydrophobic phase bound to silica gel. This column did not require time-consuming and complex extraction procedures. The RAM sorbent could also be applied in short precolumns, which are combined directly on-line with the HPLC equipment. This approach is much more convenient than that applied in the present paper. The guard column had to be changed very often in order to protect the analytical column in a sufficient way. The extraction recovery was 79.5%, RSD of 6.0%. 

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