Thiols, antioxidant function

Another example where antioxidant performance can be improved dramatically lies in the mechanochemlcally initiated addition of reactive antioxidants on rubbers (5.10) or unsaturated thermoplastics such as ABS (12). For example, using thiol antioxidants 2 and 3 as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see Equation 1) the polymer becomes much more substantive under aggressive environments. 

Most PVC stabilising systems are complex mixtures that contain a variety of additives which complement HCl removal. They include antioxidants to deal with the radicals formed in Scheme 3.5, sulfur compounds to remove hydroperoxides and olefin reactive agents to remove conjugated double bonds which are the source of colour and oxidative instability. Several functions may be found in the same molecule. Thus DOTGs are particularly powerful heat stabilisers for PVC and it has been found that the thiol produced by reaction with HCl has two additional antioxidant functions (Scheme 3.8). 

Garlic s proven mechanisms of action include (a) inhibition of platelet function, (b) increased levels of two antioxidant enzymes, catalase and glutathione peroxidase, and (c) inhibition of thiol enzymes such as coenzyme A and HMG coenzyme A reductase. Garlic s anti-hyperlipidemic effects are believed to be in part due to the HMG coenzyme A reductase inhibition since prescription medications for hyperlipidemia have that mechanism of action (statins). It is unknown whether garlic would have the same drug interactions, side effects, and need for precautions as the statins. 

Biesalski, FI. K., and Frank, J. (2002). Antioxidants in cancer therapy Is there a rationale to recommend antioxidants during cancer therapy In "Thiol Metabolism and Redox Regulation of Cellular Functions" (A. Pompella, G. Banhegyi, and M. Wellman-Rousseau, eds.), NATO Science Series, Series I Life and Behavioural Sciences. Vol. 347, pp. 252-264. lOS Press, The Netherlands. 

Coefficients correlate with the quantity of functional antioxidant groups in the studied compounds (hydroxyl or thiol), which allows the measurement of total AOA in the solutions. The AOA value of the investigated object is expressed in mole-equivalent per liter of mediator system oxidized form, reacted with probe antioxidants. 

Organotin stabilizers containing sulfur (e.g. 45) or their transformation products 96, 97 are considered as hydroperoxide-decomposing antioxidants (Al-Malaika, 1989). Within this stabilizing function, thiol 97 is oxidized by hydroperoxide in disulfide 99 and sulfenic acid 100, having peroxidolytic properties. 

Polymeric macromoleculcs of types V-VII are needed for applications requiring enhanced molecular siite (see also Chap. III). Thus, linear polymers of type V are prepared by Mannich polymerization or from bis-Mannich bases by exchange reaction, for example, with bis-thiols, as in the case of poly(ketosulfide) 501, which is useful as a high-molecular-weight antioxidant in the processing of plastics.Macromolecules of type VI are generated by the functionalization of polymers, as shown by 502, which is obtained by aminomethylation of polyacrylamide and used as flocculant in water treatment." - ... 

A vicious cycle is thus activated (Figure 4) which amplifies the inflammation stimuli at each cycle. GSH and thiol depletion may thus exert an influence on the immune functions not directly related to their role as antioxidants but as key activators of cell functions. 

The LD50 is in the range of 800-1000 ppm. HOCl, whether exogenously obtained from CI2 gas, or from an oxidative burst from neutrophils, the product reacts with a number of functional thiol groups present in enzymes (Winterboum, 1985 PuUar et al., 2002). HOCl reactivity with thiol groups could inactivate such enzymes as GSH peroxidase and catalase (Aruoma and Halliwell, 1987). Inactivation of enzymes involved in antioxidant defense system renders the cell vulnerable to OS (a disruption of redox homeostasis). 

GSH synthase, a tripeptide (glutamylcysteinylglycine), is not only a water-soluble antioxidant, but is also part of a redox buffer (Smith et al., 1996). It is found in all cells and is used for a multiplicity of cellular functions, such as protein and prostaglandin synthesis, detoxification, etc. Cytosolic concentrations of GSH range from 1 to 11 mm (Smith et al., 1996) and are 100-1000 times greater than the extracellular levels. Many proteins contain sulfhydryl groups because of their cysteine content. The content of thiols in proteins is greater than that of the pool of GSH (Torchinsky, 1981). 

The constant potential amperometric detector determines the current generated by the oxidation or reduction of electoactive species at a constant potential in an electrochemical cell. Reactions occur at an electrode surface and proceed by electron transfer to or from the electrode surface. The majority of electroactive compounds exhibit some degree of aromaticity or conjugation with most practical applications involving oxidation reactions. Electronic resonance in aromatic compounds functions to stabilize free radical intermediate products of anodic oxidations, and as a consequence, the activation barrier for electrochemical reaction is lowered significantly. Typical applications are the detection of phenols (e.g. antioxidants, opiates, catechols, estrogens, quinones) aromatic amines (e.g. aminophenols, neuroactive alkaloids [quinine, cocaine, morphine], neurotransmitters [epinephrine, acetylcoline]), thiols and disulfides, amino acids and peptides, nitroaromatics and pharmaceutical compounds [170,171]. Detection limits are usually in the nanomolar to micromolar range or 0.25 to 25 ng / ml. 

Inhibitor regenerators, which react with intermediates or products formed in the chain-stopping (termination) reaction so as to regenerate the original inhibitor or form another product capable of functioning as an antioxidant. Examples are dialkylphosphonates with hindered phenols or diphenoquinones with thiols. 

Materials that are not eflfective inhibitors when used alone may nevertheless be able to function as synergists by reacting with an oxidized form of an antioxidant to regenerate it and thus prolong its effectiveness. For example, carbon black forms an effective combination with thiols, disulfides, and elemental sulfur, even though these substances may be almost completely ineffective alone under comparable conditions. Synergistic combinations from a variety of chain terminators and sulfur compounds have been reported. A widely used combination of stabilizers for polyolefins is 2,6-di-ferf-butyl-4-methylphenol (BHT) (Figure 1.41a) with dilauryl thiodipropionate (DLTDP) ... 

Thus, the ability of DNIC with thiol-containing ligands to accelerate the healing of skin wounds (and, possibly, of other tissue injuries) is provided by the functional activity of protein-bound DNIC acting as NO donors and their ability to produce antioxidant effects on wound tissue [92,93]. 

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