Oxidative agents human studies

Probucol. Probucol is an antioxidant that is effective in lowering LDL cholesterol. Whereas probucol was known to lower cholesterol after relatively simple clinical trials (160), its mechanism of action as an antioxidant in the treatment of atherosclerosis is quite novel. Probucol has been shown to have the abiUty to produce regression of atherosclerotic lesions in animal models (161). Probucol therefore represents a novel class of pharmaceutical agent for the treatment of atherosclerosis. This effect occurs mechanistically, in part, by preventing oxidation of LDL, a necessary step in foam cell formation. This antioxidant activity has been shown in laboratory experiments and its activity in lowering LDL cholesterol in human studies is well documented (162). 

This method is also used to measure ex vivo low-density lipoprotein (LDL) oxidation. LDL is isolated fresh from blood samples, oxidation is initiated by Cu(II) or AAPH, and peroxidation of the lipid components is followed at 234 nm for conjugated dienes (Prior and others 2005). In this specific case the procedure can be used to assess the interaction of certain antioxidant compounds, such as vitamin E, carotenoids, and retinyl stearate, exerting a protective effect on LDL (Esterbauer and others 1989). Hence, Viana and others (1996) studied the in vitro antioxidative effects of an extract rich in flavonoids. Similarly, Pearson and others (1999) assessed the ability of compounds in apple juices and extracts from fresh apple to protect LDL. Wang and Goodman (1999) examined the antioxidant properties of 26 common dietary phenolic agents in an ex vivo LDL oxidation model. Salleh and others (2002) screened 12 edible plant extracts rich in polyphenols for their potential to inhibit oxidation of LDL in vitro. Gongalves and others (2004) observed that phenolic extracts from cherry inhibited LDL oxidation in vitro in a dose-dependent manner. Yildirin and others (2007) demonstrated that grapes inhibited oxidation of human LDL at a level comparable to wine. Coinu and others (2007) studied the antioxidant properties of extracts obtained from artichoke leaves and outer bracts measured on human oxidized LDL. Milde and others (2007) showed that many phenolics, as well as carotenoids, enhance resistance to LDL oxidation. 

Enhanced susceptibility to respiratory exposure to infectious agents is of considerable potential public-health significance. This has been reported to occur in mice exposed to ozone at as low as 0.08 ppm, the lowest reported effect concentration in laboratory animal studies. It seems essential, therefore, to continue and expand research on the effect of ozone and other photochemical oxidants on physiologic protective mechanisms of the lung. Appropriate dose-response studies should be included to confirm or establish the exposure concentration-time relationships that result in increased susceptibility to inhaled microorganisms, and studies of the cellular responses and mechanisms should be conducted with a view to providing methods that are applicable to epidemi-ol( c studies in oxidant-exposed human populations. 

Studies are needed to clarify the importance of age, sex, ethnicity, familial elements, nutritional factors, and pharmacologic agents in determining response to oxidants. Because people with lung disease are thought to be more susceptible to oxidant pollutants, exposure studies are needed to quantify this. Better methods for measuring or estimating the actual dose of oxidants absorbed by each subject are needed. The usual time variation in measures of human response should be evaluated... 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

The effects of sodium chlorite on membrane components and antioxidant depletion have been studied in rabbit corneal epithelial cells, human conjunctival epithelial cells, phospholipid vesicles prepared from egg yolk and GSH in solution. Incubation of phospholipid vesicles with 3.5 mmol sodium chlorite/l for up to 2 h had no effect, whereas incubation for 48 h resulted in lipid depletion and an increase in lipid oxidation. Sodium chlorite was found to be a very potent GSH oxidizing agent at a GSH/sodium chlorite ratio of 0.5, GSH was depleted after 5 min. GSH depletion was also seen in rabbit corneal epithelial cells and human conjunctival epithelial cells incubated with 3.5 mmol sodium chlorite/l or 0.55 mmol sodium chlorite/l. At 3.5 mmol/l, sodium chlorite caused rapid loss of cell viability in the corneal cells, as assessed by trypan blue staining and loss of adherence. At 0.55 mmol/l, sodium chlorite had very little effect over the first few hours but decreased viability after 24 h. The conjunctival cells appeared to be less sensitive than the corneal cells. No oxidatively modified lipids could be detected in the cells following sodium chlorite treatment, and no effects were seen in levels of cytosolic antioxidants (Ingram et al., 2003). 

Although phenolic compounds and terpenoids are very effective in cells as well as in animal models, human studies have not yet explored the hidden potential of these compounds. Many of the studies using phenolic compounds and terpenoids have used higher concentrations than can be obtained physiologically from dietary intake, suggesting a role of functional foods in prevention and treatment of diseases. In this chapter, we review the status of phenolic compounds and terpenoids as potential therapeutic agents, with an emphasis on the role of oxidative stress and the presence of redox-active plant compounds in diets. 

The cytotoxicity of LDL can also be inferred from the study by Blake et al. (1985). In this study of human cultured endothelial cells, stored sera from patients with necrotizing arteritis demonstrated an enhanced tendency to develop oxidized LDL, which correlated closely with endothelial cell cytotoxicity. This process appears to require the presence of both oxygen and transition metal ions such as iron in the presence of a reducing agent (Gebicki ef /., 1991). There is considerable evidence that transition metals are involved in cell-induced modifications of LDL including the inhibitory effects of EDTA and desfer-rioxamine (Hiramatsu et 1987). A role for Of in LDL modification by endothelial cells and fibroblasts comes from studies showing inhibition of LDL oxidation by SOD (Steinbrecher, 1988). 

Recently, the possibility to use C60 as anti-inflammatory compound has been reported (Huang et al., 2008). Fullerene-xanthine hybrids have been studied to determine if nitric oxide (NO) and tumor necrosis factor-alpha (TNF-a) production in lipopolysaccharide (LPS)-activated macrophages can be inhibited by hybrid administration, finding positive results. The presence of xanthine moiety seems to be essential for the inhibition of LPS-induced TNF-a production, while the fullerene portion ameliorates the efficiency in LPS-induced NO production blockage, leading to a new promising class of potent anti-inflammatoiy agents. It is necessary to mention also the opposite results obtained by an amino acid fullerene derivative tested on human epidermal keratinocytes at concentration from 0.4 to 400 pg/mL. 

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