Hydroperoxide levels

LOX-catalyzed oxidation of LDL has been studied in subsequent studies [26,27]. Belkner et al. [27] showed that LOX-catalyzed LDL oxidation was not restricted to the oxidation of lipids but also resulted in the cooxidative modification of apoproteins. It is known that LOX-catalyzed LDL oxidation is regio- and enantio-specific as opposed to free radical-mediated lipid peroxidation. In accord with this proposal Yamashita et al. [28] showed that LDL oxidation by 15-LOX from rabbit reticulocytes formed hydroperoxides of phosphatidylcholine and cholesteryl esters regio-, stereo-, and enantio-specifically. Sigari et al. [29] demonstrated that fibroblasts with overexpressed 15-LOX produced bioactive minimally modified LDL, which is probably responsible for LDL atherogenic effect in vivo. Ezaki et al. [30] found that the incubation of LDL with 15-LOX-overexpressed fibroblasts resulted in a sharp increase in the cholesteryl ester hydroperoxide level and a lesser increase in free fatty acid hydroperoxides. 

Hydroperoxide Levels. In thermally oxidized fats hydroperoxides are usually very low. At higher temperatures, oxidation proceeds rapidly and the rate of hydroperoxide decomposition exceeds that of hydroperoxide formation (17,18). For example, when ethyl linoleate was oxidized at 70°C, peroxide content reached a maximum of 1777 meq/kg after 6 hr then decreased to 283 meq/kg after 70 hr. At 250°C, on the other hand, peroxide value reached a maximum of only 198 meq/kg after 10 min, and was zero after 30 min. 

Experiments were in two groups. In the first, mice were irradiated as before and vitamin E content and lipid hydroperoxide levels were assayed simultaneously on skin samples. In the second, the cutaneous vitamin E content of mice was increased by dietary supplementation mice with various skin concentrations... 

In contrast to silver-catalysed cumene oxidation, the evidence concerning the mechanism of copper-catalysed reactions favours radical initiation via surface hydroperoxide decomposition. Gorokhovatsky has shown that the rate of ethyl benzene oxidation responds to changes in the amount of copper(ii) oxide catalyst used, in a manner which is characteristic of this mechanism. Allara and Roberts have studied the oxidation of hexadecane over copper catalysts treated in various ways to produce different surface oxide species, Depending on the catalyst surface area and surface oxide species present, a certain critical hydroperoxide concentration was necessary in order to produce a catalytic reaction. At lower hydroperoxide levels, the reaction was inhibited by the oxidized copper surface. XPS surface analysis of the copper catalysts showed a... 

In all cases, 100 fig borohydride (BELT) /g solution were used. Final hydroperoxide levels measured after batch treatment of the solution for 4 hr under a nitrogen atmosphere. 

Poly(ethylene-co-1,4-cyclohexane dimethylene terephthalate) The thermal and photochemical oxidation processes of a polyester based on poly(ethylene-co-l,4-cyclohexanedimethylene terephthalate) (PECT) with approximately 30% of 1,4-cyclohexanedimethanol have been studied by means of chemiluminescence [88], Also, the stabilization activity of some commercial antioxidants has been evaluated, and it has been related to hydroperoxide levels in the polymer. 

Compared with other food lipids, fish hpids are very sensitive to photooxidation, with thermooxidation being less pronounced. The lipid oxidation rate of seafood is species dependent, and is harvest and season dependent in a given species (Figure 12.4). The susceptibihty of seafood lipids to UV-catalyzed oxidation (hydroperoxide level and slope of accumulation over time of UV exposure) did not correlate with PUFA, DHA, MUFA, or SFA contents. Sprat and herring lipids showed a weak... 

Figure 13.13. Effect of dietary linoleic acid on synthesis of active eicosanoids and metabolism. Pathophysiological actions result from faster eicosanoid formation by increased hydroperoxide levels. From Lands etal. (1986).

With regard to the high (positive) value of and to the short half-life values, escape of OH and from the sphere immediately surrounding mitochondrion can be virtually excluded. Yet the neutral molecule H Oj is considered to be movable one, which can escape as from the body of the mitochondrion as well as from the cell body itself. It is comprehensible that in some tissues the actual concentrations may reach 100 pM or more as for example, in human and other animal aqueous and vitreous humors. The hydroperoxide levels at or below 20-50 pM seem, however, to have limited cytotoxicity to many cell types [32],... 

After a while, the hydroperoxide concentration reaches a level at which it begins to generate free radicals by a bimolecular degradation mechanism (RS-5 in Fig. 3.19). Reaction RS-5 is exothermic, unlike the endothermic monomolecular decomposition of hydroperoxides (RS-4 in Fig. 3.19) which needs approx. 150kJ/mol. However, in most foods, RS-5 is of no relevance since fat (oil) oxidation makes a food unpalatable well before reaching the necessary hydroperoxide level for the RS-5 reaction step to occur. RS-4 and RS-5 (Fig. 3.19) are the branching reactions of the free radical chain. 

The results in Table 6.1 show hydroperoxide levels before and after photolysis. About 70% of the PP hydroperoxides are converted into volatile products such as acetone, acetic acid and methanol. The residual 30% can be attributed to other volatile compounds (CO, CO, and so on). 

Recently, we have also been interested in examining the hydroperoxide content in retrieved components. Hydroperoxides are precursors to carbonyl formation and thus represent the oxidation potential of polyethylene [31]. The smdy of the hydroperoxide levels requires the exposure of the sections to nitric oxide (NO) for at least 16 hours to convert hydroperoxides to nitrates, which are easily detected by Fl lR spectroscopy [32]. The same regions of interest used for the oxidation analysis are used for the assessment of hydroperoxides. In this chapter, a hydroperoxide index was calculated through the thickness of microtomed sections using the area under the peak between 1600 and 1670cm divided by the area of the reference peak between 1330 and 1396cm (this is the same reference peak used to calculate the oxidation index). 

Our data do not support the hypothesis that resin and conversion techniques substantially influence the resistance of tibial inserts to in vivo oxidation [42]. After ganuna irradiation in air, oxidation and hydroperoxide levels in components fabricated from direct molded 1900H were similar to those in ram extruded GUR 415 (Figures 22.14 and 22.15). Similarly, after inert sterilization, components fabricated from 1900H were similar to molded GUR 1050 after comparable in vivo exposure. 

The thermooxidative stability of NR was studied at 150C as a function of the cure process (sulphur, sulphurless or peroxide curing) and the properties of the carbon black filler (five different carbon blacks were used). The thermooxidation was followed by iodometric analysis of hydroperoxide levels, FTIR spectroscopy and differential thermal analysis. The results were discussed and they showed that the type of curing system had an effect on the degradation profile, but the surface reactivity and structure of the carbon black had a controlling role in each system. 13 refs. 

---