Linoleic acid model system

Figure 2. Typical chromatogram of volatiles from a peanut Hpoxygenase-linoleic acid model system (A) arid a raw peanut homogerutte (B) (34)...

These results agree with the findings of Yamaguchi and Fujimaki (1974), who showed the antioxidant activity of xylose-lysine and xylose-arginine model MRPs in a linoleic acid model system. A decreased rate of lipid oxidation in cooked ground pork patties with added Glu-Lys model MRPs has been reported by Bedinghaus and Ockerman (1995). 

Literature data support the use of rice bran oil and constituents as antioxidants in food systems. Tocols and oryzanols appear to have different degrees of activity depending on the Upid system evaluated. Oryzanols are more effective in cholesterol systems and tocols the most effective in linoleic acid model systems. The combination of these components greatly enhances the activity of rice bran oil as an antioxidant. Further study is recommended to assess other lipid systems as well as synergistic activities between the oryzanols and tocols. 

B) TEARS Measurement in the Model Lipid Emulsion System. The TEARS generated in a model linoleic acid emulsion system containing Glu-Lys and Fru-Lys MRPs, in the absence of metal ions, is presented in Table 2 i.e., %AO). Unlike the oxygen depletion measurements which identified both the antioxidant or prooxidant activity of individual Glu-Lys and Fru-Lys model experiments, no potential prooxidant effect of MRPs was identified using the TBARs measurement. As such, all Glu-Lys and Fru-Lys MRPs derived from different experimental conditions, reduced lipid peroxidation in the lipid emulsion system. However, despite the finding that all experiments show antioxidant activity as assessed using TBARs endpoint measurements, many of the products derived from different Fru-Lys experiments exhibited relatively low e.g. below 5%) antioxidant activity. 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

A series of substituted diaryselenides were examined in three lipid peroxidation model systems isolated rat liver microsomes treated with Fe(II)/(ADP)/ascorbate and isolated rat hepatocytes treated with two different initiators of oxidation. In rat hepatocytes, all of the tellurides performed more effectively than the selenides. Particularly for the rat liver microsome system, the substituent effects on lipid peroxidation were consistent with what would be expected Electron-donating groups give more active compounds, while electron-withdrawing groups give poorer antioxidants. The same trends were seen for substituted diaryItellurides in inhibition of linoleic acid peroxidation in a two-phase model, where the dimethylamino... 

Haase and Dunkley (1969B) reported that although high concentrations of ascorbic acid in model systems of potassium linoleate were prooxidant, a decrease in the rate of oxidation was observed. Haase and Dunkley (1969C) further noted that certain concentrations of ascorbic acid and copper inhibited the formation of conjugated dienes, but not the oxidation of ascorbic acid, and caused a rapid loss of part of the conjugated dienes already present in the system. They theorized that certain combination concentrations of ascorbic acid and copper inhibit oxidation by the formation of free radical inhibitors which terminate free- radical chain reactions, and that the inhibitors are complexes that include the free radicals. 

It is interesting to consider the concentrations of free radicals that result from lipid hydroperoxides in an in vitro model system. For example, my group has been studying the autoxidation of linoleic acid in SDS micelles at 37°C. We initiate the autoxidation by the decomposition of an initiator, as shown in Equation 3. 

Thus, 26 molecules of linoleic acid undergo autoxidation when a single free radical is introduced into this model membrane system (96). That much damage might well be enough to destroy the membrane and produce cell lysis and death however, we must remember that in the real system, the polyunsaturated fatty acids (PUFA) would be protected by antioxidants such as vitamin E. 

Farag, R.S., Osman, S.A., Hallabo, S.A.S., Nasr, A.A. 1978. Linoleic acid oxidation catalysed by various amino acids and cupric ions in freeze-dried model systems. J. Am. Oil Chem. Soc. 55, 703-710. 

The antioxidant properties of naturally occurring anthraquinones and anthrones were evaluated using different model systems. For example, the antioxidant activity of these compounds was studied on the inhibition of peroxidation of linoleic acid. These results suggest that the antioxidant mechanism for two anthraquinones, emodin, Fig. (4) and aloe-emodin, Fig. (2), possibly depends on scavenging hydroxy radicals, while the pro-oxidant... 

The role of lipoxygenase in the production of flavor volatiles from raw peanuts was further investigated by Singleton et al. (34), In model systems containing linoleic acid, purified peanut lipoxygenase was shown to be primarily responsible for the production of flavor volatiles and the volatile profiles were almost identical to those of peanut homogenates (Figure 2). The optimum pH in the model system, for production of the flavor volatiles was shown to be between 6.5 and 7.0. Pentane but not... 

According to Kanner the presence of an ascorbic acid-cupric ion couple at relatively high concentration inhibits carotene degradation in a P-carotene linoleate model system and thereby reinforces the antioxidant activity of the carotenoid. A tendency of increased carotenoid antioxidant activity in the presence of ascorbic acid has been observed during azo-initiated oxidation of sunflower oil-in-water emulsions. ... 

Kanner, J., Pro-oxidant and antioxidant effects of ascorbic acid and metal salts in a P-carotene linoleate model system, J. Food Sci., 42, 60,1977. 

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