Catalysts, for lipid oxidation

The membrane-bound catalyst for water oxidation to O2 can be prepared via oxidation of Mn(Il) and Co(ll) salts to Mn(IV) and Co(Ill) hydroxides, respectively, in the presence of lipid vesicles. Using these catalysts and photogenerated Ru(bipy)j complex as an oxidant, it is possible to oxidize water to O2 in vesicle systems. One of such systems for O2 evolution is schematically represented in Fig. 4. 

To employ the catalysts for 02 evolution in the vesicle systems, it was essential to check whether their selectivity towards evolution of 02 remains high enough after immobilization on the lipid membrane. Shilov, Shafirovich and co-workers prepared [268-271] a membrane-bound catalyst for water oxidation by oxidation of Mn(II) salts in the presence of lipid vesicles. The Mn(IV) hydroxide catalyst... 

The membrane-bound catalysts for water oxidation can also be obtained with other transition metal hydroxides. Gerasimov et al. [272] have shown that illumination of a Ru(bpy) + — persulfate system in the presence of Co(II) and lipid vesicles results in the formation of a colloid catalyst for water oxidation, viz. Co(III) hydroxide, immobilized on the lipid membranes. The same catalyst can be obtained without illumination by Co(II) oxidation with a Ru(bpy)3+ complex in the vesicle suspension. The selectivity of water oxidation with the catalysts thus obtained depends on the nature of the membrane-forming lipid. Switching from the synthetic DPL to the natural eggL the process selectivity decreases by about two orders of magnitude due to consumption of the oxidant for oxidation of organic impurities contained in lipids of natural origin [113]. 

Redox-active metals are the initiators of perhaps greatest importance for lipid oxidation in oils, foods, and biological systems because they are ubiquitous and active in many forms, and trace quantities (electron transfers appear to be active catalysts these include cobalt, iron, copper, manganese, magnesium, and... 

Metals become potent catalysts of lipid oxidation by forming complexes through the carboxylic acid groups of any free fatty acids present in oils. In liposome systems, metal catalysis may proceed by a different mechanism in the non-polar acyl chain moiety of the phospholipid molecule from that in the polar phosphate region. Different pathways for deconposition of lipid hydroperoxides... 

Mn(IV) bound to lipid vesicles is an active catalyst for 02 evolution in the presence of one-electron oxidants, such as [Ru(bpy)3]3+ and [Fe(bpy)3]3+ where bpy is 2,2 -bipyridyl (Luneva et al., 1987 Shilov, 1997). The evaluation of the 02 forming center is discussed. It is speculated that if a lipid membrane is formed in the presence ofMn2+, the Mn2+ may be incorporated into the membrane forming the catalyst for 02 evolution from H20. 

Excess iron can lead to diabetes mellitus, faulty liver functions, and endocrine disturbance. Iron is a catalyst for oxidative damage leading to lipid peroxidation. The latest hypotheses link peroxidation to heart disease, cancer, and accelerated aging. Iron is involved in the Fenton Reaction, which catalyzes the formation of free radicals that cause excessive damage to cells and their components. 

One of the characteristics of critical illness is hypermetabolism. Trauma, burn injury, and sepsis are aU catalysts for the release of mediators that initiate and regulate the hypermetabohc response. The metabolic consequences of this response include altered carbohydrate metabolism, increased protein synthesis and degradation, and increased lipid oxidation, which ultimately result in loss of protein and lean body mass." In a previously well-nourished individual, critical illness can result in the onset of kwashiorkor-like malnutrition within 5 to 7 days. In a previously malnourished individual, critical illness can precipitate severe mixed marasmus-kwashiorkor in 3 to 5 days. In a prospective study of 129 patients admitted to the intensive care unit (ICU), 43% were malnourished." The malnourished patients had an increased length of stay in the ICU (a mean of 27 vs. 19 days) and a statistically significantly increased incidence of complications (55% vs. 40%) compared with well-nourished patients with a similar severity of illness. 

Richards, M.P. and Hultin, H.O. 2000. Effect of pH on lipid oxidation using trout hemolysate as a catalyst apossible role for deoxyhemoglobin, J. Agric. Food Chem., 48(4), 3141. 

In principle, ascorbic acid and its salts (sodinm or calcinm ascorbate) are water solnble antioxidants, not widely applicable for lipid systems but extensively nsed in beverages. In aqneons systems containing metals, ascorbic acid may also act as a prooxidant by reducing the metals that become active catalysts of oxidation in their lower valences. However, in the absence of added metals, ascorbic acid is an effective antioxidant at high concentrations. The action of ascorbic acid in lipid autoxidation is dependent on concentration, the presence of metal ions, and other antioxidants. It has been shown that ascorbates can protect plasma and LDL lipids from peroxidative damage, and it may inhibit the binding of copper ions to LDL. " In several countries, ascorbic pahnitate is used in fat containing foods due to its lipid solubility. However whether ascorbic palmitate exerts a better... 

The mechanism of initiation of lipid oxidation has been debated for many years. The most likely initiation process is the metal-catalysed decomposition of preformed hydroperoxides. The thermal oxidation of unsaturated lipids is usually autocatalytic and involves initiation by decomposition of hydroperoxides, which is generally considered metal-catalysed because it is very difficult or nearly impossible to eliminate trace metals that act as potent catalysts for reactions (3) and (4). 

Flavor deterioration of food lipids is caused mainly by the presence of volatile lipid oxidation products that have an impact on flavor at extremely low concentrations, often at the parts per billion (ppb) levels. An understanding of the sources of volatile oxidation products provides the basis for improved methods to control and evaluate flavor deterioration. The decomposition of lipid hydroperoxides produces carbonyl compounds, alcohols and hydrocarbons under various conditions of elevated temperatures and in the presence of metal catalysts. 

Many hypotheses have been advanced for these differences in oxidative susceptibility, including the oxidation potential of milks, and the action of xanthine oxidase and lactoperoxidase, which is controversial. However, there are no substrates for these enzymes in milk. The action of various metallo proteins in milk may be confused as enzymes. These metallo proteins act as powerful lipid oxidation catalysts in the presence of oxygen and redox systems involving ascorbic acid. 

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