Hydrogenation of unsaturated lipids

Addifion of oils and oilseeds to the diet is another method that has been used to reduce CH4 emissions (Beauchemin et ah, 2008 Eckard et ah, 2010). Lipid addition to the diet may reduce CH4 emissions by hydrogenation of unsaturated fatty acids, enhanced propionic acid production, and protozoal inhibition (Johnson and Johnson, 1995). Reductions in CH4 of >40% have been demonstrated with lipid supplementation... 

Other compounds that may have a protective role are vitamin K, cysteine, and ascorbate. These compounds act as alternative hydrogen donors in preference to the allylic hydrogen atoms of unsaturated lipids. 

The radical H02 formed reversibly from 02 in acidic solutions (p a = 4.7) is similar in reactivity to R02 radicals, formed by oxygen addition to alkyl and related radicals, or by H-atom loss from ROOH molecules. As radicals, they are more reactive than 02 ions and can extract active hydrogen, as in reaction [1.5] to give allyl radicals. The resulting hydroperoxide, like H202, is quite reactive, as is the allyl radical which, formally, has ca. 50% spin-density on two almost equivalent carbon atoms, as indicated. Reaction [1.5] is of major importance in certain membrane reactions, since it is one of the stages in the au-toxidation of unsaturated lipid groups (see Chapter 2). The total reaction comprises [1.5] followed by [1.6] which, in turn is followed by [1.5], These two steps constitute a chain reaction which may continue... 

The hydrogenation of unsaturated fatty acids as part of membrane stmctures of a wide range of lipid classes offers important opportunities [16, 17, 169, 170, 244 f]. Water-soluble catalysts are much more suitable than their lipophilic derivatives, as no solvent vector such as THF or DMSO is needed for the transport of the metal complexes into the membranes. Furthermore, water-soluble catalysts are easier or remove from the still-intact membrane at the end of the reaction, and even the polarity of the catalyst may be controlled by suitable choice of ligand, thus offering a bundle of advantages over monophasic hydrogenations [173]. 

One of the objectives in performing hydrogenation reactions in situ was to modulate the fluidity of biomembranes and to examine the role of membrane lipid fluidity in biochemical and physiological functions. Secondly, because membrane lipids with six or more unsaturated double bonds were found to be significant components of some membranes, such as the retinal rod membranes of the eye, the hydrogenation of these lipids was thought to be a useful tool to identify their role in these membranes. 

Foods of animal origin are suspected to contain some amount of COP formed by autoxidation. Cholesterol autoxidation is a well-established free radical process that involves the same chemistry that occurs for the oxidation of unsaturated lipids. Cholesterol contains one double bond at the carbon-5 position therefore, the weakest points in its structure are at the carbon-7 and carbon-4 positions. However, due to the possible influence of the hydroxyl group at carbon-3 and the tertiary carbon atom at carbon-5, the carbon-1 position is rarely attacked by molecular oxygen, and therefore the abstraction of an allylic hydrogen predominantly occurs at carbon-7 and gives rise to a series of A and B ring oxidation products. In the chain reaction,... 

The presence of unsaturated lipids in biological membranes confers a fluid character on the structure and this is integral to its function. Evidence for this has been established by homogeneous catalytic hydrogenation of these lipids in membrane... 

The alkyl radical of unsaturated lipids (L ) containing a labile hydrogen reacts very rapidly with molecular oxygen (k ) to form peroxyl radicals (6). This step is always much faster than the following hydrogen transfer reaction (k ) with unsaturated lipids to form hydroperoxides (7). 

This is the most widely occurring oxidation and describes the first stages of the oxidation of unsaturated lipids producing hydroperoxides as the fundamental primary products. Because this step is slow and rate-determining, hydrogen abstraction from unsaturated lipids becomes selective for the most weakly bound hydrogen. 

The alkoxyl radicals produced by cleavage reaction (10) can undergo further reactions, including (11) j8-cleavage to produce aldehydes and other decomposition product (short-chain hydrocarbons, and shorter chain fatty acids) that contribute to flavor deterioration of unsaturated lipids (Chapter 4, Figure 4.7) (12) cyclization to produce epoxy derivatives, and formation of ketone and hydroxy derivatives that may or may not contribute to flavor deterioration of unsaturated lipids (Chapter 4) and (13) hydrogen abstraction from the lipid substrate to form innocuous alcohols and a chain-carrying lipid radical. 

Keeney M (1970) Lipid metabohsm in the rumen. In PhUlipson AT (ed) Physiology of digestion and metabolism in the ruminant. Oriel Press, Newcastle, pp 489-503 Kemp P, Lander DJ (1984) Hydrogenation in vitro of a-hnolenic add to stearic acid by mixed cultures of pure strains of rumen bacteria. J Gen Microbiol 130 527-533 Kemp P, White RW, Lander DJ (1975) The hydrogenation of unsaturated fatty adds by five bacterial isolates from the sheep rumen, including a new spedes. J Gen Microbiol 90 100-114 Kepler CR, Tove SB (1967) Biohydrogenation of unsaturated fatty acids III. Purification trans-isomerase from Butyrivibrio fibrisolvens. J Biol Chem 242 5686-5692... 

It is well established by now that jjn vivo or jji vitro hydrogenation of unsaturated fatty acid residues of lipids can be carried out using complexes of platinum metal ions (mainly rhodium,ruthenium and palladium) containing as ligands various water soluble phosphines Cl], or Alizarin Red (sodium 1,2-di-hydroxyanthraquinone-3-sulfonate, QS), and this method proved to be a fruitful way for investigating the connection between membrane fluidity and the various properties and functions of... 

The reduced cytotoxicity of surfactant-coated particles may be related to a suppression of surface free radical activity (141) by surfactant. Natural surfactant is capable of scavenging hydrogen peroxide (H2O2), the oxygen free radicals (O and OH ), and radical species derived from peroxynitrite (142). In the process, peroxidation of unsaturated lipids in surfactant and degradation of surfactant pro-... 

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