Linoleic acid transformations

Lipid hydroperoxides are either formed in an autocatalytic process initiated by hydroxyl radicals or they are formed photochemically. Lipid hydroperoxides, known as the primary lipid oxidation products, are tasteless and odourless, but may be cleaved into the so-called secondary lipid oxidation products by heat or by metal ion catalysis. This transformation of hydroperoxides to secondary lipid oxidation products can thus be seen during chill storage of pork (Nielsen et al, 1997). The secondary lipid oxidation products, like hexanal from linoleic acid, are volatile and provide precooked meats, dried milk products and used frying oil with characteristic off-flavours (Shahidi and Pegg, 1994). They may further react with proteins forming fluorescent protein derivatives derived from initially formed Schiff bases (Tappel, 1956). 

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

M. Meurens, V. Baeten, S.H. Yan, E. Mignolet and Y. Larondelle, Determination of the conjugated linoleic acids in cow s rmlk fat by Eourier transform Raman spectroscopy, J. Agric. Food Chem., 53, 5831-5835 (2005). 

Figure C4.2.1 Lipoxygenase (LOX)-catalyzed transformation of linoleic or linolenic acid (R = CH3(CH2)4-, linoleic acid R = CH3CH2CH=CHCH2-, linolenic acid) showing oxidation by molecular oxygen and formation of conjugated diene hydroperoxides. These events give the basis for measurement of activity by either oxygen uptake or UV absorption at 234 nm. Also shown is the usual preference for (S)-stereospecificity of oxidation.

Ogawa, J., Matsumura, K., Kishino, S., Omura, Y., and Shimizu, S. 2001. Conjugated linoleic acid accumulation via 10-hydroxy-12-octadecaenoic acid during micro-aerobic transformation of linoleic acid by Lactobacillus acidophilus. Appl. Environ. Microbiol., 67,1246-1252. 

Lipoxygenase derived from soybean transforms linoleic acid at pH 9.2 nearly exclusively into 13S-hydroperoxy-9Z-l lE-octadecadienoic acid (13S-HPODE, Scheme 4) and linoleinic acid into 16S-9Z 12Z,14E-octadecadienoic acid (13S-HPOTE, Scheme 5) [155]. 

Epoxidation is especially efficient when the double bond and the generated peroxyl radical are localized within the same molecule Sterols are often conjugated with linoleic acid. When the linoleic acid part is transformed in an autocatalytic reaction to a peroxyl radical the closest localized double bond is that of the sterol and therefore sterol epoxides are obtained [239]. These sterol epoxides are rather toxic and can kill even rather large insects [239]. 

However, the situation may be converted in the y-3 form, in which the presence of two cw-double bonds may stabilize the chain-chain interactions of the linoleic acid leaflet, prohibiting the transformation into more stable forms of P or P as illustrated in Figure 9B. For this reason, the A5 value of y of SLS is much larger than those of Y of SOS and SRS. Furthermore, A5 of SLS y is as large as P forms of SOS. Mechanistically, the transformation from y to P or P is associated with an inclined chain arrangement with respect to the lamellar interface (10), which might be prohibited by the chain-chain interactions of the linoleoyl leaflets in SLS. 

There are other cases of inhibition of lipid enzymatic pathways by trans fatty acid isomers. The above reported mono-14 -trans isomer of arachidonic acid is inhibitor of the synthesis of thromboxane B2 and, therefore, can prevent rat platelet aggregation [52]. The transformation of mono-trans isomers of linoleic acid by rat liver microsomes showed that the 9-cis,12-trans isomer is better desaturated, whereas the 9-trans, 2-cis isomer (Scheme 6.1) is better elongated [53]. 

Typical fatty acid composition of commodity soybean oil, in comparison with the other major vegetable oils, is shown in Table 2.3. Soybean oil has a high content of linoleic acid, and a lower level of linolenic acid. These are both essential fatty acids for humans and therefore of dietary importance, but they are also the cause of oxidative instability of this oil. Processing techniques, such as hydrogenation and lipid modification through traditional plant breeding or genetic transformation, have been used to modify the fatty acid composition to improve its oxidative or functional properties. 

FIGURE 12.2 Gas chromatogram of fatty acids of total lipids in the yeasts transformed with empty vector [Polg(pYLEXl)] or the yeasts carrying the Mortierella A6-desaturase gene [Polg(pYLd6)]. Solid arrows show the presence of A6,9-16 2, A6,9-18 2, and y-linolenic acid (A6,9,12-18 3), the A6-desaturation products of A9-16 l, A9-18 l, and linoleic acid (A9,12-18 2), respectively. 

FIGURE 12.3 Effect of different media on cell mass ( ), lipid content ( ), rate of A6-desat-uration of linoleic acid to y-linolenic acid ( ), and percentage of y-linolenic acid in total lipids ([]) in the transformed Y. lipolytica. A6-Desaturation was calculated as [Product]/[Product -i-Substrate] X 100%. Each value point represents the mean of three independent experiments. In each category, values with different letters or numbers are significantly different from each other atp <. 05. 

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