Enzymatic oxidation PUFAs

The mild green-metallic-mushroom aroma of fresh fish is formed by enzymatic-oxidative degradation of polyunsaturated fatty acids (PUFA) with the participation of... 

This chapter will focus on the families of enzymes that oxidize PUFA, their cellular location, function and substrate specificity, and introduce the role they and their enzymatic products play in cellular health and pathophysiological conditions. 

Lipid Mediators from the Enzymatic Oxidation of PUFAs.174... 

LIPID MEDIATORS FROM THE ENZYMATIC OXIDATION OF PUFAs... 

LDL. The corresponding fragmented alkyl phosphatidylcholines, butanoyl GPC and butenyl GPC products are inflammatory platelet-activating factor (PAF)-like phospholipids and also named butanoyl PAF and butenyl PAF. These C-4 core aldehydes are probably the most stable and are readily analysed directly by LC-MS. Several additional and perhaps more bioactive unsaturated core aldehydes are also expected by non-enzymatic oxidation and fragmentation of PC and CE containing arachidonic, linoleic and n-3 PUFA acids (see Chapter 4) in oxidized LDL. These unsaturated core aldehydes may not have been detected because they may form covalent PC-apoprotein adducts that are not readily identified and analysed directly by LC-MS. However, these unsaturated core aldehydes may be potentially more bioactive and important than the saturated core aldehydes reported in oxidized LDL. 

Finally the lipoperoxidation process can be fully enzymatic and this is carried out by heme-containing cyclooxigenases (prostaglandin-, tromboxan- and prostacyclin-synthases) or ferrous ione-containing lipoxygenases which are oxidized arachidonic acid and another PUFA by means of free radical mechanism [4,5] as can be seen in Figure 2. 

FA distribution in TAG as well as in phospholipids affects the physical properties, lipolitic and oxidative stability, and nutritional availability of lipids. In many TAG, the FA are arranged in a nonrandom distribution. In plants, monoenoic FA and PUFA are dominant at a sn-2 position (Orthoefer, 1996). In pig depot fat and in cow s milk, the TAG sn-2 position is occupied by palmitic acid. The distribution is also different in cattle and sheep depot fats (Love, 1996). In blubber seals, long-chain n-3 PUFA are esterified rather in sn-l,3 positions, whereas, in muscle, TAG in the sn-2 position that is typical for the lipid muscles of nearly all fish (Ackman, 1994). Enzymatic syntheses of structured TAG containing dY-5,8,ll,14,17-eicosapentaenoic acid (EPA) and dY-4,7,10,13,16,19-docosahexaenoic acid (DHA) in the sn-2 position with medium-chain FA at the end positions are particularly interesting (Halldorsson... 

LOX-catalyzed lipid oxidation differs from the free radical reaction by the formation of hydroperoxides at a certain position of the chain of, most often, a free FA. Lipoxygenases use molecular oxygen to catalyze the stereo- and regiospecific oxygenation of PUFA with -cisA- dY-pentadiene moieties. LOX react enzymat-... 

The fish muscle prooxidative system, both enzymatic and nonenzymatic, is more active than that in homeotherms in addition, susceptibility of lipids to UV-catalyzed oxidation is higher (see Chapter 8, Table 8.1). Enzymic NADH-Fe systems were more active than nonenzymic ascorbate-Fe systems in lipid oxidation of cod sarcoplasmic reticulum (Soyer and Hultin, 2000). The most active lipoxygenases (LOX) are in the gill and skin of fish (Hsieh et al., 1988). Fish gill tissue contains two separate LOX (12- and 15-LOX), each being active toward PUFA (AA, EPA, DHA) but exhibiting different hydroperoxide addition sites. LOX from sardine skin were... 

A number of natural and synthetic antioxidants are available to the food industry. These can be used on their own or in blends acting in synergy with each other to inhibit oxidation and to slow down the eventual onset of the oxidizing process. However, the only certain way to stop oxidation is to remove the oxygen supply completely (Lohger, 1983). Enzymatic hydrolysis of the lipid phase in the product and breakdown of the TAG structure followed by oxidation of long-chain n-3 PUFA is another issue to be aware of. 

The enzymatic and tree radical peroxidation ot PUFAs which contains at least three double bonds, like AA and DHA, could lead to malondialdehyde (MDA). This product can be generated by thromboxane synthase, but a report trom the Biomarkers ot Oxidative Stress Study (BOSS) showed that peripheral levels ot MDA derive primarily trom nonenzymatic peroxidative degradation... 

The PUFAs are excellent substrates for enzymatically mediated oxidation reactions. These reactions lead to a plethora of biologically active natural products that have been isolated from plants, animals, and the marine enviromnent. Very often EPA (6) and arachidonic acid (8) are substrates for these oxidations. One significant example is the eicosa-noid class of natural products that constitute many pharmacologically active oxygenated PUFAs. Two such examples are the prostaglandins and the leukotrienes presented in more detail in the succeeding text. These two classes of... 

---