MECHANISMS OF FATTY ACID OXIDATION
Three different mechanisms are able to induce lipid peroxidation:
1 - autoxidation by free radical reaction
2 - photo-oxidation
3 - enzyme action
This is a radical-chain process involving 3 sequences:
Initiation, propagation and termination.
1.1 - Initiation
In a peroxide-free lipid system, the initiation of a peroxidation sequence refers to the attack of a ROS (with sufficient reactivity) able to abstract a hydrogen atom from a methylene group (- CH2-), these hydrogen having very high mobility. This attack generates easily free radicals from polyunsaturated fatty acids. .OH is the most efficient ROS to do that attack, whereas O2.- is insufficiently reactive.
This peroxidation process is inhibited by tocopherols, mannitol and formate. The presence of a double bond in the fatty acid weakens the C-H bonds on the carbon atom adjacent to the double bond and so makes H removal easier.
The carbon radical tends to be stabilized by a molecular rearrangement to form a conjugated diene.
Under aerobic conditions conjugated dienes are able to combine with O2 to give a peroxyl (or peroxy) radical, ROO..
1.2 - Propagation
As a peroxyl radical is able to abstract H from another lipid molecule (adjacent fatty acid), especially in the presence of metals such as copper or iron, thus causing an autocatalytic chain reaction. The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide). This reaction characterizes the propagation stage.
Probable alternative fates of peroxyl radicals are to be transformed into cyclic peroxides or even cyclic endoperoxides (from polyunsaturated fatty acids such as arachidonic or eicosapentaenoic acids).
1.3 - Termination
Termination (formation of a hydroperoxide) is most often achieved by reaction of a peroxyl radical with a-tocopherol which is the main lipophilic "chain-breaking molecule" in the cell membranes. Furthermore, any kind of alkyl radicals (lipid free radicals) L. can react with a lipid peroxide LOO. to give non-initiating and non-propagating species such as the relatively stable dimers LOOL or two peroxide molecules combining to form hydroxylated derivatives (LOH). Some bonds between lipid peroxides and membrane proteins are also possible.
As singlet oxygen (1O2) is highly electrophilic, it can react rapidly with unsaturated lipids but by a different mechanism than free radical autoxidation. In the presence of sensitizers (chlorophyll, porphyrins, myoglobin, riboflavin, bilirubin, erythrosine, rose bengal, methylene blue...), a double bond interacts with singlet oxygen produced from O2 by light.
Oxygen is added at either end carbon of a double bond which takes the trans configuration. Thus, one possible reaction of singlet O2 with a double bond between C12 and C13 of one fatty acid is to produce 12- and 13-hydroperoxides.
The lifetime of singlet O2 in the hydrophobic cell membrane is greater than in aqueous solution. Furthermore, photo-oxidation is a quicker reaction than autoxidation since it was demonstrated that photo-oxidation of oleic acid can be 30 000 times quicker than autoxidation and for polyenes photo-oxidation can be 1000-1500 times quicker. Similar effects have been described in liposomes and in intact membranes.
The inhibition of photosensitized oxidation is efficiently inhibited by carotenoids, the main protective role played by these compounds in green plants. The inhibitory mechanism is thought to be through an interference with the formation of singlet oxygen from the oxygen molecule. In contrast, tocopherols inhibit this oxidation by quenching the previously formed singlet oxygen, this forms stable addition products. Unexpectedly, It was shown that carotenes are efficient inhibitors in vegetal oils only if tocopherols are also present to protect the former (Frenkel EN et al. Lipids 1979, 14, 961).
3 - Enzymatic peroxidation
Lipoxygenase enzymes (from plants or animals) catalyze reactions between O2 and polyunsaturated fatty acids, such as arachidonic acid (20:4 n-6), containing methylene interrupted double bonds.
When 20:4 n-6 is the substrate, these hydroperoxides are known as HpETEs which can be transformed into hydroxy products (HETEs).
These HETEs are also formed directly via cytochrome P450 induced reactions (mono-oxygenases) and sometimes also via cyclooxygenase enzymes.
Six hydroperoxides (5-, 8-, 9-, 11-, 12-, and 15-HpETE) are known to be formed from arachidonic acid in animal cells. Dihydroperoxy compounds (DiHpETEs) may also be formed via the action of 5- and 15-lipoxygenases. These compounds are important metabolic intermediates but are also bioactive.
Cyclooxygenase enzymes (in plants and animals) catalyze the addition of molecular oxygen to various polyunsaturated fatty acids, they are thus converted into biologically active molecules called endoperoxides (PGG, PGH), intermediates in the transformation of fatty acids to prostaglandins.
Among the cytochrome P450 catalyzed reactions, the fatty acid epoxygenase activity produces epoxide derivatives. Those formed from 20:4 n-6 (5,6-, 8,9-, 11,12-, 14,15-EpETrE) have been shown to have prominent biological activities. Furthermore, these mono-epoxides are susceptible to be metabolized into di-epoxides, epoxy-alcohols or oxygenated prostaglandins.