Lipoxygenases, enzymatic peroxidation

Lipid peroxidation may proceed by both enzymatic and nonenzymatic pathways. Enzymatic peroxidation is catalyzed by enzymes such as lipoxygenases and cyclooxygenases (COXs) and is considered in Chapter 26. One of the most important questions in the study of the mechanisms of lipid peroxidation is the characteristic of an initiation stage. Obvious... 

Holmes et al. (1998) performed two enzymatic reactions, the lipase-catalyzed hydrolysis of y>-nitrophenol butyrate and lipoxygenase-catalyzed peroxidation of linoleic acid, in w/c microemulsions stabilized by a fluorinated two-chained sulfosuccinate surfactant (di-HCF4). The activity of both enzymes in the w/c microemulsion environment was found to be essentially equivalent to that in a water/heptane microemulsion stabilized by Aerosol OT, a surfactant with the same headgroup as di-HCF4. The buffer 2-(A-morpholino)ethanesulfonic acid (MES) was used to fix the pH in the range 5-6. 

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

In this chapter the generation of free radicals, mainly superoxide and nitric oxide, catalyzed by prooxidant enzymes will be considered. Enzymes are apparently able to produce some other free radicals (for example, HO and N02), although their formation is not always rigorously proved or verified. The reactions of such enzymes as lipoxygenase and cyclooxygenase also proceed by free radical mechanism, but the free radicals formed are consumed in their catalytic cycles and probably not to be released outside. Therefore, these enzymes are considered separately in Chapter 26 dedicated to enzymatic lipid peroxidation. 

In 1990, Triantaphylidds and coworkers reported on the preparative enzymatic synthesis of hnoleic acid (135) hydroperoxide 24a using soybean lipoxygenase-1. In this dioxygenation asymmetry is induced by the catalyst, the enzyme. The reaction was later used by Dussault and also by Baba and coworkers as key step in the preparation of more complex peroxides. The enzyme is a non-heme iron dioxygenase which catalyzes the incorporation of dioxygen into polyunsaturated fatty acids to yield E,Z conjugated diene hydroperoxides 24a-d. With this enzymatic method, the hydroperoxide 24a could... 

JAs are derived from linolenic acid via an octadecanoid pathway consisting of several enzymatic steps (Figure 36). Multiple compartments in plant cells participate in JA synthesis. The early steps of this pathway occur in chloroplasts, where linolenic acid is converted to OPDA by means of the three enzymes lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC).867-869 Linolenic acid is oxygenated by 13-LOX producing a peroxidized fatty acid 13-hydroperoxylinolenic acid. The product is subsequently metabolized by AOS to an unstable compound allene oxide. Allene oxide is sequentially converted by AOC to produce OPDA. An alternative pathway from another trienoic fatty acid, hexadecatrienoic acid (16 3), is present in chloroplasts.870 In this pathway, dinor OPDA is produced instead of OPDA. OPDA and dinor OPDA are transported into the peroxisome. An ABC transporter involved in this transport was identified in... 

Fats or raw materials that serve as a source for fatty acids are frequently employed in process flavourings. During the flavour reaction, thermal peroxidation of lipids such as triglycerides, fatty acids and phospholipids occurs. This non-enzymatic lipid oxidation, also called autoxidation, leads to a very complex mixture of reaction products, and has to be regarded separately from the enzymatic lipid oxidation which occurs at low temperature and is catalysed by lipoxygenases. 

Gardner, H.W., Soybean lipoxygenase-1 enzymatically forms both (9S)- and (13S)-hydro-peroxides from linoleic acid by a pH-dependent mechanism, Biochim. Biophys. Acta, 1001, 274, 1989. 

Yamamoto. S. Enzymatic lipid peroxidation, reactions of mammalians lipoxygenases. Free Radical Biology and Medicine, 10,149-159.1991. 

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