Lipoxygenase oxidation

Lipoxygenase-Catalyzed Oxidations. Lipoxygenase-1 catalyzes the incorporation of dioxygen into polyunsaturated fatty acids possessing a l(Z),4(Z)-pentadienyi moiety to yield ( ),(Z)-conjugated hydroperoxides. A highly active preparation of the enzyme from soybean is commercially available in purified form. From a practical standpoint it is important to mention that the substrate does not need to be in solution to undergo the oxidation. Indeed, the treatment of 28 g/L of linoleic acid [60-33-3] with 2 mg of the enzyme results in (135)-hydroperoxide of linoleic acid in 80% yield... 

Lipoxygenase-Catalyzed Oxidations. Lipoxygenase-1 catalyzes the incorporation of diuxygen into polyunsaturated fatty acids possessing a l(Z).4(Z)-pentadienyl moiety to yield ( ).conjugated hydroperoxides. A highly active preparation of the enz.yme from soybean is commercially available in purified form. 

This review concentrates on instances where both substrate and prod-uct of an individual enzyme are known and where the enzymatic reaction has been studied in a cell-free system. Enzymes discussed elsewhere in this book (mostly oxidative lipoxygenase, polyphenol oxidase, peroxidase) are omitted. 

The basic chemistry of enzyme catalyzed oxidation of food lipids such as in cereal products, or in many fruits, and vegetables is the same as for autoxidation, but the enzyme lipoxygenase (LPX) is very specific for the substrate and for the method of oxidation." Lipoxygenases are globulins with molecular weights ranging from 0.6-1 x 10 Da, containing one iron atom per molecule at the active site. 

Fruits and vegetables (e. g., pineapple, apple, pear, peach, passion fruit, kiwi, celery, parsley) contain unsaturated Cn hydrocarbons which play a role as aroma substances. Of special interest are (E,Z)-l,3,5-undecatriene and (E,Z,Z)-1,3,5,8-undecatetraene, which with very low threshold concentrations have a balsamic, spicy, pinelike odor. It is assumed that the hydrocarbons are formed from unsaturated fatty acids by P-oxidation, lipoxygenase catalysis, oxidation of the radical to the carbonium ion and decarboxylation. The hypothetical reaction pathway from linoleic acid to (E,Z)-l,3,5-undecatrieneis shown in Formula 5.25. 

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]. 

Peroxidation is also catalyzed in vivo by heme compounds and by lipoxygenases found in platelets and leukocytes. Other products of auto-oxidation or enzymic oxidation of physiologic significance include oxysterols (formed from cholesterol) and isoprostanes (prostanoids). 

Grosch, W. and Laskawy, G., Co-oxidation of carotenes requires one soybean lipoxygenase isoenzyme, Biochim. Biophys. Acta, 575, 439, 1979. 

The biological membranes that surround cells and form the boundaries of intracellular organelles contain polyunsaturated fiitty acids, which are susceptible to oxidation. This reaction is used under controlled conditions by enzymes, such as the lipoxygenases or cyclooxygenases, within cells to produce oxygenated lipids, which can act as mediators of inflammation (Smith and Marnett, 1991 Yamamoto, 1992). Such compounds are characterized by their high potency and specificity in their interaction with cells (Salmon, 1986). While these enzymatic reactions... 

Figure 2.2 Oxidation of human LDL by lipoxygenase and exposure to copper. The oxidation of human LDL was monitored by the increase in absorbance at 234 nm after the addition of soybean lipoxygenase (LO) at t = 0 min (— and —) followed by the addition of Cu (10 fiM) at t = 90 min to one LO-treated sample (—) and the control (-). Other conditions were exactly as described in Jessup et af. (1991).

An example of an experiment in which LDL has been treated with 15-lipoxygenase and the oxidation monitored by the formation of conjugated diene is shown in Fig. 2.2. In the absence of transition metal, a rapid increase in absorbance occurs, with no lag phase, which ceases after a period of about 90 min under these conditions. If copper is added to promote LDL oxidation at this point, LDL treated with lipoxygenase oxidizes at a faster rate with a short lag phase when compared to the control. During this procedure there is only a minimal loss of a-tocopherol and so we may ascribe the shortened lag phase to the increase in lipid peroxides brought about by lipoxygenase treatment. A similar result was found when LDL was supplemented with preformed fatty acid hydroperoxides (O Leary eta/., 1992). 

Cellular lipoxygenases have been implicated as possible enzymatic mediators of endothelial cell-dependent oxidation of LDL. Inhibitors of lipoxygenase, but not cyclooxygenase, have been shown to be effective inhibitors of LDL oxidation using rabbit endothelial cells (Parthasarathy etal., 1989). Interestingly, a phospholipase A2 activity intrinsic to apo-B has also been implicated in the endothelial cell-dependent modification of LDL (Parthasarathay et al., 1985). 

Kanner, J., Harel, S. and Granit, R (1992). Nitric oxide, an inhibitor of lipid oxidation by lipoxygenase cyclooxygenase and hemoglobin. Lipids 27, 46—49. 

O Leary, V.J., Darley-Usmar, V.M., Russell, L.J. and Stone, D. (1992). Pro-oxidant efiects of lipoxygenase derived peroxides on the copper initiated oxidation of low density lipoprotein. Biochem. J. 282, 631-634. 

Parthasarathy, S., Wieland, E. and Steinberg, D. (1989). A role for endothelial cell lipoxygenase in the oxidative modification of low density lipoprotein. Proc. Nad Acad. Sci. USA 86, 1046-1050. 

Sparrow, C.P. and Olszewski, J. (1992). Cellular oxidative modification of low density lipoprotein does not require lipoxygenases. Proc. Natl Acad. Sci. USA 89, 128-131. 

Sparrow, C.P., Parthasarathy, S. and Steinberg, D. (1988). Enzymatic modification of LDL by purified lipoxygenase plus phospholipase Ai mimics cell mediated oxidative modification. J. Lipid Res. 29, 745-733. 

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