Photooxidation lipids

FIGURE 12.4 Seasonal variation in susceptibility to oxidation of Baltic herring lipids. Lipid photooxidation rate ( b ) of Baltic herring caught in different months from y = a + bx y-PV mg 0/100 g lipids x-time (min.) exposure of lipids to UV. (From Kolakowska et al., unpublished data and data adapted from Kobakowska, A. et al., 1992, Quality Assurance in the Fish Industry, Huss, H.H., Jakobsen, M., and Liston, J., Eds., Elsevier, Amsterdam, 81.)... 

Likewise, photochemical reactions of N-oxides are considered in the following, not the use of N-oxides in photochemistry. Therefore, the use of nitrones, in particular of cyclic polysubstituted cyclic nitrones such as 5,5-dimethylpyrroline N-oxide (DMPO, 8, see Equation 99.4), as radical traps for oxygen-centered radicals, in particular the hydroxyl radical or the superoxide anion,is not discussed here. In fact, these nitrones are used for determining the mechanism of photoinduced oxidation reactions, for example, in lipid photooxidation " or in oxidative DNA damage but are not undergoing a photochemical reaction under such circumstances. 

Since ascorbate reduces photooxidation of lipid emulsions and multivitamin preparations (see Figure 4) [19], Lavoie et al. [34] studied the formation of oxidative by-products of vitamin C in multivitamins exposed to light. They found that the loss of ascorbic acid in photoexposed multivitamin preparations was associated with the generation of products other than dehydroascorbate and 2,3-diketogulonic acid, which are the usual products of vitamin C oxidation. The authors showed that hydrogen peroxide at concentrations found in TPN solutions induced the transformation of dehydroascorbate into new, biologically active compounds that had the potential to affect lipid metabolism. They believe that these species have peroxide and aldehyde functions [35]. 

The lack of inhibition of lysis by catalase in the resealed vesicles prompted the proposal that the precursor which reacted with O to form the lytic species was not H2O2 but instead a lipid hydroperoxide the marked sensitization of the vesicles to the lytic effects of O , which prior photooxidation produced, was consistent with this proposal. Lysis of bovine red cells exposed to Oj , however, was inhibited by superoxide dismutase but not by catalase. ... 

Ultraviolet light damages proteins as well as DNA. Residues of Trp, Tyr, His, Cys, and Met are especially susceptible to photolysis, or photooxidation by 02, or by singlet oxygen. Also damaged are unsaturated lipids, porphyrins, flavins, etc. Kynurenic acid (Fig. 25-11) and urocanic acid (Eq. 14-44), an important ultraviolet filter in skin,196 are also decomposed by light. 

This process continually generates lipid free radicals. The formation of nonradical products resulting from the combination of two radical species can terminate this chain reaction or propagation. Alternatively, unsaturated lipids can form hydroperoxides by reacting with singlet oxygen produced by sensitized photooxidation, which is a non-free-radical process. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

Oxidation of lipids, in addition to the free radical process, can be brought about by at least two other mechanisms—photooxidation and enzymic oxidation by lipoxygenase. The latter is dealt with in Chapter 10. Light-... 

During this process, the position and geometry of the double bond may change. The hydroperoxide mixtures produced by autoxidation and photooxidation are not the same, indicating that different mechanisms are involved. Free radical oxidation can be promoted or inhibited. Deliberate promotion speeds the polymerization of drying oils, and strenuous efforts are made to inhibit the onset of rancidity in edible oils. Frankel has recently reviewed this topic in depth (41) see also (1) for an extensive discussion of oxidation of food lipids. 

Antioxidents Lipid oxidation is influenced by many factors the medium, oxygen concentration, temperature, light, degree of unsaturation, and metal ions among others. In the presence of oxygen, oxidation cannot be entirely prevented nor can it be reversed, but it can be inhibited, delaying the buildup of oxidized products to unacceptable levels. Antioxidants can interact with several steps of free-radical or photooxidation. Their performance is medium and concentration dependent and requires care as they can also act as prooxidants under some conditions (51). 

Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 ... 

Hsu ER, Gebert MS, Becker NT, Gaermer AL. Effects of plasticizers and titanium dioxide on the properties of poly(vinyl alcohol) coatings. Pharm Dev Technol 2001 6(2) 277-284. Kakinoki K, Yamane K, Teraoka R, et al. Effect of relative humidity on the photocatalytic activity of titanium dioxide and photostability of famotidine. J Pharm Sci 2004 93(3) 582-589. Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 75-80, 82. 

The targets of photooxidative reactions may be proteins, lipids, DNA, RNA, and/or cell membranes (Straight and Spikes, 1985). In vitro tests can be designed to determine the specific sites of damage to the various ocular compartments (i.e., lens and retinal epithelial cells and photoreceptor cells) and the products of those reactions. Table 11.2 presents a summary of additional biochemical and photophysical techniques that can be performed to predict the potential for and extent of in vivo phototoxicity more accurately. 

Some food constituents are sensitive to photooxidation. Oil and fats are the most sensitive food constituents for photooxidation because they contain a large number of double bonds in their structures and probably due to the greater solubility of molecular oxygen in the lipid phase as compared to the aqueous phase 1,2). Flavor compounds especially five membered heterocyclic flavor compounds such as alkylfuran, alkylpyrrole and alkyloxazole compounds easily undergo photooxidation reaction (3)... 

Capsanthin (Figure 1) is the most abundant carotenoid in the paprika spice. The concentration of capsanthin is about IS90 mg per kg of dry matter and 41 54% of total carotenoids in paprika 18,19). The antiphotooxidative effect and kintics study of capsanthin on soybean oil has not been well studied. In addition, the study of the antiphotooxidative effect of carotenoids mostly focused on lipid or fatty acids. The antioxidative effect of carotenoids on the photooxidation of flavor compounds has not been well studied, either. 

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