Radical Reactions

Excess amounts of radicals, molecules having unpaired electrons, e.g., peroxyls (ROO"), can be created in tissues exogenously, e.g., by light exposure, or endogenously, e.g., by overexercising. Radicals react with lipids, proteins, and DNA causing damage, which possibly contributes to disease symptoms and aging. The special properties of the polyene chain make carotenoids susceptible to elec-trophilic attack, resulting in formation of resonance-stabilized radicals that are less reactive.

Three possible reactions can occur with carotenoids.

1. Adduct formation (CAR. + R"! R-CAR•); these products should be stable because of resonance in the polyene structure. If the radical were a lipid per-oxyl, this reaction (CAR + ROO"! ROO-CAR") would prevent further propagation (chain-breaking).

2. Hydrogen atom abstraction (CAR + R"! CAR" + RH), where a hydrogen atom is taken from the carotenoid allylic to the polyene chain, leaving a resonance-stabilized carotenoid radical.

3. Electron transfer (CAR. + R"! CAR"+ + R"), which has been reported in plant and cyanobac-terial photosystems using laser flash photolysis of Photosystem II.

In many cases, the products formed are colorless, thus revealing the bleaching effect of many oxidants on carotenoids. Further oxidation of the carotenoid or carotenoid radical can occur as in studies of soybean (Glycine max) and recombinant pea (Pisum sativum) lipoxygenase-mediated cooxidation of carotenoids and polyunsaturated fatty acids. Approximately 50 breakdown products of ¡-carotene were detected. This large number of products seems to indicate a random attack along the polyene chain of ¡-carotene by a linoleoylperoxyl radical. Studies using potassium permanganate, a metalloporphyrin (a P450 enzyme center mimic), and autooxidation have been performed with lycopene, resulting in formation of a number of apo-lycopenals and apo-lycopenones. However, only two metabolites of lycopene have been identified in human plasma, 2,6-cyclolycopene-1,5 diols A and B (Figure 1). Additionally, seven metabolites of the carotenoids lutein and zeaxanthin have been detected in human tissues.

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