Antioxidant Protection from UV- and Light-Stress Related to Carotenoid Structures

Abstract

This review summarizes studies of protection against singlet oxygen and radical damage by carotenoids. The main focus is on how substitutions of the carotenoid molecules determine high antioxidant activities such as singlet oxygen quenching and radical scavenging. Applied assays were carried out either in vitro in solvents or with liposomes, and in a few cases with living organisms. In the latter, protection by carotenoids especially of photosynthesis against light- and UV-stress is of major importance, but also heterotrophic organisms suffer from high light and UV exposure which can be alleviated by carotenoids. Carotenoids to be compared include C₃₀, C₄₀ and C₅₀ molecules either acyclic, monocyclic or bicyclic with different substitutions including sugar and fatty acid moieties. Although some studies are difficult to compare, there is a tendency towards mono and bicyclic carotenoids with keto groups at C-4/C-4' and the longest possible polyene structure functions to act best in singlet oxygen quenching and radical scavenging. Size of the carotenoid and lipophilic substituents such as fatty acids seem to be of minor importance for their activity but hydroxyl groups at an acyclic end and especially glycosylation of these hydroxyl groups enhance carotenoid activity.

Keywords: UV-stress; anti-oxidants; carotenoids; light-stress; lipid peroxidation; radicals; singlet oxygen; structure activity relationship.

Figure 1

Formation of reactive oxygen species and their reaction with carotenoids. (A) Photosensitized formation of singlet oxygen, (B) radical formation, (C) reactions of carotenoids with singlet oxygen or radicals and regeneration by ascorbate.

Figure 2

IC₅₀ values of carotenoids as ¹O₂ quenchers assayed according to [20]. (A) C₃₀ 4,4’-diapolycopene derivatives [20,21,22,23], (B) monocyclic carotenoids [24], (C) astaxanthin and β-carotene for comparison [23].

Figure 3

Comparison of ¹O₂ quenching activities of different carotenoids. Arrows indicate increasing activity, brackets indicate similar activity.

Figure 4

Protection by carotenoids against peroxyl radicals (A), against UV with α-terthienyl as the sensitizer (B), carotenoid structures mentioned in Section 3.1 and not shown in other figures (C). AMVN: 2,2′-azobis(2,4’-dimethylvaleronitrile; AAHP: 2,2’-azo-bis(2-amidinopropane) hydrochloride.