Antioxidant activity of carotenoids

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Abstract

Carotenoids are pigments which play a major role in the protection of plants against photooxidative processes. They are efficient antioxidants scavenging singlet molecular oxygen and peroxyl radicals. In the human organism, carotenoids are part of the antioxidant defense system. They interact synergistically with other antioxidants; mixtures of carotenoids are more effective than single compounds. According to their structure most carotenoids exhibit absorption maxima at around 450 nm. Filtering of blue light has been proposed as a mechanism protecting the macula lutea against photooxidative damage. There is increasing evidence from human studies that carotenoids protect the skin against photooxidative damage.

Introduction

Carotenoids are among the most common natural pigments, and more than 600 different compounds have been characterized until now, with β-carotene as the most prominent (Olson and Krinsky, 1995). Carotenoids are responsible for many of the red, orange, and yellow hues of plant leaves, fruits, and flowers, as well as the colors of some birds, insects, fish, and crustaceans. Only plants, bacteria, fungi, and algae can synthesize carotenoids, but many animals incorporate them from their diet. Carotenoids serve as antioxidants in animals, and the socalled provitamin A carotenoids are used as a source for vitamin A. Carotenoids attracted attention, because a number of epidemiological studies have revealed that an increased consumption of a diet rich in carotenoids is correlated with a diminished risk for several degenerative disorders, including various types of cancer, cardiovascular or ophthalmological diseases (Mayne, 1996). The preventive effects have been associated with their antioxidant activity, protecting cells and tissues from oxidative damage (Sies and Stahl, 1995). Carotenoids also influence cellular signaling and may trigger redox-sensitive regulatory pathways (Stahl et al., 2002).

Section snippets

Structures of carotenoids

The unique structure of carotenoids determines their potential biological functions and actions (Britton, 1995). Most carotenoids can be derived from a 40-carbon basal structure, which includes a system of conjugated double bonds. The central chain may carry cyclic end-groups which can be substituted with oxygen-containing functional groups. Based on their composition, carotenoids are divided in two classes, carotenes containing only carbon and hydrogen atoms, and oxocarotenoids (xanthophylls)

Antioxidant activity––singlet oxygen quenching, peroxyl radical scavenging

As an attribute to aerobic life the human organism is exposed to a variety of different prooxidants capable to damage biologically relevant molecules, such as DNA, proteins, carbohydrates, and lipids (Sies, 1986; Halliwell, 1996). Among the various defense strategies, carotenoids are most likely involved in the scavenging of two of the reactive oxygen species, singlet molecular oxygen (1O2), and peroxyl radicals. Further, they are effective deactivators of electronically excited sensitizer

Cooperative effects of carotenoids with other antioxidants

The antioxidant defense system of the organism is a complex network and comprises several enzymatic and non-enzymatic antioxidants (Sies, 1993). It has been suggested that interactions between structurally different compounds with variable antioxidant activity provides additional protection against increased oxidative stress. Vitamin C, for instance, the most powerful water-soluble antioxidant in human blood plasma, acts as a regenerator for vitamin E in lipid systems (Niki et al., 1995).

Photoprotection in humans

In biological systems, light exposure leads to the formation of reactive oxygen species which are damaging to biomolecules and affect the integrity and stability of subcellular structures, cells and tissues (Stahl and Sies, 2001; Krutmann, 2000). Photooxidative processes play a role in the pathobiochemistry of several diseases of light-exposed tissues, the eye and the skin.

Age-related macular degeneration is a major cause for irreversible blindness among the elderly in the Western world and

Conclusion

Carotenoids are efficient antioxidants protecting plants against oxidative damage. They are also part of the antioxidant defense system in animals and humans. Due to their unique structure it can be suggested that they possess specific tasks in the antioxidant network such as protecting lipophilic compartments or scavenging reactive species generated in photooxidative processes. They may further act as light filters and prevent oxidative stress by diminishing light exposure. The possible role

Acknowledgments

H.S. is a Fellow of the National Foundation of Cancer Research (NFCR), Bethesda, MD.

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