NEBRASKA REDOX BIOLOGY CENTER EDUCATIONAL PORTAL

Nebraska Redox Biology Center Educational Portal


Beta-carotene

Carotenoids are pigmented fat-soluble compounds synthesised by plants and photosynthetic micro-organisms. Carotenoids are present in many foods, including fruit, vegetables, and fish. These compounds are rosponceble for red and yellow colors of some fruits and vegetables, birds, insects and marine invertebrate. The carotenoids are distributed widely in nature and have wide range functions, for example, light harvesting and photoprotective roles in photosynthesis and ROS scavenging. There are more then 600 different carotenoids identified to date. Beta-carotene is a precursor of vitamin A and the most studied carotenoid [ 1, 2, 3, 4 ].

Cleavage of beta-carotene is catalyzed by beta-carotene 15,15'-dioxygenase resulting in production retinol (vitamin A) [ 5 ].


Carotenoids may scavenge radicals in initial step of reaction that involves one or more of the intermediate steps. Three different scavenging mechanisms are proposed: [ 2, 6, 7, 8 ].

CAR + ROO. → CAR.+ + ROO- ; (Electron transfer reaction)
CAR + ROO. → CAR. + ROOH ; (Hydrogen abstruction)
CAR + ROO. → ROO-CAR. ; (Addition)

The resulting carotenoid radicals have distinct properties and in some cases inter-conversion between the different types of radicals may be possible. Thus, antioxidant properties of carotenoids are related to radical scavenging processes including electron transfer and consequent carotenoid free radical production [ 6, 7, 8 ].

Carotenoid radicals can be reduced by alpha-tocopherol and ascorbic acid which might account for the observed interactions in vitamins A, C and E systems. The resulting alpha-tocopherol and ascorbyl radicals can be recycled by reaction with ascorbic acid and NADH-semidehydroascorbate reductase enzymes: [ 9, 10, 11, 12 ].

CAR.+ + ASCH2 → CAR + ASCH.+ + H+ ;
CAR.+ + ASCH- → CAR + ASCH.- + H+ ;
CAR.+ + TOH → CAR + TO. + H+ ;

Finally, aintioxidant and possible prooxidant function of beta-carotenoids and of their metabolites in the human body are dependent on wide range of factors other than the basic chemical properties of these molecules. Exact reaction mechanisms involving beta-carotenoids remain to be resolved and current knowlage is limited by beta-carotene intermediate radical formation in reactions with ROS. The nature of the biological environment may influence its beta-carotenoids reactions through their interactions with ROS or with other antioxidants [ 2, 6, 7, 8 ].

There is no clear evidence to support the hypothesis that carotenoids may act as prooxidants in biological system. Hovewer, they may act as a prooxidant under conditions of high carotenoid and oxygen concentrations in experimental sysytems [ 2, 13 ]. Furthermore, recent studies and clinical trials have shown that beta-carotene supplementation has little or no beneficial effect on human health and may increase the incidence of lung cancers in smokers, and may have deleterious effects on cardiovascular disease and incidence of tumours as well. High dose of carotenoids was administered in these trial resulting in high plasma carotenoid levels compared to individuals on a normal healthy diet. Thus, prooxidative effect of beta-carotene may contribute to the observed negative effects [ 2, 13, 14, 15 ].

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