Carotenoids are a group of more than 600 naturally occurring pigments, of which only about 50 can be bioconverted to vitamin A. Because they are highly insoluble in water, carotenoids circulate in the blood with lipids in lipoproteins. In cells, they are present in cell membranes, while liver and adipose tissue are the major tissues where intact carotenoids accumulate. Carotenoids have a 40-carbon skeleton with a series of conjugated double bonds that can be cyclized on each end (Figure 21.3). p-Carotene is the primary provitamin A carotenoid in the human diet. Oxidative cleavage of p-carotene by the 15-15'-dioxygenase enzyme at the central carbon-carbon double bond yields retinal, which can be oxidized to the retinoid (retinoic acid) or reduced to retinol.21 The only


9-cis nelinoie acid

COOH lycopene

COOH lycopene

FIGURE 21.3 Structures of ß-carotene, all-trans retinoic acid, 9-cis retinoic acid, and lycopene. ß-Carotene is a provitamin A carotenoid that can undergo central cleavage to form two molecules of retinal. Retinal can undergo further metabolism to form the retinoids, all-trans retinoic acid and 9-cis retinoic acid, which regulate gene expression via nuclear retinoic acid receptors. Lycopene is a nonprovitamin A carotenoid.

function of P-carotene currently recognized in humans by the Food and Nutrition Board of the Institute of Medicine is to act as a source of vitamin A in the diet.22 Vitamin A plays essential roles in visual function, immune system function, and cell growth and differentiation. The retinoic acid isomers, 9-cis- and all-trans-retinoic acid are retinoids that regulate gene expression through nuclear retinoic acid receptors (RAR and RXR).

In plants, carotenoids have an important antioxidant function as effective quenchers of singlet oxygen formed during photosynthesis. Singlet oxygen is an oxidant most often generated by photosensitization reactions, although it can also be generated during lipid peroxidation.15 Although important for plants, the importance of singlet oxygen quenching to human health is less clear. The ability of P-carotene to inhibit lipid peroxidation is dependent on the system studied. In organic solvents, P-carotene inhibited lipid peroxidation at low oxygen (O2) concentrations comparable with those in most tissues, but it acted as a pro-oxidant at very high O2 concentrations.23 The prooxidant effect was seen at 100% oxygen, but not at ambient conditions (21% O2) or at tissue concentrations, which are much lower.24 P-Carotene did not protect lipids in LDL25 or liver microsomes26 from oxidation in vitro at low or high O2 concentrations, and evidence that P-carotene is an effective antioxidant in humans is limited.27'28 At present, it is unclear whether P-carotene plays an important antioxidant role in humans in vivo, and it is possible that its biological effects are exerted through mechanisms unrelated to its antioxidant activity.29

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