ABSTRACT
From a biological perspective, carotenoids can be the most easily classified as those that possess provitamin A activity, and those that do not. From a chemical perspective, carotenoids are generally classified as the hydrocarbon carotenoids, of which the prototype would be β-carotene, a C40H56 hydrocarbon containing 2 betaionone rings and 11 conjugated double bonds, and the xanthophylls, oxygenated carotenoids such as lutein, C40H56O2, found in all green leaves, with hydroxyl groups on each of the rings. Both compounds can be found in μM concentrations in human serum and are obtained from consumption of carotenoid-containing fruits and vegetables (1). Carotenoids can also be straight-chain molecules, such as lycopene, C40H56, the red pigment found in tomatoes. The principal structural features of carotenoids are the possession of a methyl-substituted, conjugated double-bond system, reflecting their synthesis from isoprene units in the plant (2). Until recently, carotenoids were considered to have two major biological functions: first, the possession of a β-ionone ring by the provitamin A carotenoids allows their bioconversion in mammals to retinoids-compounds with essential roles in vision and in normal growth and differentiation (3); second, the conjugated double-bond system possessed by all carotenoids allows them to be effective lipid-phase antioxidants which can protect cell organelles from free-radicals, believed to be a major source of DNA damage (4). As discussed later, there is now evidence that carotenoids can directly regulate the expression of genes protective against
carcinogenesis and inflammation. The role of carotenoids in the biosynthesis of retinal, the visual pigment in the retina is a separate and discrete function, which will not be considered further here. Instead, we will concentrate on the role of carotenoids as antioxidants and as modulators of gene expression.
