Definition: A member of a class of carotenoids, yellow-red pigments (lipochromes) widely distributed in plants and animals, notably in carrots, and closely related in structure to the xanthophylls and lycopenes and to the open-chain squalene; of particular interest in that they include precursors of the vitamins A (provitamin A carotenoids). Chemically, they consist of 8 isoprene units in a symmetric chain with the 2 isoprenes at each end cyclized, forming either α-carotene or β-carotene (γ-carotene has only one end cyclized). The cyclic ends of β-carotene are identical β-ioninelike structures; thus, on oxidative fission, β-carotene yields 2 molecules of vitamin A. The cyclic ends of α-carotene differ in that one is an α-ionone and the other a β-ionone; on fission, α-carotene, like γ-carotene, yields 1 molecule of vitamin A (a β-ionone derivative).
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