beta-Carotene

β-Carotene is an organic, strongly colored red-orange pigment abundant in plants and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 carbons. Among the carotenes, β-carotene is distinguished by having beta-rings at both ends of the molecule. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate. β-Carotene is an organic, strongly colored red-orange pigment abundant in plants and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 carbons. Among the carotenes, β-carotene is distinguished by having beta-rings at both ends of the molecule. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate. β-Carotene is the most common form of carotene in plants. When used as a food coloring, it has the E number E160a.:119 The structure was deduced by Karrer et al. in 1930. In nature, β-carotene is a precursor (inactive form) to vitamin A via the action of beta-carotene 15,15'-monooxygenase. Isolation of β-carotene from fruits abundant in carotenoids is commonly done using column chromatography. It can also be extracted from the beta-carotene rich algae, Dunaliella salina. The separation of β-carotene from the mixture of other carotenoids is based on the polarity of a compound. β-Carotene is a non-polar compound, so it is separated with a non-polar solvent such as hexane. Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is very lipophilic. Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known provitamin A carotenoid. Others include α-carotene and β-cryptoxanthin. Carotenoid absorption is restricted to the duodenum of the small intestine and dependent on class B scavenger receptor (SR-B1) membrane protein, which is also responsible for the absorption of vitamin E (α-tocopherol). One molecule of β-carotene can be cleaved by the intestinal enzyme β,β-carotene 15,15'-monooxygenase into two molecules of vitamin A. Absorption efficiency is estimated to be between 9 and 22%. The absorption and conversion of carotenoids may depend on the form of β-carotene (e.g., cooked vs. raw vegetables, or in a supplement), the intake of fats and oils at the same time, and the current stores of vitamin A and β-carotene in the body. Researchers list these factors that determine the provitamin A activity of carotenoids: In the molecular chain between the two cyclohexyl rings, β-carotene cleaves either symmetrically or asymmetrically. Symmetric cleavage with the enzyme β,β-carotene-15,15'-dioxygenase requires an antioxidant such as α-tocopherol. This symmetric cleavage gives two equivalent retinal molecules and each retinal molecule further reacts to give retinol (vitamin A) and retinoic acid. β-Carotene is also cleaved into two asymmetric products; the product is β-apocarotenal (8',10',12'). Asymmetric cleavage reduces the level of retinoic acid significantly. Since 2001, the US Institute of Medicine uses retinol activity equivalents (RAE) for their Dietary Reference Intakes, defined as follows: