β-Carotene
β-Carotene (beta-carotene) is an organic, strongly colored red-orange pigment abundant in fungi,[7] 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.
Not to be confused with beta-keratin.
Dietary β-carotene is a provitamin A compound, converting in the body to retinol (vitamin A).[8] In foods, it has rich content in carrots, pumpkin, spinach, and sweet potato.[8] It is used as a dietary supplement and may be prescribed to treat erythropoietic protoporphyria, an inherited condition of sunlight sensitivity.[9]
β-carotene is the most common carotenoid in plants.[8] When used as a food coloring, it has the E number E160a.[10]: 119 The structure was deduced in 1930.[11]
Isolation of β-carotene from fruits abundant in carotenoids is commonly done using column chromatography. It is industrially extracted from richer sources such as the algae Dunaliella salina.[12] 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.[13] Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is lipophilic.
Provitamin A activity[edit]
Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known provitamin A carotenoid.[8] Others include α-carotene and β-cryptoxanthin.[8] Carotenoid absorption is restricted to the duodenum of the small intestine. One molecule of β-carotene can be cleaved by the intestinal enzyme β,β-carotene 15,15'-monooxygenase into two molecules of vitamin A.[8][14][15]
Absorption, metabolism and excretion[edit]
As part of the digestive process, food-sourced carotenoids must be separated from plant cells and incorporated into lipid-containing micelles to be bioaccessible to intestinal enterocytes.[8] If already extracted (or synthetic) and then presented in an oil-filled dietary supplement capsule, there is greater bioavailability compared to that from foods.[16]
At the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class B, type 1 (SCARB1). Absorbed β-carotene is then either incorporated as such into chylomicrons or first converted to retinal and then retinol, bound to retinol binding protein 2, before being incorporated into chylomicrons.[8] The conversion process consists of one molecule of β-carotene cleaved by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene, into two molecules of retinal.[8] When plasma retinol is in the normal range the gene expression for SCARB1 and BCO1 are suppressed, creating a feedback loop that suppresses β-carotene absorption and conversion.[16]
The majority of chylomicrons are taken up by the liver, then secreted into the blood repackaged into low density lipoproteins (LDLs).[8] From these circulating lipoproteins and the chylomicrons that bypassed the liver, β-carotene is taken into cells via receptor SCARB1. Human tissues differ in expression of SCARB1, and hence β-carotene content. Examples expressed as ng/g, wet weight: liver=479, lung=226, prostate=163 and skin=26.[16]
Once taken up by peripheral tissue cells, the major usage of absorbed β-carotene is as a precursor to retinal via symmetric cleavage by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene.[8] A lesser amount is metabolized by the mitochondrial enzyme beta-carotene 9',10'-dioxygenase, which is encoded by the BCO2 gene. The products of this asymmetric cleavage are two beta-ionone molecules and rosafluene. BCO2 appears to be involved in preventing excessive accumulation of carotenoids; a BCO2 defect in chickens results in yellow skin color due to accumulation in subcutaneous fat.[17][18]
Industrial sources[edit]
β-carotene is industrially made either by total synthesis (see Retinol § Industrial synthesis) or by extraction from biological sources such as vegetables, microalgae (especially Dunaliella salina), and genetically-engineered microbes. The synthetic path is low-cost and high-yield.[33]