Β-Carotene part of beta-carotene 15,15'-dioxygenase activity
No risk for hypervitaminosis A
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. The conversion process consists of one molecule of β-carotene cleaved by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BC01 gene, into two molecules of retinal. When plasma retinol is in the normal range the gene expression for SCARB1 and BC01 are suppressed, creating a feedback loop that suppresses absorption and conversion. Because of these two mechanisms, high intake will not lead to hypervitaminosis A.
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