Figure 1.

Central and eccentric cleavages of β-carotene. Oxidative cleavage of β-carotene at the 15, 15′ double bond is catalyzed by the enzyme β-carotene 15, 15′-dioxygenase 1 (BCO1) and yields two molecules of retinal (β-apo-15-carotenal). Cleavage at other double bonds leads to the formation of β-apocarotenals and β-apocarotenones. The cleavage at the 9′, 10′ double bond is catalyzed by β-carotene 9′10′-dioxygenase 2 (BCO2) and yields β-apo-10′-carotenal and β-ionone. Eccentric cleavage at other double bonds occurs nonenzymatically or may be enzyme-catalyzed. In cells, the β-apocarotenals can be reduced to the corresponding β-apocarotenols by aldehyde reductases and/or alcohol dehydrogenases. The β-apocarotenals can also be oxidized to the corresponding β-apocarotenoic acids by aldehyde dehydrogenases. (Figure 1 from [4] with permission).