FIGURE 11.

Proposed scheme for the metabolism of provitamin A carotenoids. β-Carotene is converted by BCO1 in the cytoplasm. β-Cryptoxanthin and other asymmetric carotenoids are transported to mitochondria. In mitochondria, BCO2 converts β-cryptoxanthin by oxidative cleavage at the C9,C10 double bond yielding β-apo-10′-carotenal and 3-hydroxy-β-ionone. β-Apo-10′-carotenal is reduced to the corresponding alcohol and transported to the cytoplasm by yet to be identified proteins as indicated by the question marks. β-Apo-10′-carotenol can then be esterified by LRAT and/or converted by BCO1 by oxidative cleavage to all-trans-retinal.