Molecular Analysis of Vitamin A Formation: Cloning and Characterization of β-Carotene 15,15′-Dioxygenases

β-Carotene 15,15′-dioxygenase cleaves β-carotene into two molecules of retinal and is the key enzyme in the metabolism of carotene to vitamin A. Although the enzyme has been known for more than 40 years, all attempts to purify the protein to homogeneity or to clone its gene have failed until recently, when the successful cloning and sequencing of cDNAs encoding enzymes with β-carotene 15,15′-dioxygenase activity from Drosophila (J. von Lintig and K. Vogt, 2000, J. Biol. Chem. 275, 11915–11920) and chicken (A. Wyss et al., 2000, Biochem. Biophys. Res. Commun. 271, 334–336) were reported. Very soon it became clear, that we have cloned two members of a new family of carotenoid cleaving enzymes. Overall homologies are very high, certain amino acid stretches almost identical. Thus, β-carotene 15,15′-dioxygenase can be considered as evolutionarily well conserved. These findings open up wide perspectives for further analysis of this important biosynthetic pathway, concerning basic and medical research as well as biotechnological aspects related to vitamin A supply, which are discussed here.