The in vivo cleavage of carotenoids into retinoids in chlamydomonas reinhardtii

A variety of carotenoids have been incorporated in vivo into a carotenoid mutant of the unicellular alga, Chlamydomonas reinhardtii. The mutant can synthesize retinal from exogenous carotenoids. In this way the first step in biosynthesis of retinoids, namely how β-carotene is converted into retinal, has been studied. Since retinal forms with opsin the photoreceptor for phototaxis in Chlamydomonas, the action-spectral peaks of the phototaxis restored by carotenoid incorporation were used to predict the products formed by the enzyme that cleaves these carotenoids. The data suggest that the physiologically relevant cleavage of β-carotene into retinal is central rather than excentric. When apocarotenoids are substrates, the enzyme targets the double bond located a constant distance away from the carbonyl group on the acyclic end and, consequently, retinal is not produced. Interestingly, dehydro-analogues containing a triple bond are preferentially cleaved and oxidized at the triple bond relative to the corresponding analogue with only double bonds.