Isolation and Partial Characterization of an 18 kDa Carotenoid-Protein Complex from Carrot Roots

A carotenoid-protein complex was isolated, purified, and partially characterized from carrot juice by detergent treatment followed by ammonium sulfate precipitation, gel filtration, and ion exchange chromatography. Results from SDS-urea-PAGE analysis indicated that the complex contained a major protein subunit with a molecular mass of 18 kDa. HPLC analysis of the pigments associated with this complex indicated that β-carotene, α-carotene, and lutein were the major carotenoids present along with small amounts of phytoene and ζ-carotene. Carrot chromoplast subfractionation and sucrose density centrifugation also resulted in a major carrot pigment fraction which was enriched in the 18 kDa protein. The 18 kDa protein-containing-carotenoid complex is the major complex associated with noncytosolic carrot pigments within the carrot chromoplasts. The relative concentrations of both 18 kDa protein and total carotenoids present in chromoplasts indicate that the carotenoid-protein complex is only associated with a small percentage of total carrot carotenoids. It is thus unlikely that the 18 kDa protein influences carotenoid bioavailability from carrots. This carotenoid-protein complex may function enzymatically in carotenoid synthesis.