Two Carotenoid Oxygenases Contribute to Mammalian Provitamin A Metabolism

Background: Mammalian genomes encode two carotenoid oxygenases, but their contributions to vitamin A homeostasis remain undefined. Results: Mammals employ symmetric and eccentric cleaving carotenoid oxygenases to convert different provitamin A carotenoids to vitamin A. Conclusion: Both carotenoid oxygenases contribute to vitamin A production. Significance: Carotenoids are the major source for vitamin A in the human diet. Mammalian genomes encode two provitamin A-converting enzymes as follows: the -carotene-15,15-oxygenase (BCO1) and the -carotene-9,10-oxygenase (BCO2). Symmetric cleavage by BCO1 yields retinoids (-15-apocarotenoids, C-20), whereas eccentric cleavage by BCO2 produces long-chain (>C-20) apocarotenoids. Here, we used genetic and biochemical approaches to clarify the contribution of these enzymes to provitamin A metabolism. We subjected wild type, Bco1(-/-), Bco2(-/-), and Bco1(-/-)Bco2(-/-) double knock-out mice to a controlled diet providing -carotene as the sole source for apocarotenoid production. This study revealed that BCO1 is critical for retinoid homeostasis. Genetic disruption of BCO1 resulted in -carotene accumulation and vitamin A deficiency accompanied by a BCO2-dependent production of minor amounts of -apo-10-carotenol (APO10ol). We found that APO10ol can be esterified and transported by the same proteins as vitamin A but with a lower affinity and slower reaction kinetics. In wild type mice, APO10ol was converted to retinoids by BCO1. We also show that a stepwise cleavage by BCO2 and BCO1 with APO10ol as an intermediate could provide a mechanism to tailor asymmetric carotenoids such as -cryptoxanthin for vitamin A production. In conclusion, our study provides evidence that mammals employ both carotenoid oxygenases to synthesize retinoids from provitamin A carotenoids.