BCO2 (beta-carotene oxygenase 2) is a mitochondrial dioxygenase that catalyzes the asymmetric oxidative cleavage of carotenoids at the 9,10 and 9',10' carbon-carbon double bonds 1. The enzyme demonstrates broad substrate specificity, cleaving both carotenes (β-carotene, lycopene) and xanthophylls (zeaxanthin, lutein, β-cryptoxanthin) to produce ionones and apocarotenoids 12. BCO2 functions as a gatekeeper of carotenoid homeostasis, with genetic deletion leading to enhanced tissue accumulation of carotenoids like zeaxanthin 3. In the retina, BCO2 is expressed in photoreceptor inner segments, retinal pigment epithelium, and ganglion cell layers, where it undergoes mitochondrial processing from a 579-amino acid precursor to a 65 kDa mature protein 4. Notably, human BCO2 exhibits reduced catalytic activity compared to mouse BCO2, with 10-40 fold weaker binding affinities for xanthophyll carotenoids, which may explain the unique accumulation of macular pigments in primate retinas 5. Polymorphisms in BCO2 have been associated with ischemic stroke risk, suggesting broader physiological roles beyond carotenoid metabolism 6. The enzyme plays crucial roles in vitamin A production, oxidative stress regulation, and maintenance of retinal health 24.