sentence: These are the beta-carotene 15,15 ' - monooxygenase 1 ( BCMO1 ) responsible for the symmetric cleavage of β - carotene into two molecules of retinal [ 4-6 ] , and the beta-carotene oxygenase 2 ( BCO2 ) responsible for the asymmetric cleavage of β - carotene into β - apo-10 ' - carotenal ( C ) and β - ionone ( C ) [ 7 ] .