Substrate specificity of a recombinant chicken β-carotene 15,15'-monooxygenase that converts β-carotene into retinal

The recombinant beta-carotene 15,15'-monooxygenase from chicken liver was purified as a single 60 kDa band by His-Trap HP and Resource Q chromatography. It had a molecular mass of 240 kDa by gel filtration indicating the native form to be tetramer. The enzyme converted beta-carotene under maximal conditions (pH 8.0 and 37A degrees C) with a k (cat) of 1.65 min(-1) and a K (m) of 26 mu M and its conversion yield of beta-carotene to retinal was 120% (mol mol(-1)). The enzyme displayed catalytic efficiency and conversion yield for beta-carotene, beta-cryptoxanthin, beta-apo-8'-carotenal, beta-apo-4'-carotenal, alpha-carotene and gamma-carotene in decreasing order but not for zeaxanthin, lutein, beta-apo-12'-carotenal and lycopene, suggesting that the presence of one unsubstituted beta-ionone ring in a substrate with a molecular weight greater than C-30 seems to be essential for enzyme activity.