The Reaction Mechanism of the Enzyme-Catalyzed Central Cleavage ofβ-Carotene to Retinal

Abstract

Ein kleiner Paradigmenwechsel: Das Enzym, das die zentrale Spaltung von β‐Carotin 1 zu Retinal 2 katalysiert, ist entgegen der bisherigen Annahme keine Dioxygenase. Die Ergebnisse der Inkubation des Substratanalogons α‐Carotin in Gegenwart von stark angereichertem 17O2 und H218O belegen einen Monooxygenase‐Mechanismus.

Utilization of Dioxygen by Carotenoid Cleavage Oxygenases

“…In the former case, the non-O 2 -derived oxygen atom found in the reaction is expected to originate from an active site-bound water molecule. A number of studies using isotopically labeled O 2 and H 2 O were carried out to address this question for CCOs, but the issue still remains debatable (11)(12)(13)(14)(15). A factor complicating the interpretation of such experiments is the exchange of oxygen incorporated into the nascent products with that of bulk water during the sample workup and analysis.…”

Carotenoid metabolism in mammals, including man: formation, occurrence, and function of apocarotenoids

“…Originally, the enzyme was thought to be a dioxygenase ( 46,47 ). However, one study presented evidence that it acts as a monooxygenase ( 58 ). In this latter proposed reaction, a number of points still need to be clarifi ed, such as the nature of the metal complex involved in the epoxidation as well as the extents of oxygen exchanges between species during the 7.5 h enzyme reaction at 37°C.…”

The Human Enzyme That Converts Dietary Provitamin A Carotenoids to Vitamin A Is a Dioxygenase

“…BCO1 was given the Enzyme Commission (EC) number 1.13.11.21 in 1972, designating a dioxygenase (9), 29 years before the first report of an oxygen labeling experiment. A monooxygenase mechanism was proposed for BCO1 in 2001 (10). In that study, ␣-carotene, purified chicken BCO1 and horse liver alcohol dehydrogenase were incubated in an 85% 17 O 2 -95% H 2 18 O environment.…”