ROLE OF OXYGEN IN THE VITAMIN-K-DEPENDENT CARBOXYLATION REACTION - INCORPORATION OF A 2ND ATOM OF O-18 FROM MOLECULAR OXYGEN-O-18(2) INTO VITAMIN-K OXIDE DURING CARBOXYLASE ACTIVITY

Vitamin K in its hydroquinone form, vitamin KH2, is a cofactor for the enzyme that carboxylates the N-terminal glutamates in six proteins of the blood-clotting cascade. Vitamin KH2 is transformed to vitamin K oxide concurrently with the carboxylation leading to gamma-carboxyglutamate. When vitamin KH2 is treated with O-18(2) in the presence of rat liver microsomes, the product, vitamin K oxide, carries a full atom of O-18 at the epoxide oxygen. This paper reports the partial incorporation of almost 20% of a second atom of O-18 at a carbonyl oxygen of vitamin K oxide. Control reactions demonstrate that exchange with (H2O)-O-18 under the reaction conditions is too slow to account for the additional increment of O-18. It is concluded that the second O-18 arises directly from molecular oxygen and that its incorporation is an integral part of the mechanism of action of vitamin K.