Characteristics of the vitamin K-dependent carboxylating system in human placenta

Abstract

Gamma-carboxyglutamic acid, formed during the post-translational vitamin K-dependent carboxylation of glutamic acid residues in polypeptides has been identified not only in coagulation factors II (prothrombin), VII, IX and X [1–4], but also in several other plasma proteins [3,5,6] and in protein of bone [7,8] and kidney [9]. In rat liver, carboxylation is mediated through an enzyme system located in the microsomal membrane [10]. The enzyme system requires CO 2, O 2 and the reduced (hydroquinone) from of the vitamin, as well as a suitable substrate [10,11]. Rat liver microsomes also convert vitamin K 1 (phylloquinone) to its stable 2,3-epoxide [12]. Several studies suggest a link between carboxylation and the formation of the epoxide [12–14]. In on of these [14], a survey of rat tissues for vitamin K 1 epoxidation revealed that, in addition to liver, this activity was also possessed by kidney, bone, spleen and placenta. In preliminary experiments, vitamin K-dependent carboxylating systems have been found in rat and chick kidney [9], in chick bone [15] and in rat spleen and placenta (unpublished observations). In this communication, we describe some of the basic characteristics of the vitamin K-dependent carboxylating system as found in human placental microsomes.