title: 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 CO2, O2 and the reduced (hydroquinone) form of the vitamin, as well as a suitable substrate [10,11]. Rat liver microsomes also convert vitamin K1 (phylloquinone) to its stable 2,3-epoxide [12]. Several studies suggest a link between carboxylation and the formation of the epoxide [12--14]. In one of these [14], a survey of rat tissues for vitamin K1 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carboxylating systems have been found in rat and chick kidney [9], in chick bone [15] and in rat spleen and placenta (unpublished observations).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.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002264:Carboxylic Acids / Q000378:metabolism*; D005260:Female; D006801:Humans; D008861:Microsomes / Q000378:metabolism; D008862:Microsomes, Liver / Q000378:metabolism; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D051381:Rats; D014812:Vitamin K / Q000008:administration & dosage / Q000378:metabolism*
chemical_list: D002264:Carboxylic Acids; D014812:Vitamin K; D002118:Calcium