In the liver vitamin K epoxide, which is produced during the posttranslational carboxylation of proteinbound glutamic acid residues, is recycled by the action of one or more dithiol-dependent reductases. In vitro synthetic dithiols may serve as a cofactor for these enzymes, but the physiological reluctant has not yet been found. In this paper we report that in vitro the commercially available thioredoxin/thioredoxin reductase from E. coli can replace the synthetic dithiols during the various reactions of the vitamin K cycle. Based on the assumption that in vivo thioredoxin also plays a role in the regeneration of vitamin K hydroquinone from the epoxide, an extension of the generally accepted vitamin K cycle is proposed.
