Bovine liver vitamin K-dependent carboxylase was compared with that obtained from human liver and placenta. Human liver microsomal preparations contained more endogenous substrate than did bovine preparations, but no differences were found between the two types of hepatic enzyme. This observation demonstrates that the bovine liver carboxylating enzyme system is a good model system which will help us to understand vitamin K action in man. Placental carboxylase differed from the liver systems because only vitamin K hydroquinone and not vitamin K quinone could be used as a coenzyme for the carboxylation reaction. Obviously, vitamin K reductase was absent in these preparations.
