The Reduction of Vitamin K, by an Enzyme from Dog Liver*

Most of the research concerning the biological role of vitamin Kr (2methyl-3-phytyl-l , 4naphthoquinone) has been concerned with its function in blood clotting (1) ; however, substances which possess vitamin K activity are found in organisms such as bacteria and higher plants (1,2) which have no blood-clotting system. The role of the vitamin in these organisms has not been established but may be related to one of the unique properties of naphthoquinones, viz. the ease with which they undergo oxidation and reduction. The possible participation of a naphthoquinone in electron transport was suggested by a previous study (3, 4), in which an enzyme from Escherichia coli was described which catalyzed the transfer of electrons from the reduced pyridine nucleotides to menadione (Zmethyl-1 ,4-naphthoquinone) and was accordingly designated pyridine nucleotide-menadione reductase. This activity was also found in mammalian and plant tissues. Vitamin Kr, tested as an emulsion, could not substitute for menadione as an electron acceptor. Whether this was due to the unsuitable nature of the emulsion or due to the specificity of the enzyme was not determined. In order to determine whether vitamin Kr could serve as an electron carrier, various agents were used to solubilize it for testing with a menadione reductase from dog liver. Of the agents tried, BRIJ35 (polyoxyethylene lauryl alcohol) served well in solubilizing vitamin K1 so that it could replace menadione in the assay system. The present report is concerned with the purification and properties of an enzyme from dog liver which catalyzed the reduction of vitamin Ki.