Vitamin K-dependent carboxylase. Partial purification and properties of the enzyme-substrate complex

The vitamin K-dependent carboxylase extracted from rat liver microsomes by 3-([3-cholamidopropyl] dimethylammoniol)-1-propane sulfonate detergent solution has been partially purified by chromatography on Ultrogel AcA-34 followed by carboxymethyl-Sepharose chromatography and pentapeptide affinity chromatography. The carboxylase appears to be composed of two proteins, the enzyme and endogenous substrate as judged by the incorporation of 14CO2 into trichloroacetic acid insoluble protein. The apparent Km for Phe-Leu-Glu-Glu-Leu as carboxylation substrate is approximately 3 mM. 2,3,5,6-Tetrachloro-4-pyridinol at 10 microM inhibits 90% of the enzyme activity, whereas maximal stimulation (1.7-fold) by pyridoxal 5'-phosphate is obtained at 1 mM and by Mn2+ at 5 mM. The stimulation by pyridoxal 5'-phosphate and by Mn2+ are not additive. The carboxylation of Phe-Leu-Glu-Glu-Leu at 20 degrees C is linear for 90 min. Vitamin K1 plus NADH do not replace vitamin K1 hydroquinone, indicating that vitamin K reductase is not part of this purified carboxylase-substrate complex. Vitamin K epoxidase activity co-elutes with the carboxylase complex. Some 400-fold purification from microsomes has been obtained to yield enzyme preparations with a specific activity of approximately 17,000 pmol of CO2 fixed into peptide/mg of enzyme protein, which is some 15-fold greater than any previously reported enzyme preparation from rat liver microsomes.