Direct measurement of vitamin K-dependent enzymes in various isolated and cultured tumor and non-tumor cells.

A modification of the assay for vitamin K-dependent carboxylase is described with which the enzyme could be detected in relatively low amounts of cells (n = 10(6)). Using this assay, we could demonstrate vitamin K-dependent carboxylase activity in hepatocytes, renal tubular cells, osteoblasts, endothelial cells and macrophages, but not in lymphocytes or platelets. The cultured tumor cells UMR-106, B16 and 5583 also contained vitamin K-dependent carboxylase activity. Vitamin K epoxide reductase activity was demonstrated only in cells where vitamin K-dependent carboxylase activity was present. The tumor cells possessed remarkably less K epoxide reductase activity than the normal cells. When cells were cultured in medium containing warfarin, the K epoxide reductase activity was found to be decreased and the amount of non-carboxylated precursor proteins had increased, suggesting an analogous vitamin K mechanism as in liver.