Abstract
Vitamin K is a substrate for the enzyme catalyzing the carboxylation of specific glutamyl residues to gamma-carboxyglutamyl residues in hepatic precursors of a limited number of plasma proteins, including prothrombin. The gamma-carboxylation of these proteins can be blocked by the anticoagulant warfarin; and in the bovine and human, warfarin treatment results in the secretion of under-gamma-carboxylated forms of prothrombin into plasma. In the rat, this response is not seen, but plasma prothrombin concentrations are drastically decreased. This response has now been studied in rat hepatoma (H-35) cells in which prothrombin secretion is decreased 90% by incubation in the presence of warfarin. Neither prothrombin mRNA levels nor the apparent rate of prothrombin message translation were decreased when cells were cultured in the presence of warfarin rather than of vitamin K. The pool of intracellular prothrombin precursors is increased threefold by warfarin treatment, and this pool is rapidly secreted when vitamin K is administered. In contrast, continued incubation in the presence of warfarin resulted in the degradation of 60% of this pool in 24 hours. When transport of secretory proteins to the golgi apparatus was blocked with Brefeldin A, this precursor pool was gamma-carboxylated in the presence of vitamin K and no degradation occurred. Lysosomal enzyme inhibitors did not block the degradation, and the data suggest that, in rat hepatocytes, under-gamma-carboxylated prothrombin is specifically targeted to a pathway of protein degradation located in the endoplasmic reticulum.
