Abstract
Prothrombin synthesis and secretion were studied in a human hepatoma cell line (Hep G2) incubated with 35S-methionine for 2 to 24 hours at 37 degrees C. Extracellular and intracellular prothrombin were detected by immunoprecipitation with affinity-purified antiprothrombin antibody. Incorporation of 35S-methionine into prothrombin was monitored by counting specific bands excised from 10% sodium dodecyl sulfate- polyacrylamide gel electrophoresis (SDS-PAGE). Prothrombin represented 0.3% to 0.7% of total newly synthesized protein secreted into the media. Warfarin had no effect on total prothrombin synthesis (extracellular plus intracellular). However, warfarin inhibited secretion of newly synthesized prothrombin by 58% to 73% over a 2 to 4 hour period. This was accompanied by the intracellular accumulation of an immunoprecipitable species of prothrombin of 78 kd, 6 kd less than extracellular prothrombin. At the end of the 4-hour incubation with warfarin, intracellular prothrombin increased from 44% to 82% (twofold) of total prothrombin, whereas extracellular prothrombin decreased from 56% to 19% (threefold) of total prothrombin. After 24-hour incubation with warfarin, intracellular and extracellular immunoprecipitable prothrombin approached control values. Deglycosylation of extracellular and intracellular prothrombin with hydrofluoric acid (HF) resulted in a decrease in mol wt for both species to 66 kd. Endoglycosidase-H treatment, which digests “early mannosyl” residues, resulted in a decrease in the mol wt of the intracellular species of 8 kd with no effect on the extracellular species. Thus, the lower mol wt intracellular species that accumulates following early warfarin treatment is due to the presence of incompletely processed carbohydrate chain. The data are compatible with the hypothesis that optimum glycosylation and secretion require Vitamin K-dependent carboxylation.
