Prothrombin biosynthesis: characterization of processing events in rat liver microsomes

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Abstract
Plasma and hepatic microsomal forms of rat prothrombin have been compared by sodium dodecyl sulfate-polyacrylamide electrophoresis and isoelectric focusing. The major prothrombin species that accumulated in the microsomes of rats treated with warfarin had a MW of 78,500 and a pI in 8 M urea of 6.3-6.5. Plasma prothrombin had a MW of 83,500 and a pI of 5.3-5.7. Microsomes from normal rat liver contain a second pool of precursor with a MW of 83,500, and digestion with the glycosidase Endo H indicated that this form has been processed to contain complex carbohydrates, while the MW 78,500 form is a high mannose form and is the substrate for the vitamin K dependent carboxylase. Treatment of rats with tunicamycin revealed that glycosylation was not essential for carboxylation or secretion from the liver. Comparison of the aglyco forms of prothrombin and its precursors suggests that the intracellular forms contain a basic, MW .apprx. 1500 peptide that is missing from the plasma form of prothrombin.