Prothrombin biosynthesis: characterization of processing events in rat liver microsomes

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• Prothrombin is a key protein in blood coagulation.
• Vitamin K-dependent carboxylation is essential for prothrombin function.
• Warfarin is an anticoagulant that inhibits vitamin K metabolism.

Purpose of the Study:

• To compare plasma and hepatic microsomal forms of rat prothrombin.
• To investigate the characteristics of prothrombin precursors in rat liver microsomes.
• To elucidate the role of glycosylation in prothrombin processing and function.

Main Methods:

• Sodium dodecyl sulfate-polyacrylamide electrophoresis (SDS-PAGE) for molecular weight determination.
• Isoelectric focusing (IEF) for charge characterization.
• Endo H digestion to assess glycosylation status.
• Tunicamycin treatment to study the effect of glycosylation inhibition.

Main Results:

• Warfarin-treated rats showed a major microsomal prothrombin species (78,500 MW, pI 6.3-6.5) distinct from plasma prothrombin (83,500 MW, pI 5.3-5.7).
• Intracellular precursors (83,500 MW) in normal rat liver microsomes contain complex carbohydrates, while the 78,500 MW form is a high mannose precursor and substrate for vitamin K-dependent carboxylase.
• Glycosylation is not essential for carboxylation or hepatic secretion of prothrombin, as shown by tunicamycin treatment.
• Aglyco forms suggest intracellular prothrombin precursors possess a basic peptide (approx. 1500 MW) absent in plasma prothrombin.

Conclusions:

• Warfarin induces the accumulation of a specific, partially processed intracellular prothrombin form.
• The 78,500 MW microsomal prothrombin is a direct substrate for vitamin K-dependent carboxylation.
• Intracellular prothrombin processing involves modifications, including the potential addition and removal of a basic peptide, and differential glycosylation, independent of carboxylation and secretion.