Proteolytic activation of protein C from bovine plasma

Abstract

Protein C is a vitamin K dependent protein present in bovine plasma. It is a glycoprotein (MW approximately 62,000) composed of a H chain (MW 41,000) and a L chain (mW 21,000). The H chain has an amino-terminal sequence of Asp-Thr-Asn-Gln and contains nearly 3/4 of the carbohydrate. The L chain has an amino-terminal sequence of Ala-Asn-Ser-Phe. Incubation of protein C with either factor X activator from Russell''s viper venom or trypsin resulted in the cleavage of an Arg-Ile bond between residues 14 and 15 of the H chain. Concomitant with this cleavage was the formation of a serine enzyme which was inhibited by diisopropyl phosphofluoridate. Liberation of the tetradecapeptide decreased the MW of the H chain from about 41,000 to 39,000 and resulted in the formation of a new amino-terminal sequence of Ile-Val-Asp-Gly in the H chain. No change in the molecular weight of the L chain was observed during the activation reaction. Protein C, like the 4 vitamin K dependent coagulation proteins apparently exists in plasma in a precursor form and is converted to a serine protease by hydrolysis of a specific Arg-Ile peptide bond. The biological substrate for the enzymatic form of protein C and the physiological mechanism whereby protein C is converted to a serine enzyme are not known.