Proteolytic cleavage of vitamin K-dependent bovine plasma protein S by α-thrombin and plasma serine protease

It is known that protein S, a vitamin K-dependent plasma protein, isolated from a human source, gives a closely spaced doublet on sodium dodecyl sulfate-polyacryl amide gel electrophoresis after reduction and that this heterogeneity in molecular size results from a limited proteolysis of protein S mediated by α-thrombin in human species. We found here that a-thrombin also rapidly converted single-chain bovine protein S to a nicked form, which consisted of the NH₂ segment containing γ-carboxyglutamic acid and the COOH-terminal large segment bridged by a distilfide linkage(s). These two segments were isolated and chemically characterized after S-alkylation of the nicked protein S. The results suggest that the α-thrombin-catalyzed hydrolysis of protein S probably occurs at a peptide linkage (Arg-Ser) located in the NH₂ portion. The conversion of single-chain protein S to the nicked form was also mediated by plasma kallikrein and plasmin, in addition to α-chymotrypsin and trypsin. However, the α-thrombin-catalyzed conversion did not occur when calcium ions were added to the reaction mixture.