Walter Kisiel,* 4 William M. Canfield, § Lowell H. Ericsson, and Earl W. Davie abstract: Protein C is a vitamin K dependent glycoprotein which exists in bovine plasma as a precursor of a serine enzyme. Incubation of this plasma protein with a-thrombin at an en-zyme-to-substrate weight ratio of 1: 50 resulted in the cleavage of an Arg-Ile bond between residues 14 and 15 of the heavy chain of the molecule and the formation of activated protein C. The heavy chain of this serine enzyme contains the activesite sequence of-Leu-Cys-Ala-Gly-Ile-Leu-Gly-Asp-Pro-Arg-Asp-Ala-Cys-Gln-Gly-Asp-S££-Gly which is homolo-gous with the corresponding regions of a number of plasma serine proteases. Activated protein C markedly prolongs the kaolin-cephalin clotting time of bovine plasma but not thatof

I^ otein C is a vitamin Kdependent protein present in bovine plasma (Stenflo, 1976). It is a glycoprotein composed of a heavy chain (mol wt 41 000) and a light chain (mol wt 21 000) held together by a disulfide bond (s). The light chain of protein C is homologous in its amino-terminal sequence with that of the four vitamin K dependent coagulation factors. The latter four proteins existin plasma as zymogens and are converted to serine proteases by limited proteolysis (Davieand Fujikawa, 1975).