Protein S and Thrombosis*

Thrombosis is a serious problem in the U nited States. Activated protein C is a vitamin K-dependent serine protease that seems to be an im portant regulator of the hemostatic process. Protein S is another vitamin K-depen dent protein that is essential for the expression of the anticoagulant activ ity of activated protein C. Evidence suggests that defects in either of these proteins may be related to the developm ent of thrombosis. Therefore, the accurate m easurem ent of these proteins is im portant in the study and in the treatm ent of thrombosis. Sixty percent of the protein S found in hum an plasma is bound to the com plem ent com ponent C4b-binding protein, and when in this complex, protein S is inactive. The remaining 40 percent is the free, functional form of protein S. This paper describes the m easurem ent of both free and total protein S by two types of assays. The first assay involves immunoelectro-phoresis, also known as the Laurell Rocket m ethod. The second assay is a capture enzyme linked imm unosorbent assay (ELISA). The two assays are compared as to relative ease and cost